Correctly calculating impedance of a biconical antenna and impedance matching - Page 1
Forum section: electronics / rf-microwave

author: polemon
author_rank: Contributor
timestamp: August 29, 2017, 06:15:44 pm
content: Hello, I'm planning to build a biconical antenna, for a center frequency of around 162MHz, and a flare angle of around 30° (both upper and lower cones are identical.I've read in John D. Kraus' "ANTENNAS" (2nd edition), that the impedance of an infinite biconical antenna is Z_r = 120 * ln( cot(theta/2) ), where theta is the half-flare angle, so in my case around 15°.What I found confusing, however, is how to calculate the impedance for a finite (i.e. an actual, practical antenna). Since the center frequency shall be around 162MHz, I imagine a quarter wavelength cone length is in order, but the book states this might not be the case. The rest of the book kinda goes over the top of my head when I really just require a formula to calculate the impedance depending on the flare angle and length of the cone. Is there a book or somewhere where I might find that?Furthermore, the book doesn't really go into detail how the flare angle influences bandwidth. In its extreme form, (flare angle 0°) the antenna is basically just a regular half-wave dipole, but the larger the flare angle, the wider the band of the antenna. How exactly the angle relates to the band widening, is not explained.And last but not least, How do I connect my Antenna to a 50 Ohm RG8 coax cable, when the impedances don't match. According to my (rather mediocre) calculations, the impedance of the antenna is much higher than 50 Ohm, roughly about 100 Ohms, How do I match them up properly? I imagine I need some sort of balun if I want to attach the lower cone to the shield of the coax and the upper cone to the core?

author: CopperCone
author_rank: Super Contributor
timestamp: August 29, 2017, 07:55:53 pm
content: I made some bicones before. The formulas for understanding them are difficult and involved alot of integrals and calculus.What I remember is that if you make it as tall as wide, it should be about 50 ohms. Also, some people said if you want it to be more broad band, is to avoid the balun and feed it from one of the cone bottoms (this is what I did). I made mine twice as tall as wide however, by accident. It is like 3 feet tall by 1.5 foot wide.Based on what you say, it should be for like 900MHz ?Also, I made a smaller bicone thats approximately 2.5 inches by 8 inches, and I found that when they are plugged into a SA, the gain of the antennas is very similar past like 300MHz.Both are fed through the bottom. The large one I cut the top off the one of the cones, soldered a BNC connector in there (good for 4 GHz), mounted the two cones on spacers seperated by fiberglass rod, and soldered the tip of the BNC connector to the tip of one of the cones.The smaller one I used rg141 hardline to do the same thing.

author: polemon
author_rank: Contributor
timestamp: August 29, 2017, 09:42:24 pm
content: Well, the calculus and the maths for calculating the characteristics isn't a big deal to me, it's just that I can't find these formulas that I need.I cannot find one single source explaining the characteristics depending on flare angle, cone length, etc.Btw, the center frequency should be at 162MHz, as I've stated in my first post, I don't need them to be exceptionally wide band, around 100MHz - 300MHz would be more than sufficient.Same thing for impedance depending on flare angle, cone length, etc.Also, once I have these numbers in place: how should I construct the feeder? How should the coax be connected to the cones? Should it be crimped, soldered, etc? How large and what sort of spacer should I put between the two tips of the cones?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 29, 2017, 10:52:56 pm
content: There oughta be formulas out there, somewhere. I don't know offhand, though.If nothing else, minimum is 1/4 wave as you guess, but the impedance is going wonky down there, because of reflection from the edges. That might be a reason why impedance and bandwidth are harder to find. I'd guess 50% more length than needed would get you pretty reasonable performance, but I don't have measurements to put behind that.HF limit is determined by symmetry (how much does the radiation pattern break up?) and how ideal the apex is. Probably you could use a pretty hokey structure, wireframe if it's easier, or foil wrapped on cardboard say, to get a modest (few octave) bandwidth.Tim

author: polemon
author_rank: Contributor
timestamp: August 30, 2017, 12:49:54 am
content: Well yeah, like I said, the textbook I've referenced kinda deals with it to some extent, but it also leaves out the details.I've checked the current version of the ARRL handbook, and there's really no talk about biconical or discone antennas, unfortunately.Now that I'm revisiting the material as I'm writing this, it seems the resistance is minimal and reactance is almost 0 when the length of the cone is 1/4 lambda. However the book is confusing when it comes to the formulas, as it claims they're only usable for small angles of around 3°, where the rest of the data for its diagrams is coming from, is not stated. The book assumes "thin cones" in almost all its mathematical parts, which isn't really helping.There are diagrams in that book (the ANTENNAS book), but they're of such coarse resolution, that they're not really usable for deducing values.

author: CopperCone
author_rank: Super Contributor
timestamp: August 30, 2017, 02:11:19 am
content: https://www.slideshare.net/yariz16/design-and-application-of-biconical-antennathis is useful and concise

author: polemon
author_rank: Contributor
timestamp: August 30, 2017, 02:38:40 am
content: Quote from: CopperCone on August 30, 2017, 02:11:19 amI found this too, but without much explanation, I find myself poking in the blind a lot. The input impedance is "explained" in just one slide, there are no hints to what a Legendre polynomial or a Hankel function is. Also, I'd kinda like to see perhaps one example or so.Also, when the input impedance is complex, how do I deal with the imaginary part?cdev: I'd love to see the papers you referred to in your post (that you've since deleted).EDIT:There also seems to be an error on slide 4: In the slide it's Z_0 = 60 ln x cot(alpha / 4)The variable 'x' is used only in this instant and is not explained anywhere else. The entire formula is entirely different to my other literature, where the characteristic impedance is Z_k = 120 ln ( cot(alpha / 4) )(theta = alpha/2 so that part checks out)EDIT 2:How the gap 'g' affects the characteristics and how to calculate that, isn't explained anywhere. Not even as a footnote formula.

author: CopperCone
author_rank: Super Contributor
timestamp: August 30, 2017, 03:03:46 am
content: I thought it was a multiplication symbol lolI think I traced their references on their formula, and one was behind a pay wall, and it sounded too complicated to pay for, or maybe you had to contact a university to make a photocopy for you or something, and you needed to be in a university (feels familiar)And I could never find anything on the gap other then some emperical studies from some universities. No formula I found includes it.If you google around it seems that some where less then 2mm is good. Having too wide a gap seems to mess it up. But its just VSWR and Gain plots with different gap sizes.if someone is in a university, feel free to send me a special PM lolhttp://ieeexplore.ieee.org/document/1701411/i am not paying for papa and kings old ass publication, their probably in the ground by now, and im probobly never gonna attempt solving somethign that has a legrende polynomial, henkel function and a complex auxiliary function in it

author: cdev
author_rank: Super Contributor
timestamp: August 30, 2017, 03:24:23 am
content: If you want 50 ohms without it being quite a bit wider than it is tall, you should consider using conductive spheres instead of cones.Both will be 50 ohms, however the wide short antenna is going to have advantage as far as gain out to the horizon. It will also be as much as five times wider than the bispherical for an equivalent lower cutoff frequency. So, kind of difficult to work with if made out of metal sheeting.You can make it substantially smaller by enclosing it in a sphere of dielectric material. You could also make it out of wires instead of a continuous sheet.The great thing about frequency independent antennas is you don't have to do the math.If you follow the geometry, they just work.This appears to be a source for the formula..C. H. Papas, and RWP King, "Radiation fromwide-angle conical antennas fed by a coaxial line,"Proceedings. IRE , vol. 39, pp.49?51, Jan. 1951.

author: polemon
author_rank: Contributor
timestamp: August 30, 2017, 03:36:04 am
content: Quote from: CopperCone on August 30, 2017, 03:03:46 amWell, ln takes an argument, just multiplying ln doesn't make much sense, so I believe 'x' is supposed to be a variable.Quote from: cdev on August 30, 2017, 03:24:23 amhaving a flattened bicone is fine with me, as it'll give me wider range (and the application is ground based) so having a flare angle of 90° or more is fine with me, when it gives me a "flattened donut" pattern.But the thing is also understanding. I rather understand what I'm doing so I can build those antennas to my own spec, etc. I'm gonna try to get a version of the ARRL Antenna handbook and see what it says about it, if at all.

author: cdev
author_rank: Super Contributor
timestamp: August 30, 2017, 04:25:13 am
content: What is the intended use? General purpose radio communications between ground based stations?For AIS and NOAA weather radio reception, you would likely always want your gain at the horizon.For a single frequency band you might as well make a co-linear antenna, which will give you much more gain.A biconical antenna is for broadband use and is similar to a discone but has better performance at the horizon than a discone. The angle enclosed by the radiating elements roughly describes the half power points in the pattern.

author: polemon
author_rank: Contributor
timestamp: August 30, 2017, 04:32:56 am
content: Quote from: cdev on August 30, 2017, 04:25:13 amReceiving transponders of ground vehicles. I need an omnidirectional antenna, and using a vertical biconical seemed to be the best bet. I could use just a verdical dipole, but I wanted something slightly more sophisticated. Also, I've never built a bicone before, so I wanted to have a go at it.

author: cdev
author_rank: Super Contributor
timestamp: August 30, 2017, 04:46:14 am
content: Its actually got to be quite a bit wider than it is tall to be 50 ohms exactly. A few companies manufacture them (and they are very expensive)The proportions are similar to those of a hatbox.

author: polemon
author_rank: Contributor
timestamp: August 30, 2017, 05:01:33 am
content: Yeah, I know, Schwarzenbeck makes very good measurement antennas, but they're several hundred Euros each.I was thinking of making one like this:http://www.schwarzbeck.de/images/ANT/BICON/1790/resizedimages/Schwarzbeck%20RE%201790%20.jpgonly more squat with the wires. The problem is: I still don't know the actual angle. Trial and error is one thing, bu I kinda want to be exact, too.

author: CopperCone
author_rank: Super Contributor
timestamp: August 30, 2017, 06:17:55 am
content: it looks like a profesional photo, you can probably measure it with a protractor with less then 5% error.That one looks quite a bit taller then it is wide. Unless you mean the individual cones.

author: Neganur
author_rank: Unknown
timestamp: August 30, 2017, 07:01:01 am
content: PS, I think you can usually access IEEE papers from a library.(At least here in Finland, you don't need to be a student to be a customer at university libraries)

author: polemon
author_rank: Contributor
timestamp: August 30, 2017, 12:51:50 pm
content: Quote from: CopperCone on August 30, 2017, 06:17:55 amThis was just an example of how my antenna should look with the rods, etc. The flare angle on mine can be whatever, if it's like you say that a flatter cone arrangement gives me better width lateral range, I'd go for that.Finding accurate information on calculating impedance for it turns into an enterprise similar to looking for a great white whale...EDIT:One other thing: judging from the pictures and the descriptions of the Schwarzenbeck antennas, they contain some sort of balun and/or impedance matching element.

author: cdev
author_rank: Super Contributor
timestamp: August 30, 2017, 03:01:20 pm
content: A balun at the feed point is quite desirable for a biconical antenna.Then you can dispense with the ferrite beads, they aren't necessary.You can use a 1:4 or a 1:1 RF transformer.Any mismatch will not be corrected by a 1:1 transformer. a 4:1 transformer may be a better pick. They both help. If you use a balun, you get a much quieter signal, without junk added by the feed line imbalance.I use a biconical to receive NOAA VHF hi and low UHF and a balun makes a big difference.Use a broadband part. If you make your own transformer use an appropriate iron powder core.#61 material is I think what they use in CATV baluns. You could use a CATV balun. They are very cheap and widely available.You really will notice that a balun vastly improves the quality of the signal. This is really noticeable for example with FM stereo reception. (in the US now many FM broadcast stations split their signals between vertical and horizontal polarization)

author: polemon
author_rank: Contributor
timestamp: August 30, 2017, 06:21:35 pm
content: Quote from: cdev on August 30, 2017, 03:01:20 pmThe frequency range you use it at pretty much the exact range I need it to: 162MHz. Could you give me the dimensions of your biconical? And what sort of balun did you use?Also, when using a CATV balun: the F-connector part goes to the antenna, and the 8p8c part goes to the N-connector and the coax, right? (or is it the other way round?)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 30, 2017, 09:15:40 pm
content: I'm fond of this type balun:Note that there are two twisted pairs (Zo ~ 100 ohms), in parallel at the BNC (50 ohm), and in series at the antenna (Zo ~ 200 ohm?), with the center tap to ground. (Note that one pair technically doesn't need ferrite beads, but the other does. Doing both does reduce CM currents. Or grounding the tap minimizes CM voltage.)I don't have much for equipment to test this against (like a proper TEM horn, or conical, or..), but it seems to cover a pretty good range, despite its simple design (the wires fan out about half a meter each side, at the angle seen here, and there are some cross wires to keep them in place).Tim

author: cdev
author_rank: Super Contributor
timestamp: August 30, 2017, 10:28:21 pm
content: Here are some baluns and an unun showing the details of how they are wired. These both show how to make em.You can use a switchable balun/unun to give you better power transfer and clean up junk.. They may also help smooth out the response and they may even give you some clues in figuring out the impedance of an antenna.For VHF/UHF use an appropriate material and of course fewer turns.The first two are of a nice switchable HF balun that is made by Elecraft and it has very low loss.I copied it and I often use that for SWL. But you will notice that the design that both of us use is the second of the two switchable choices in the Elecraft balun.The elecraft uses #43 material..For VHF you should use #61 material.I would just pick up a CATV balun. Maybe you already have one.Its "figure 2".

author: polemon
author_rank: Contributor
timestamp: August 31, 2017, 05:01:11 am
content: Quote from: cdev on August 30, 2017, 10:28:21 pmI'll probably go for the CATV solution, but just to clarify: I've attached a picture of a CATV balun. Now, the RJ45 / 8p8c side is the unbalanced, and the side where the F-connector is, is the balanced one, right?Also, you mentioned in your earlier post, that you use a bicone antenna to receive NOAA weather information at around 162MHz, which is in the same vicinity where I need them, could you please explain the dimensions of your antenna? I.e. height, width, length of the rods, etc. Length of the rods and flare angle of the cones would suffice. I'd greatly appreciate that.EDIT:I've since gotten a hold of the 22nd edition of the ARRL Antenna Handbook, unfortunately, no mention of bicone. It goes slightly into discone antennas at one point, but not into biconical. That aside, all measurements are in imperial, which slightly grinds my gears...

author: cdev
author_rank: Super Contributor
timestamp: August 31, 2017, 05:59:33 am
content: I meant the other kind of balun, also I have a splitting headache, so I hope you don't mind my pointing you tohttps://www.researchgate.net/publication/254050053

author: polemon
author_rank: Contributor
timestamp: August 31, 2017, 07:32:14 am
content: Quote from: cdev on August 31, 2017, 05:59:33 amHmm, OK, those were the only baluns I can find when searching for "CATV balun". The paper is a bit much, but I'll try to figure out stuff that I need. It's where that one presentation a couple posts up took their information from, apparently. However in this paper, the formulas actually make sense.The one thing I'm most confused about is if "kl" is supposed to be "k * l" where k = 2?/? and l is the length of the cone. Other than that, I think I'll use a 120° flare angle. How exactly I should manufacture it, especially how the rods should attach to the feed I'm not quite sure about, but I think I'm in the process of wrapping my head around it.

author: cdev
author_rank: Super Contributor
timestamp: August 31, 2017, 03:31:14 pm
content: Basically, assume that the frequency where the elements of the cones would be a dipole is your effective lower frequency limit. (although actually the "wideness" of the conductor extends the frequency a bit lower)120 degrees is around what I am currently using, and it works. (I use that because it would be a major PITA for me now to make it any wider and keep it where I can easily grab it and turn it sideways when i need to do that for FM DX, which I do fairly often.)Taking the same (wire) elements and changing the angle to 60 degrees would require a much larger structure - larger in terms of width, to hold it. (Think, maybe a meter wide, huge)Re transformers, I would look at non-consumer level (professional offerings) RF transformers if you want to receive the highest frequencies. (which CAN be done with this kind of antenna)Do a parametric search for whatever would work for you package wise.A good (SMT or commercial/consumer) iron powder based 4:1 balun properly implemented often can and does work at 1575/1602 MHz - - this varies from one balun to the next- but if you go much higher the loss becomes much larger with all the inline baluns including the ones Ive made myself with #61 cores. This probably has to do with the size of the transformer winding and wavelength/self resonance. the higher you go the smaller the transformer you need. (If anybody here reading this can explain why in a concise way that would be great.)Therefore, you should use the smallest RF transformer you can get away with, the best ones for broadband are really tiny. Do a parametric search, identify some candidates and get them to try out.You should make a small testing jig with each of your transformers and an SMA connector. Keep your leads short. Take care not to use lossy wire. Use low loss insulated very thin wire.As far as shape.. Copy commercial antennas proportions. Visuals of the pictures of successful commercial antennas will show the parameters they use.You can easily take a design and scale it.No need to be too precise, that's the great thing about these antennas.And most importantly, have fun and learn stuff.

author: cdev
author_rank: Super Contributor
timestamp: August 31, 2017, 03:45:58 pm
content: If you are building an antenna to go outdoors I would simply buy a commercial antenna. There is a company in the UK which makes and sells biconical antennas - they cost around $75 shipped to the US. Alternatively consider discones as they seem to be much more popular and have structural advantages.You have an obligation with anything pointy and metallic that's mounted outdoors for it to be safe and physically able to handle wind situations.Also, wherever you live, you likely have neighbors. Commercial antennas have advantages as far as compatibility with norms of behavior in towns, zoning etc. Suppose if in a few years you want to build a tower or something, you don't want to have alienated your neighbors.It also likely helps to be a help in emergencies or similar. Keeping a sturdy antenna working through storms is likely much easier if its built to be a survivor.Certain brands of discones consistently survive "superstorms" with no damage. functionality intact.Others fall apart on their own, no storms required. If dislodged by wind, one of those rods could easily injure somebody.

author: polemon
author_rank: Contributor
timestamp: August 31, 2017, 09:13:44 pm
content: Hmm, yeah, I've only constructed directional narrow band antennas to this point.I was under the impression, that a balun must be precise to its frequency tuning, this doesn't seem to be the case, however.Calculating the actual impedance of the antenna (the biconical antennas, that is) is still quite difficult in the scheme of things, but given there are wider range baluns, this doesn't seem to be as much of an impact. I've got the basic shape of the antenna down. But I've never made a balun myself, I'd have to research this further. I was under the impression, that one of the cheap CATV baluns found on ebay would be a quick solution, but as you said this isn't exactly the case.I'll see if I can make an adequate balun for the frequency range I need it to.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 31, 2017, 09:49:30 pm
content: Quote from: polemon on August 31, 2017, 09:13:44 pmWell, a resonant balun does.If you were using 1/4 wave stubs and whatnot, yeah, those are fairly narrow, like 10 or 20% bandwidth.At these frequencies, a transformer balun is very reasonable. Keep the wire length short (less than 1/4 wave at the maximum frequency of interest), and use whatever core size and type is required to get the necessary magnetizing impedance (Zmag >> Zo so it doesn't load down the line). #43 and #61 ferrites, and several powdered iron mixes, are quite suitable core materials.QuoteAlso, since the bandwidth is wide, it's not that you're going to get screwed with reactance from a mismatch: the antenna is more or less resistive in its passband, whatever that resistance happens to be. If you have 2:1 VSWR, so what, you'll have it over the whole band, so it's just insertion loss. I mean, YES, once it goes up a length of cable, you get dips due to exactly that -- standing waves! But if you can sweep the antenna, this should be obvious, and you can add a little breakout connector to test different resistors in series or parallel with the antenna, to see how far off it is. Then wind a transformer for that match, and you'll have, who knows, maybe 1.3:1 VSWR instead, what better could you ask for?QuoteThe balun I pictured above should be pretty obvious, just from the picture; it's the same as the 1:4 Guanella TLT design also pictured above. You don't need anything fancier than that.Well, give or take if you need a super low VSWR and an oddball ratio, that is.Tim

author: cdev
author_rank: Super Contributor
timestamp: August 31, 2017, 11:31:19 pm
content: You can use an inexpensive 4:1 75 ohm to 300 ohm balun. That's ideal. They are made for VHF/UHF. The issues I brought up aren't relevant until you are trying to receive frequencies in the GHz range. You should be fine at 162 MHz (and for quite a bit above that).I just had to redo mine because its also supporting the weight of the bottom half of the antenna.If you are using your antenna on a non-SDR, non computer-based receiver, or probably also witha Raspberry pi-connected one, (which is a lo smaller) you probably would not have all those problems with computer noise on the feedline and might not even need a balun, or could simply use a couple of common mode chokes clipped around the feedline (or coil your coax tightly around some form at least a half dozen or more times to create some inductance to block common mode junk) and get an adequate cleanup.

author: dazz1
author_rank: Frequent Contributor
timestamp: March 29, 2018, 09:27:02 am
content: HiI found this image :http://www.gtemcell.com/wp-content/uploads/2012/02/DSC02335.jpgof a matching network.I am guessing that there is some feature hidden under the heat shrink that is key to making this work.Just curious to know how this might work.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 29, 2018, 12:41:40 pm
content: Looks like a normal 2:1 balun. Ground wire seems like a red herring. Connections should be just soldered, no hidden components. Black thing ought to be ferrite, but it's strange that it's not a full (magnetic) circuit, so it may have poor balance at low frequency.Tim

author: dazz1
author_rank: Frequent Contributor
timestamp: March 31, 2018, 08:33:24 pm
content: HiI don't see a balun. The coil former seems to be too large to be ferrite so I think the coils are mostly air-cored. I see 2 co-axial coils of coax which indicates these are for common mode rejection. I suspect there is a small ferrite bead under the shrink-wrap. I don't know what the earth is/is not connected to. It seems to be the only point the coax is earthed to the enclosure. It appears that both coax cables are connected directly to the antenna feed point.

author: dazz1
author_rank: Frequent Contributor
timestamp: April 07, 2018, 02:32:26 am
content: HiI assumed that the signal is carried by the coax centre conductor. If so, the coils might block common mode noise on the outside of the shield. If the signal is carried by the coax shield, then this might be a balun. If so, I am not familiar with this topology.Dazz

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 07, 2018, 03:02:55 pm
content: The first joint may have the coax in anti-parallel. The second joint has the coax in series, I think.It wouldn't be correct to say that the signal is carried wholly one way or the other. There will be unbalanced currents; how much depends on the winding, and core (if any).Tim

author: dazz1
author_rank: Frequent Contributor
timestamp: July 05, 2023, 10:12:05 pm
content: HiI have got to the point of making my biconic antenna that I need to design a balun.I was planning on using a Robert balun but when I looked in depth, the Roberts balun was designed for thin dipoles, not a wide band biconic.I found this website that simulates a biconic antennahttps://au.mathworks.com/help/antenna/ug/vhf-uhf-biconical-antenna-for-testing-applications.html#VHFUHFBiconicalAntennaForTestingApplicationsExample-8. The dimensions are very close to my actual antenna. I have 6 not 12 elements. The plots of impedance and return loss are likely to be very similar to my antenna.The impedance plot shows reactance is negative (capacitive) below about 700MHz, and positive (inductive) above about 800MHz. So I need to find a balun that will match.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 08, 2023, 10:30:52 am
content: HiI have been doing some research on research of wide band baluns for biconic antenna. For such a common antenna you would think there is plenty of good practical references out there. What I am finding is that there is little published info to guide a diy build.I have found a reference that indicate a current balun is more balanced than a voltage balun. One antenna maker uses a modified Marchand balun but a Guanella balun could be a better choice.My antenna will only be used to receive, not to Tx, so I don't have to worry about current saturation and power limits.The Tchebycheff tapered balun transformer would have the bandwidth but making and testing one would be a challenge. I think I would need to make a length of large diameter rigid coax.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 17, 2023, 05:19:35 am
content: The problem I have hit with making/buying a suitable wide band balun is the tyranny of distance. The cost of buying a part here is swamped by the cost of shipping across the planet. Buying a couple of ferrite beads or ready-made baluns is expensive and economy shipping takes ages to get things here.I tore apart the Gigabit ethernet connector of a dead motherboard to repurpose the ferrite toroid cores. I figure if they are good enough for Gigabyte data, they might be OK for low level RF to 300MHz or so. Now I just need to find some enamel wire the correct size.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 18, 2023, 03:11:09 am
content: Here are the Ethernet ferrites that I hope to repurpose for the biconic antenna.They are buried within conformal coating, so that will have to come off.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 23, 2023, 09:45:20 am
content: HiI decided to try and figure out how the baluns for Ethernet were wired to see if I could learn anything applicable to antenna applications. I understand why baluns are needed, but I have not played around with them before. Ethernet drives balanced pairs which are similar to driving a dipole. Ethernet needs galvanic isolation, but antenna don't. I will never be using the biconic antenna for susceptance testing, so power limits of small toroids are not a factor.I removed most of the conformal coating from one of the Ethernet toroid pairs to be able to figure out how they were wound as shown in the attached diagram. These coils are small. I was working with a microscope to figure out how they are connected.I think toroid A is probably driven by a differential balanced pair of line drivers. Toroid A looks like it works as a choke to eliminate common mode RF. If so, I think the dots are wrong.Toroid B appears to be a transformer and balun with a single coil driving two balanced pairs. All 4 wires are twisted together so the turns ratio between any of them is 1.If the two coils on A are push/pull driven, and they drive 2 coils on B, I think the impedance transformation is also 1 : 1.The toroids appear to be different materials.Testing balun configs is going to be a challenge. I will need to make a test rig to get consistent results.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 23, 2023, 04:11:37 pm
content: Usually B is driven, push-pull, since the stub length is shorter that way. Indeed it's 1:1, and the PHY (transceiver device) is 50+50 ohms output (usually a current-sinking push-pull amplifier into termination resistors), and the line is 100 ohm differential.CMRR specs are rather modest, actually, but the CMC is an important part of achieving that, and it's so rated up to 100MHz.Tim

author: dazz1
author_rank: Frequent Contributor
timestamp: July 23, 2023, 09:50:23 pm
content: Quote from: T3sl4co1l on July 23, 2023, 04:11:37 pmThe pcb traces to A go to the connector pins. So it appears that B is the driven end.Attached is my proposed method of testing the balun separate from the antenna.I'd like to attach an instrument to the output load side of the balun, but I don't have a balanced to unbal test load. I probably need a balun to do that.

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: July 24, 2023, 07:27:52 am
content: I am surprised there isn’t a mention of using antenna modelling software on this thread. The hugely capable EZNEC Pro is now offered for free by the author.https://www.eznec.com/SJ

author: dazz1
author_rank: Frequent Contributor
timestamp: July 24, 2023, 10:02:28 am
content: Quote from: Solder_Junkie on July 24, 2023, 07:27:52 amI first used NEC back in the 1980s including a version that implemented a genetic algorithm some time in the 1990s to evolve an antenna design over generations.I wasn't aware of ezNEC front end but I haven't looked for it. I have added the link to my bookmarks for future reference.I haven't simulated an antenna because I found enough practical design info to arrive at a design the looks like the others.My immediate problem is producing a wideband balun for a known antenna. To the best of my knowledge, there is no simulation software useful for balun design, especially when I don't know the properties of the ferrite material I have repurposed.There is surprisingly little applicable published research on the topic. There is lots of cook-book type solutions, but not a lot in the way of actual this is the design process.For a simple wideband balun on a toroid, I can choose the:type (eg. Marchard, Guanella etctwists per cmturns around the toroidIf I use a coax based design, the physical dimensions increase the risk of self resonance within the antenna bandwidth.If I use a tiny toroid, I can't use coax and power throughput is limited.The number of combinations to test even a few options starts looking exponential very quickly.Open to suggestions.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 24, 2023, 03:41:08 pm
content: Well, it's not very fancy. The core presents some shunt impedance depending on turns and its permeability, and the windings (typically arranged as transmission lines) determine the HF cutoff. Which, a Guanella has bandwidth limited by construction (TL cross-section, cross-talk, and how they're interconnected), while a Ruthroff is limited by turn length, and more conventional transformer designs are further limited by uncontrolled impedances, CM coupling, and layer effects (when applicable).So, you need enough inductance to get the LF cutoff, enough impedance to have acceptable power dissipation or insertion loss (and a bit of mismatch), and reasonable windings for the impedances desired. Which understandably gets difficult when the impedance is very low (it's a lot of work to just make a low-Z TL at all), and very difficult when high (imagine how much free space you need around 600-ohm twin lead, now make a transformer out of it!).Tim

author: dazz1
author_rank: Frequent Contributor
timestamp: July 25, 2023, 01:16:29 am
content: I still have the problem of measuring the performance of the balun. I don't have a VNA, but even if I did, I would still need an adapter/sensor between the DUT balun and the instrument.Just wondering about modifying a cheap SWR bridge to form a balanced input on one port. That would allow me to insert the modified bridge between the balun and the antenna, like a super balun.I haven't thought about this too much so there is probably good reasons for not doing it. It would allow a balun DUT to be placed between an RF sig-gen and the super balun used as a sensor.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 25, 2023, 03:19:51 am
content: How about making a DM+CM conversion box, with exceptional construction (small transformers with shields?), and characterizing that in the normal mode: drive one CM/DM port, terminate all others, measure amplitude; swap output port/terminator, measure opposite phase amplitude; etc. Bonus points for measuring phase as well and verifying phase match (0° CM, 180° DM, +/- some margin).Typically you use 1:1 baluns for such a beast, and padding resistors to make up the difference (CM goes into 25 ohm, DM goes into 100).Ref:https://www.eevblog.com/forum/projects/diy-dm-cm-seperator-for-emc-lisn-mate/Tim

author: dazz1
author_rank: Frequent Contributor
timestamp: July 25, 2023, 09:07:12 am
content: Quote from: T3sl4co1l on July 25, 2023, 03:19:51 amI like that.There is a lot to digest on the linked thread, plus the links within the thread. I will need to take some time to soak it all in before I decided what to do.Adding phase would be a challenge but a really useful feature. Imbalance (phase error) is a significant error if present in a biconic antenna.

author: nctnico
author_rank: Super Contributor
timestamp: July 25, 2023, 11:07:24 am
content: Quote from: dazz1 on March 29, 2018, 09:27:02 amThis looks like a wideband coaxial transformer. Unfortunately it seems a lot of useful information is hidden behind paywalls. This looks like an interesting paper:https://www.researchgate.net/publication/263635575_Wideband_Impedance_Transformer_Using_a_Coaxial_CableThis looks interesting though and much like the picture:https://pa0fri.home.xs4all.nl/Ant/Balun/balun%20eng.htmIMHO it is not going to help using random ferrite cores and just try stuff without doing some reading first.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 25, 2023, 11:37:26 am
content: Quote from: nctnico on July 25, 2023, 11:07:24 amMaybe good info hidden behind expensive paywalls. Cost does not equal quality. I have in the past handed over (employer) money to get rubbish in return.I have done a lot of reading. My balun folder is shown below. These are the files I have kept and not all I have read.The most promising paper is attached. A lot of the modern stuff is planar super duper high frequency stuff that is not relevant. The really good info is closely held by manufacturers.I agree that experimenting with unknown cores is less than ideal but if I order a $0.01 component, I pay about $USD15 shipping and wait for weeks. At least with the Ethernet cores, I know they have been selected with a similar application, power and frequency range. They are not just entirely random choices. I did spend some time thinking about what I could repurpose.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 25, 2023, 06:20:13 pm
content: Haha, that paper is just dripping with inexperience; but it's exactly what I would expect given its origin: senior design project, undergrad level. You pick a topic, you're barely told anything about it, and told to get to work solving it!The frequency range is entirely unspecified, and neither analysis not discussion of results is present. I suspect their results are equivalent to a helical transmission line through air -- those core memory cores (wow, blast from the past!) are specifically made with substantial hysteresis to yield the logic function necessary. Meaning they have very low initial permeability. Similar results I bet would've been had on an air core. GHz is also above the range where ferrite does much; they could've just looped transmission line through the air with similar results. Likewise, if they had considered which frequency range to prioritize, they could've chosen a more appropriate Microcircuits part; or at least tested several and seen how they respond, not just one and given up on the idea! (Granted, you don't get any budget either, so, buying fromMicrocircuitsis rather hard to justify.)Even basic stuff, like the permeability test fixture, wasn't explored, or justified. Either it works or it doesn't; test a range of parts and compare results to the datasheet. Is it a systematic error or just not representative at all? How could it be changed to be more representative? How do professionals do it? (Well, obviously they just buy the fixture:https://www.keysight.com/us/en/product/16454A/magnetic-material-test-fixture.htmlBut, one might use that as a jumping-off point for appnotes and papers, and develop an understanding of how it's done.)(I say this, having done a senior design project myself, looking back on it. So, part hindsight, but also anyone with experience can, I think, identify these traits and shortcomings of the paper.)But I get the similarity; you're in a similar position, I suppose. Beware that you're balancing the familiar feeling of the search for knowledge, against the certainty of knowledge attained (but knowledge in the abstract, that is often ever so hard to communicate to those outside of that state of knowledge). That is, you identify with the author's inexperience, but you may not have much to learn from the author, or with the correct conclusions. (These are not peer-reviewed, remember; at best they're graded by a professor!) Conversely, it's difficult learning from direct sources, because you lack the stepping-stones of knowledge that they followed to attain that knowledge (or, even if available, those stepping-stones just don't produce the same understanding for you).As for basic parts availability -- RS operates in .au, don't they? Well, maybe their shipping or minimum order is still >= $15, I don't know, but surely something domestic couple-days delivery is available. Offhand I see a half-dozen electronics suppliers that target .au (not necessarilyare in.au, like Digikey), though I don't know what of them are professional/wholesale versus individuals/one-offs. And if so, which ones have any selection of ferrite beads. But it seems like there ought to be something.As for salvage: pulse transformers, common mode chokes, and EMI cores (including beads on cables) are usable here. CMCs and EMI cores kind of less so, because they tend to have either low-frequency priority (CMCs are generally more about getting high impedance for SMPS fundamental -- 100s kHz), or damping (EMI cores are lossy in the 10s to 100s MHz, and may not have all that much impedance or inductance overall for transformer purposes). But given more size, or number of turns, that's still doable.Power cores (ferrite) are usable as well, but watch out for air gaps. Small gaps can be sanded down flat with a lapping plate (e.g. stick a sheet of SiC sandpaper to a sheet of glass or other flat surface, and grind with that; or if you happen to have a diamond sharpening stone, those are very nice), but large gaps, you'll end up running out of space around the bobbin, unless you want to build your own rather than salvage the one it came with. (Or you broke it in the process of salvage...)Ferrite cores can be unglued by heating. Baking to ~200C is a good idea, softens the varnish. If you break the core, it can still be superglued back together (but beware the pieces might not fit exactly anymore, adding air gap; lapping the faces flat again helps maximize effective permeability).Powder cores (usually bicolor toroids, but also blue or black, or other colors) have low mu and cutoff frequency, so generally aren't usable for pulse transformer duty.Ethernet transformers are indeed in the pulse/CMC/EMI category so are fine here; as long as you don't need more winding area or flux capacity, heh.Tim

author: dazz1
author_rank: Frequent Contributor
timestamp: July 26, 2023, 04:19:24 am
content: I understand the limitations of following the work of an under-grad project. I earned 3 degrees so been there, done that, more than once. I don't have experience making baluns so I am climbing that learning curve.My old professor's old professor used to say, it ain't true if it ain't measured.I want to be able to measure the performance of any balun I make to close loop on the design process. At present, I don't have the instrumentation to measure a balanced source/load. In addition to measuring the performance of a balun, it would be useful for measuring dipole antenna, like a biconic.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 26, 2023, 06:25:12 pm
content: Ahhh, got it. I've no indication of peoples' level of knowledge here other than what's immediately mentioned (or occasionally remembered from prior conversation) so I tend to include such details in case they are relevant, and conversely are superfluous when not, which is fine.So what's missing? Make a resistance bridge, perhaps?Like I said, I would test it by wiring up terminations and measuring pairs of ports. Prove that the gain between ports is as it's supposed to be (one port (normal mode) goes into +/-1/2 at CM/DM ports; CM/DM go into +/-1/2 (normal) at the balanced ports), CM is easy enough to measure (set up a 0° splitter into the balanced ports), and, that doesn't completely constrain what DM gain/phase can be but it greatly narrows it down. Just going through the permutations of ports should be illuminating. Preferably this would be done as a vector measurement, but even doing magnitude on the spec or even scope is something.You could further test the balance by making a 180° splitter; this could be as simple as a 0° splitter into an inverter (1:1 transformer), plus a TL to balance the delay. You could for example take two equal lengths of coax, wire one to the splitter (compensation delay, nothing else), and wire the other to the splitter but cut it in half in the middle to swap +/-, and stack ferrite cores all along it (or wind it around a core). The outputs of which can be tested with a resistor divider from one port to the other, the midpoint of which should read zero (zero CM = zero phase/amplitude error in the balun; there could still be a differential gain error where both vary proportionally but this can be constrained by measuring each port normal-mode).Or you could construct two DM/CM splitters and verify their cascaded balance. See: measuring one antenna by building two and measuring their combined transfer function.Tim

author: dazz1
author_rank: Frequent Contributor
timestamp: July 26, 2023, 09:42:47 pm
content: Quote from: T3sl4co1l on July 26, 2023, 06:25:12 pmI am not sure but the DM circuit by@Jay_Diddy_Blooks like it could have potential. CM mode would be useful. Phase difference would be a bonus. If possible, lossless and frequency independent.https://www.eevblog.com/forum/projects/diy-dm-cm-seperator-for-emc-lisn-mate/msg3117742/#msg3117742I really need a DIY instrument grade balun. Measuring balanced loads should be a routine task with "standard" methods.I am nearing completion of two identical biconic antenna you can see at this link:https://www.eevblog.com/forum/rf-microwave/diy-rf-emc-biconic-antenna/msg4369435/#msg4369435I have made two specifically so I can calibrate them back-to-back. The antenna break down to parts for easy storage in a cheap Aliexpress gun case I haven't purchased yet. By far the most difficult part to make is the cone shaped sockets at the apex of the cones. These needed the accuracy and capability of a milling machine to make.I acquired an old Electro Metrics EMC-30 receiver you can see here:https://www.eevblog.com/forum/testgear/electro-metrics-em-30-emc-receiver/msg4832825/#msg4832825It was designed to drive one or two 6-way antenna switches under GPIB control. I am building a 6-way solid state antenna switch unit based on an Aliexpress board fitted with obsolete HMC252 ic.I have the parts and my version of the PCBs for the Xyphro GPIB USB adapter here:https://www.eevblog.com/forum/testgear/open-source-gpib-adapter/msg4902662/#msg4902662So this is a collection of projects that are progressing in parallel.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 12, 2023, 09:09:07 am
content: HiI have been continuing my quest to design/build a balun for the biconic antenna.I found this reference:https://www.keysight.com/us/en/assets/7018-06840/application-notes/5950-3000.pdfwhich basically says you need a calibrated balun in order to measure a DUT balun. That approach leaves unanswered the question of how was the first balun calibrated.So if making an instrument grade balun to test and calibrate a DUT balun is not practical, then another approach is required.The other option would be a differential probe. There are a number of expensive commercial solutions, and some nice DIY probes. This is one of the better ones I have seen:The gain can be increased by amending the voltage divider.One of the key performance measures for a balun is balance. Unless the balun is truly balanced, it will distort the measured antenna beam pattern. So maybe a better option is to use two separate single ended amplifiers, one on each leg of the balun output. If the amplifiers are exactly matched, any imbalance in the balun would be easy to see on the X-Y mode of an oscilloscope.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: October 12, 2023, 04:48:44 pm
content: Quote from: CopperCone on August 30, 2017, 03:03:46 amGo to a University library and you probably can get the paper. Might have to pay for a copier.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 14, 2023, 09:34:52 am
content: HiI connected up an ethernet balun betwen a balanced 50ohm load and the unbalanced DUT port of a balanced bridge on the spectrum analyzer. Measurements of the wire indicate this should not be closely matched to the 50ohm test load (a pair of SMD 100R resistors).I have hit a problem with this setup. I get a measure of VSWR s11 but I don't know how much loss is in the balun.Attached is the return loss of the balun. The toroid is supposed to be connected as a 1:1 Marchand balun but I made a mistake and it is wired as a transformer with a primary/secondary. It provides a reference against which I can compare proper baluns with.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 14, 2023, 11:57:22 pm
content: OK so I reconfigured the Ethernet toroid and coil as a transmission line. Everything is the same except for the connection of the twisted pair.This produced a significant improvement in return loss.This is still wound with the 0.1mm enamel wire for Ethernet, which I calculate has an impedance of somewhere in the range of 75R to 100R.There should be significant room for improvement.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 15, 2023, 03:27:27 am
content: HiI have tried a completely different approach.I had a spare toroid so I tried a 1:1 Guanella-balun. The results are attached.At first I tried just fitting a 50R test load onto the end of the wound coax, but I realised that that the current was hermetically sealed inside the coax. I then created a test board with the 50R load exposed so any common mode current could reach the outside of the coax shield. The humps and bumps are entirely due to resonance along the length of the cable. These bumps are present even with a straight length of coax is terminated with a 50R matched load. The return loss is high enough to reach the limits of the balanced RF bridge.I am speculating that some readers might be wondering if I should be using a VNA and the reason is simply that they have unbalanced ports.In theory, the matched current flowing within the coax should not see any impedance from the toroid. The counter-would split coil should produce a net average zero magnetic field within the toroid. So in a perfect world, there would be zero loss through the balun.In the real world, nothing is perfect but when I compare the return loss plots for:a straight piece of coax terminated with 50R andthe same piece of coax, wound to form a Guanella-balun, and terminated with 50RI don't see any difference in the return loss plots. I am going to try and measure insertion loss through the balun, with my modest selection of test equipment.

author: A.Z.
author_rank: Super Contributor
timestamp: October 16, 2023, 05:25:18 am
content: Quote from: dazz1 on March 29, 2018, 09:27:02 amlooks like a Guanella 4:1, see this one (for HF)https://www.m0pzt.com/blog/4to1-current-balun/if so notice that in such a config, the CM impedance is almost halved with respect to the one presented by the 1:1Regarding core material, a decent starting point to choose it may behttps://toroids.info/HTH

author: dazz1
author_rank: Frequent Contributor
timestamp: October 16, 2023, 09:11:24 am
content: HiI make no claims about knowing much about baluns. This is all a bit of a voyage of discovery for me.The ferrite I am using is this one:Fair-Rite Products Corp. 5961001701 type 61.https://www.digikey.com/en/products/detail/fair-rite-products-corp/5961001701/8594128I just happen to have a pair in my parts inventory.I did a test today. The objective was to measure coax insertion loss through the toroid.I connected a coax cable (identical to the one wound through the toroid) from the TG output to the RF input of the Spectrum Analyser.I normalised the plot.Then I swapped in the cable wound through the toroid.The plot was absolutely identical to the normalized plot.So this proves that differential current flowing within the coax is completely unaffected by the path it follows through the ferrite coils, or not.In theory, the inside of the coax is like being inside a Faraday shield. You would expect any differential currents within the coax to be immune to anything outside. What did surprise me is just how close to theoretical perfection the plot was.I am not saying this is a good test, but it is one that I could do.It seems that I have wound a common choke rather than a balun.My biconic antenna should have an impedance of about 50ohm. so I definitely don't want a 4:1 balun. It would be a bad match.The thing I have wound lets internal coax differential currents flow without any affect.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 16, 2023, 11:08:15 am
content: To test common mode impedance, try grounding the opposite end of the coax. That is, tie the signal pin to ground and let the shield float; a 1:1 inverting autoformer. Put a termination on the far side and measure return loss, which will be the additional shunt impedance of the common mode. Also compare to impedance or mu', mu'' curves of the core.Tim

author: dazz1
author_rank: Frequent Contributor
timestamp: October 17, 2023, 07:32:31 am
content: HiI am thinking of taking a more direct comparative path to measuring impedance s21.The attached shows my proposed test rig. The plan to apply the signal to the coax shield of the Device Under Test (DUT), leaving the center conductor floating.The return path would be a sheet of aluminium.To conduct the test, I would fit the straight cable and normalize the spectrum analyzer.The fit the cable plus toroid choke and measure the difference between the results.The test would compare the impedance of the a DUT cable shield only, This should be near zero.and then it the DUT cable with balun. The difference would be the balun impedance. The difference should be better than -25dB.For a one-off test with minimal junk box materials rig, that should be sufficient.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 18, 2023, 08:59:05 am
content: HiI made a fixture to measure coax choke performance.The fixture is made from aluminium clad composite. This is a plastic core between two layers of aluminium.Two sma connectors are electrically bonded to the aluminium both sides.Two sma connectors are electrically isolated. I milled away the aluminium both sides to reveal the black plastic core.I used pieces of pcb to connect the signal pin to the shield of each pair. The pcb stops the sma connectors spinning when tightened.To use the fixture, I connected to the spectrum analyzer TG output and RF input. I linked the common mode sma connectors with a short length of coax, and normalised the spectrum analyser.I then connected the choke in place of the short coax to measure the difference, being the attenuation by the choke.The plot characterizes the choke. The choke seems to be quite well behaved. There are no resonant spikes and dips. I have not done anything to try to make it perform better. Not sure if I need to.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 19, 2023, 07:40:43 am
content: HiI decided to experiment today.I double stacked the ferrite cores. I used two cores, one stacked on the other, then wound 10 turns (same as before).The results are attached.In band, the added core does improve performance.Out of band, there is definitely resonance. The well behaved flat response is not there.Resonance causes phase shift and that will create imbalance in the signal received from two ends of a biconic antenna.It would be reasonable to conclude that the added inductance is offset by the added inter-winding capacitance. Definitely a case of more is less.Next I will try reducing the number of turns to see if I can keep a flat frequency response with high attenuation.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 20, 2023, 04:28:34 am
content: HiI reduced the turns count to 8 and got the attached results.The obvious resonance has gone and the response is reasonably flat for the first 100MHz. Beyond that not so good.I'd prefer to stay with a single core because I already have a pair. If I go for a stacked core solution, I will need another pair of ferrite toroids.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 20, 2023, 09:11:10 am
content: Interesting, does the resonant frequency correspond to any dimensions in circuit, winding length perhaps?Tim

author: dazz1
author_rank: Frequent Contributor
timestamp: October 20, 2023, 10:37:27 am
content: HiI haven't got that scientific about it to identify the primary cause of anything.There appears to be a compromise.Increasing inductance = greater resonance.Decreasing inductance = greater imbalance.The double stack ferrite makes it harder to wind tight turns on the coils = greater leakage inductance which will increase differential insertion loss.So the answer is that there is probably no right answer, just some answers better than others.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 22, 2023, 07:22:53 am
content: HiI tried a larger toroid made of mysterium. I wound 10 turns.The performance is shown in the plots.The first plot (PNG1) goes out to 500MHz. It shows the balun common mode impedance. It gets a bit lumpy out of band but usable.The second plot (PNG2) goes out to 100MHz. It also shows the balun common mode impedance. This is significantly better than all previous tests. There is more attenuation at lower frequencies. The response is flat from 10MHz to 90MHz consistent with no resonance within those frequencies.The last plot (PNG3 ) shows the insertion loss. Flat and less than 1dB out to 500MHz.I suspect the 100MHz resonance is due to the length of unwound cable.So,double stacking cores is, in this case, inferior to a larger single core.I started with tiny ferrites but bigger is better.More ferrite is better than more turns.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 22, 2023, 08:48:06 am
content: HiI added 2x 10dB attenuators at the input/output of the coax choke test fixture to dampen any resonance due to surplus cable lengths. The aim being to see if the instrumentation was shaping the frequency response.The attached plots show no major difference. Therefore the shape of the plots is mainly due to the balun and the associated surplus cable. The coax between the fixture and the SA is not a significant influence on the measured results.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 23, 2023, 03:02:28 am
content: HiThe coax I am using to wind turns on the ferrite core is a constant length and longer than required for each balun.I tested the coax alone without a core. I used pegs to minimise the area enclosed by the single turn of coax. This was to minimise signals received as a loop antenna and to minimise loop inductance.The attached plot show that the cable alone has a frequency response that is a long way from flat.This coax alone has a significant effect on the frequency response when the coax is wound over a ferrite. The result is that when I wind the final version of the balun, the coax will be as long as it needs to be, that is shorter than all of the prototypes.The coax cable I am using is good up to about 100MHz. A shorter cable should have a higher frequency bandwidth.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 23, 2023, 03:08:56 am
content: HiJust reduced the turns on the large mysterium ferrite from 10 to 8 turns.Not a lot of change compared to the 10 turn version. As before, above 100MHz, the frequency response is dominated by the coax.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 23, 2023, 12:39:25 pm
content: I imagine the floor (consistently -30dB) is due to radiation from the wire itself, or the turns. Being in the 1.5kohm range isn't too bad, considering.All that should be changing with turns is the LF cutoff, and the floor once core loss is less than radiation resistance (parallel equivalent).Tim

author: dazz1
author_rank: Frequent Contributor
timestamp: October 24, 2023, 02:27:00 am
content: HiAll of the testing indicates that the length of coax where the signal is on the shield, and not the centre conductor, defines the upper frequency response/limit.I want to get the upper frequency limit to around 300MHz.Testing results indicate I need to reduce the length of the coax cable that I wind around the core to about 1/3rd of it's current length. To achieve this I will need to use the smaller size core.I don't have a cable assembly that long (short) so I need to sort that.

author: dazz1
author_rank: Frequent Contributor
timestamp: October 25, 2023, 04:19:48 am
content: HiTo test whether a 300MHz bandwidth of the balun is achievable, I used a (too) short coax cable assembly to wind too few turns on the smaller ferrite.The outcome is that the balun has a reasonably flat response out past 500MHz. The resonance at about 175MHz is not due to the balun.If I use a longer cable to get more turns, the attenuation is expected to improve.So this option is still looking viable out to 300MHz.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 25, 2023, 02:55:42 pm
content: Pretty poor though (10dB?). Got any clip-on ferrites? This is probably the range where you need a single-turn winding on a loooong core. Alternately, make a new cable with beads all up on it before crimping the connectors.Tim

author: dazz1
author_rank: Frequent Contributor
timestamp: October 25, 2023, 08:05:02 pm
content: Quote from: T3sl4co1l on October 25, 2023, 02:55:42 pmThe poor common mode attenuation was expected because of the low number of turns. I was testing the hypothesis that reducing the cable length would increase the bandwidth. The results prove that to be true.Quote from: T3sl4co1lLots of them, the split clip-on type. I was planning on using them on the coax that connects the antenna to the receiver. The aim being to attenuated re-radiation of external signals picked up by the coax shield.Quote from: T3sl4co1lI haven't finished looking at all of the options for a coax/toroid balun yet. I plan to try RG178 coax. At 2mm diameter, it will be easier to wind tightly through the smaller toroid. That will reduce stray (differential) inductance to minimise insertion loss. I will be able to wind more turns. There will be less inter-winding capacitance. The length of the coax will be less for a given number of turns. The coax losses will be greater, but the insertion loss should still be low for such a short length.The fixture for common mode testing has proven to work well except for the short cables that connect to the SA TG out and RF in. I am thinking of making a new fixture that connects directly to the SA connectors.I don't make up my own coax assemblies. It is far cheaper, easier and slower to buy them ready made from China. That's what I plan to do for the RG178 assemblies. They won't arrive until some time around December so while I am waiting, I will work on another project.

author: dazz1
author_rank: Frequent Contributor
timestamp: November 06, 2023, 10:07:43 pm
content: I have reason to suspect that my simple common mode test fixture is distorting the measurements because of the lengths of coax resonating.The fixture links the inner signal conductors to/from the spectrum analyser to the outer conductors of the DUT. It provides a direct measure of the attenuation of common mode currents on the outer shield of the DUT. I have not seen this test fixture described in any literature. There is probably a good reason. Either it is a dumb idea that doesn't work very well, or I am a genius. Unlikely to be the latter.Given the apparent success of the simple CM test fixture I have made an improved version. This version connects directly to the front panel connectors of my spectrum analyser (SA). Very short coax then connects the input/output connectors.The fixture is based on aluminium clad composite. A black plastic core is sandwiched between two layers of aluminium.The fixture includes a calibration assembly. The calibration assembly makes the shortest possible conductor between the two SMA DUT connectors. The SA normalises against this calibration assembly. The assembly only connects the connector shields. There is no signal on the inner coax conductor.I need a second N-type to SMA adapter before I can fully test the new CM test fixture.I have considered using a twisted pair to wind the toroid but I cannot find short lengths of cable that would produce a 50ohm impedance. I could use two Ethernet pairs (100ohm each) in parallel. The coax measurements show low insertion loss, good matching and easy to build.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 07, 2023, 02:33:06 am
content: Note that you're making a 1/4 wave monopole here, very roughly speaking: you have unbalanced metal above a ground plane, coupled to a signal line. Radiation depends on impedance along the length of the thing, so it's not going to be a good antenna with a lot of turns and ferrite in there, but the fact remains it's a blob of mostly-conductive stuff hanging out, and there will be radiation resistance as part of the measurement.What's more, because it's unbalanced, and the "plane" is small, there will be plenty of feedline currents -- you should see a few dB of dependence on their position and length.For best results, extending those "planes" to a full metal enclosure around the DUT would be a good idea, to contain those CM (displacement) currents, eliminate radiation (it's reflected back to the DUT), and improve consistency. (There should be a peak/notch evident, corresponding to reflection from the enclosure -- cavity modes; these can be ignored, or if the DUT doesn't need to be too big, a small enclosure can be used to push it up higher.)Tim

author: dazz1
author_rank: Frequent Contributor
timestamp: November 07, 2023, 08:38:11 am
content: Quote from: T3sl4co1l on November 07, 2023, 02:33:06 amI saw coax/antenna type resonance in the 1st version of the CM test fixture. The calibration link was a 15cm sma coax assembly plus the coax connections to the SA that acted like an antenna. I could see external radio signals in the plots. The quarter wave length of the entire test fixture was well inside the DUT frequency band of interest. I reached a point in the testing where I could not differenciate between DUT characteristics and measurement errors.The ver 2 fixture is as short as practical . The calibration link length is equivalent to a 1/4 wavelength antenna @ about 1.5GHz, well outside the 0Hz-300MHz frequency band of interest.Quote from: T3sl4co1l on November 07, 2023, 02:33:06 amI agree that enclosing the fixture would eliminate pickup of external signals (eg. cell phones, radio stations etc) but would add cavity reflections. I have access to a local shielded chamber (not anechoic) that I would use if necessary. Building a shielded fixture would be the option of last resort.My expectation is that the version 2 fixture should be "good enough" to make measurements that have sufficient certainty to make informed design decisions. I will need to do some testing to find its limits. Given my modest range of test equipment, I think this is the best I can do with what I have.

author: A.Z.
author_rank: Super Contributor
timestamp: November 07, 2023, 10:09:31 am
content: Quote from: T3sl4co1l on October 25, 2023, 02:55:42 pmAgreed, a decent W2DU style choke/balun should work pretty well at those frequencies, just a matter of choosing the right material(s) and adjusting the number of elements

author: dazz1
author_rank: Frequent Contributor
timestamp: November 07, 2023, 10:54:02 am
content: None of the references found for the W2DU balun, or any other type, provide a method for measuring the (im)balance of a balun.It could be done by applying a signal generator to the unbalanced side, then connecting the balanced outputs to the X-Y inputs of an oscilloscope or VNA. A perfect balun would display a 45 degree line/vector.My scopes are rated to 100MHz and not the 300MHz I want to measure.

author: tautech
author_rank: Super Contributor
timestamp: November 12, 2023, 07:22:10 am
content: Quote from: dazz1 on November 07, 2023, 10:54:02 amI can help with that.Down your ways in a couple weeks but without wheels if you can collect from Karori.

author: dazz1
author_rank: Frequent Contributor
timestamp: November 12, 2023, 08:04:56 am
content: Quote from: tautech on November 12, 2023, 07:22:10 amIt would be good to catch up again. You can tell me all about the latest Siglent test equipment I want, but can't justify or afford.Right now, I can't get my balun to work to 300MHz, so even if I had the scope, it wouldn't help.It is the usual conflict between wants and needs.I am no balun expert, but as I have learned more about their actual measured performance, it leads me to have doubts about claims of baluns I see on the Internet. No measured data, flawed measurement methodology, and other issues raise these doubts. To avoid the same issues I have invented or modified measurement methods to fit within the capabilities of my modest inventory of test equipment. Necessity being the mother of invention and all that. So far the data is telling me that the critical part of an EMC antenna is the balun, and not the antenna.It appears that the most important characteristic of the balun is balance. Without balance, the beam pattern will be distorted and the measured values of emission will be entirely unreliable. Not good for an EMC antenna. Measuring balance is made to be difficult by the fact that most (nearly all?) RF test equipment is unbalanced.I have ordered a N-type to SMA adapter from China, but that is not scheduled to arrive until Jan 2024. I won't be able to fully test my new RF imbalance fixture until then. Progress on this project is in burst mode.

author: dazz1
author_rank: Frequent Contributor
timestamp: November 16, 2023, 03:13:32 am
content: OK I have made another small step forward. The new direct screw-on common mode test fixture is now complete.As a reminder, this test fixture takes the unbalanced signal from the connector centre pin of the tracking generator output, and puts that signal onto the shield of the coax of the Device Under Test (DUT). The signal is then shifted back onto the centre pin of the input of the spectrum analyzer.The DUT is a toroid with coax windings to make, in this case, a 1:1 balun.Balanced currents within the coax are not affected by the toroid core. Balanced currents have a low insertion loss through the coax and toroid windings.Any imbalance in the balun results in common mode currents flowing on the outside of the coax shield. These are affected by the ferrite toroid. The toroid presents a high impedance to the exterior common mode currents, especially compared to the parallel path along the inside of the coax, balanced with the currents flowing on the coax centre wire.The test fixture applies an unbalanced current to the exterior of the DUT coax/ferrite core to measure the impedance and insertion loss. High loss (>20dB) is good because that blocks common mode current (imbalance) and allows differential currents (balanced) to flow. No signal is applied to the centre conductor of the DUT coax.The aim of building the Mk2 version of the CM test fixture was to minimise parasitics, including pickup of local transmitters in the area. The fixture includes a reference link that is as close as practical to being a short circuit.Given the modest range of test equipment I have, this is the best method of measuring common mode current I can devise. I have not seen this method applied by anyone anywhere else but I would be surprised if someone else has not done something the same way.The attached plot shows the insertion loss of the CM test fixture compared to a short length of coax. The coax was connected and the spectrum analyser was normalised. The test fixture was then fitted, complete with shorting link. The attached image shows the insertion loss out to 500MHz but performance remains good to much higher freqs. The loss is low, and notably, there is no sign of any external rf Tx pickup. Nor is there any sign of resonance or other bad behaviour.The plot shows the new improved test fixture achieved the design aims. No resonance. Very low insertion loss. No detectable external interference.

author: dazz1
author_rank: Frequent Contributor
timestamp: November 16, 2023, 03:23:01 am
content: With the new test fixture and a thin coax assembly wound on the smaller toroid, the attached plot was produced.The attached plot shows acceptable performance out to 200MHz. There is no indication of any resonance or other bad behaviour.This approach looks promising. I have made no attempt to optimise this DUT. The next step will be to reduce the number of windings to trade attenuation for bandwidth. There are other things I can do, like change the ferrite material, or add very small toroid beads over the coax.

author: dazz1
author_rank: Frequent Contributor
timestamp: November 16, 2023, 04:23:34 am
content: I thought I would investigate a coil with no ferrite.I wound a few turns around a piece of PTFE just see what would happen.As you might expect, the coil performance was poor. No resonance, but not much attenuation either.

author: dazz1
author_rank: Frequent Contributor
timestamp: November 16, 2023, 04:26:05 am
content: And I also tried reducing the turns on the ferrite toroid.

author: dazz1
author_rank: Frequent Contributor
timestamp: November 18, 2023, 04:43:06 am
content: Next test was to retest my "reference" balun with the new text fixture. My reference balun is just an arbitrary design that I use to compare with any other design.

author: dazz1
author_rank: Frequent Contributor
timestamp: November 18, 2023, 04:54:24 am
content: This test reduces the number of turns with the intention of reducing parasitics. The aim being to increase the 20dB bandwidth out to 300MHz.The trade off is that the lowest frequency increases.I may be able to improve the lower frequency attenuation by changing the toroid material. This is still the best prototype yet. It maintains >20dB attenuation up to about 350MHz. Performance above 350MHz is well behaved.I think I am now at the knee in the curve leading to diminishing returns. Any significant changes might lead to small improvements. I would like to improve the low frequency bandwidth. Looking at the datasheets, changing to the Fair-rite grade 53 might achieve an improvement without wrecking the upper bandwidth.

author: dazz1
author_rank: Frequent Contributor
timestamp: November 18, 2023, 05:23:13 am
content: I thought I better just confirm the insertion loss with the "new" thin coax I am now using.As you can see from the plot, the insertion loss is flat and insignificant. This proves that the differential balanced current flowing inside the coax is not affected by the windings through the toroid.I do have a balanced bridge, but it would not provide a direct reading of insertion loss. The balanced bridge introduces measurement errors.

author: dazz1
author_rank: Frequent Contributor
timestamp: November 20, 2023, 02:56:27 am
content: I have thought about my next move.I think I have gone as far as I can with what I have on-hand, which just happened to be about the right size ferrite toroid and the right material (61).When I look at the Fair-rite website, (https://fair-rite.com/materials/) the material 31 is recommended for a frequency range of 1-300MHz, exactly the range I want.The other material properties vary widely compared to 61. I need to figure out what effects these material differences will have on balun core size and turns etc.Then I need to order the parts. I need to order enough parts to avoid paying shipping costs. That means there will be another pause in progress on this project until I need to order enough parts.

author: tautech
author_rank: Super Contributor
timestamp: November 20, 2023, 03:06:39 am
content: I have in the past requested a free sample.......https://fair-rite.com/request-a-sample/

author: dazz1
author_rank: Frequent Contributor
timestamp: November 20, 2023, 03:53:22 am
content: Quote from: tautech on November 20, 2023, 03:06:39 amFree is always good. I will try that.I have been considering ways of further testing the balun. I know the text book method is with a VNA but I don't have one of those and, in this particular case, I don't think it is the best method.The attached drawing shows the proposed test setup. This is to illustrate the concept. The final circuit will need more stuff. This test setup will provide a direct visual comparison of imbalance in phase and amplitude in a way that a normal Smith chart can't. I know that use of Lissajous patterns is far older than I, but the availability of wide bandwidth oscilloscopes makes them usable at frequencies that previously required specialist RF equipment, like a VNA.The reason why I don't think a VNA is the right instrument for this type of test is that typical VNAs are two port devices. The proposed setup is a 3 port system, with direct read out of imbalance. In addition to phase and amplitude imbalance, it will also show losses (insertion loss). A perfect lossless balun will display a straight 45 degree line. Any loss will display hysteresis with an internal area proportional to losses.If I sweep the frequency, I will be able to produce a Smith chart showing the difference in phase and amplitude between the two ports. The main issue with this setup is that the comparison is linear, not logarithmic. That will adversely affect the dynamic range of the displayed comparison.I don't have a 300MHz (~500MHz) scope, but I may be able to get access to one a short distance from where I live.My ancient Wavetek 2520A RF signal generator has GPIB so I can sweep the frequency. It has been calibrated to 0.1dB across the whole frequency range.If you think there is a better way of doing this, let me know.

author: tautech
author_rank: Super Contributor
timestamp: November 20, 2023, 06:23:28 am
content: You could always measure one leg of the balun with your analyzer and save it as a reference trace to compare the other balan leg against.Or failing that, View (freeze) the analyzer result and activate a 2nd trace (C&W) for the other leg of the balan.Or when I'm down next time with wheels bring along a 4 port VNA.....Looking forward to that coffee you suggested we have in a couple of weeks.

author: dazz1
author_rank: Frequent Contributor
timestamp: November 20, 2023, 06:45:13 am
content: Quote from: tautech on November 20, 2023, 06:23:28 amIf you did that, I would just drool all over it like my dog waiting for dinner.

author: dazz1
author_rank: Frequent Contributor
timestamp: November 26, 2023, 09:05:52 am
content: Based on this video:https://www.youtube.com/watch?app=desktop&v=SuOEWapx62cit would seem that my ancient 100MHz HP54645D is usable out to about 300MHz, the planned bandwidth of the biconic antenna. The amplitude is attenuated, and no doubt there will be phase shift, but for my purposes, this should be indicative. So long as both DSO channels have the same amplitude and phase shift, I should still see usable Lissajous patterns.

author: dazz1
author_rank: Frequent Contributor
timestamp: December 02, 2023, 08:05:07 am
content: Fair-Rite have come to the party and offered samples so my R&D can continue.I have asked for a toroid of similar dimensions to the part I already have (5961001701), type 61 material.The key difference with the requested part (2631801202) is the material type 31.The µ for 31 is 10x greater than for 61. 10 turns on a 61 toroid should be the same as 1 turn on a 31 toroid.or10 turns on a 31 toroid should provide a lower frequency -20dB point than a 61 toroid.The penalty is that 31 has a lower upper frequency limit, but that should be OK. The type 61 toroid balun I have developed already has a greater bandwidth than the antenna I have built. The balun has a wider bandwidth than equivalent commercial biconic antennae. So you might wonder why I don't stop and have a beer in celebration of my self-acknowledged geniusSo while the theory tells me the 31 material will be a Cinderella solution, the practice might be a little different. I anticipate I will need to play around with the number of turns to trade bandwidth for attenuation. Just like before.I have built two identical antenna so I can calibrate them Once I have done that, one antenna will be spare. I was planning on selling the second biconic but I doubt anyone will be willing to pay enough for it. If anyone was to ask the price for me to make/sell one of these biconics, that question alone would suggest they can't afford it. So far I have a waiting list of zero.If I keep it, I can very easily and cheaply make another set of elements that plug-in to create a biconic with a different frequency range. That will be a lot easier if I can make an ultra-wideband balun so I can change-out elements without changing the balun.The cost/effort of trying the 31 toroid is cheaper/easier/faster than making another set of hubs.I have started to order the parts for my cheap-as 3 port, X-Y balun balance tester. It might take a while for those items to get through the Christmas crush of presents clogging the mail system.The more I learn about baluns, the more skeptical I am of the performance of some of the commercial biconics I see advertised. Any unbalance in the performance of the balun will distort the beam pattern. Few, if any, antenna datasheets include a plot of the actual measured 3D beam pattern. I have seen simulated beam patterns, no doubt based on theoretically perfect baluns. Some might say it doesn't matter because the antenna only has to receive in one direction. That chain of thought ignores the effects of the beam not pointing where it should. If I was in the market for an EMC antenna, I would be focusing on the measured beam pattern and not relying on the data sheet and brand name.I am expecting another pause on this project until I get the 31 toroids and build the 3-port balun balance tester.Fluffy cats and cute pets always attract lots of views so here are mine.

author: dazz1
author_rank: Frequent Contributor
timestamp: December 02, 2023, 10:24:22 pm
content: I thought I would measure the workable bandwidth of my Phillips PM3070 100MHz scope. I also wanted to test if I needed amplifiers in my 3 port balun balance tester.I connected up my ancient Wavetek 2520 RF sig gen to the Phillips scope. The connection to the scope was terminated with a 50ohm load adapter to avoid false readings from reflections.The photo shows the scope display with a 13dBm input at 250MHz. This is the end limit. At any higher frequency, it would not trigger.I found that the trigger was the weakest link. As the frequency increased, the signal amplitude had to be increased to trigger.At 250MHz there was significant amplitude reduction. For my purposes, this is of no consequence because I will be looking at the X-Y phase relationship in Lissajous plots.There will undoubtedly be phase shift through the scope circuits, but as long as the lag is the same on both channels, no problem.So the Mod 1 version of my balun balance tester should work comfortably to 200MHz with the Phillips scope.I ran similar tests with the HP 54645D 100MHz scope. As expected, there was significant amplitude roll-off at higher-than-spec frequencies.The HP 54645D could comfortably display 250MHz signalsand still workable at 300MHz (but a little noisy).This scope reached the limits 330MHz @ 13dBm. The displayed signal was noisy and the scope could burst into full on aliasing.For both vintage scopes the test results are impressive and more than adequate for my balun balance tester (BBT). I will not need amplifiers in the circuit. It seems that, as usual with the RF things I make, the performance of the BBT will be defined by the physical construction. No electronics required.

author: dazz1
author_rank: Frequent Contributor
timestamp: December 11, 2023, 07:34:12 am
content: I received the parts I needed for my 3 port balun balance tester.These included a pair of identical 50R BNC > BNC terminations to plug into the scope inputs.For test purposes, a Tee was the DUT. This has perfect balance.This test was to figure out the limits of the tester to identify measurement errors and just see if it works.The first attachment shows the xy plot at 100MHz. The 45 degree diagonal trace shows perfect balance. Any loss would appear as a loop rather than a straight line.At 350MHz, 13dBm the scope struggles but gives a stable, readable and useful trace. Notice here that the line is no longer at 45 degrees indicative of an imbalance. Some experimentation showed that one channel has slightly different gain to the other. As long as I know this, I can compensate for it.In the xy mode, there is no trigger so I could still see a small but just usable trace a 400MHz@13dBm. Not bad for a 100Mhz scope.The next step is to configure the balun for 3 ports. These are:1. signal and shield to RF sig-gen output2. signal to scope X input3. shield to scope Y input.If the balun is doing its job, the X-Y trace should remain at 45 degrees with no loop across the frequency range.The Fair-rite offer of free parts is too expensive to accept. Free parts, yes. Free shipping, no.It is cheaper for me to pay for the parts and get free shipping from Digi-key, element14 etc.

author: dazz1
author_rank: Frequent Contributor
timestamp: January 02, 2024, 07:56:02 pm
content: There is an interesting post here:https://www.eevblog.com/forum/rf-microwave/diy-rf-emc-biconic-antenna/75/showing the balun for a TekBox Biconic antenna.What would be more interesting is to measure the performance of this balun.

author: dazz1
author_rank: Frequent Contributor
timestamp: January 13, 2024, 04:20:34 am
content: HiI want 100R terminators for my 3-port balun balance tester. For the tests above, I used standard 50R terminators for the X-Y inputs. These reduce the input impedance seen by the balun to 25R. Using 100R terminators will look like 50R to the balun.If I could buy 100R terminators, I expect they will be way too expensive. I turned to the huge shop that Aliexpress is and found cheap sma attenuators. The BNC equivalent are about 3x the price. I figured I could buy the sma attenuators and modify them to make DIY 100R terminators.Given the price, I had low expectations on quality and performance. What I found inside surprised me.The attenuator is one-piece brass body construction with a sleeve to cover the insides. This sleeve is a press fit.Inside is a ceramic substrate with laser trimmed resistors. Measurement of the resistors indicates they are accurate. The substrate is soldered to the body. Easy to remove. Based on physical inspection, the 8GHz bandwidth is plausible.It is possible that these are rejects from a production line for a name-brand manufacturer. If they are, I can't see or measure any obvious defect.The photo of the attenuator in pieces was the result of destructive exploration. Now that I know how to open these up, they will be ideal candidates for making 100R terminators.

author: dazz1
author_rank: Frequent Contributor
timestamp: February 04, 2024, 02:53:00 am
content: Another ant step forward today.I received the SMA attenuator to replace the butchered one. I am back to a pair of these.I made a tool to remove the sleeve from the attenuators without damaging them.I replaced the attenuation part with a straight-through wire and a 100R resistor to ground.I now have two 100R scope terminators. My scope has 13pF inputs so I should match this with inductance in series with the 100R resistor. I have chosen not to do this because it will be difficult to make the impedance of both DIY modded scope terminators.The photos show the two completed sma terminators. The tool flexs to open around the sleeve. When gently closed, the tool grips the edges of the sleeve. The aluminum punch fits into the sma socket to push the body out. The same tool is used to install the sleeve. The tool works without damage or marks.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 04, 2024, 04:20:55 am
content: Mind, it's probably not full capacitance. You'll want to do a reflection measurement to determine what the actual input impedance is at high frequencies. It's rated xx pF so that, in combination with a probe of yy pF, you get a 10x divider, but both of those actually have ESR, and the equivalent circuit trends towards a low-Z probe at HF.Of course, what they actually are, they don't tell you, and won't be consistent from instrument to instrument, so you'll have to measure.In any case, you'll likely find inductance is not necessary, or that less is necessary to achieve whatever peaking is possible [than would be calculated from the mid-frequency pF value].Tim

author: dazz1
author_rank: Frequent Contributor
timestamp: February 04, 2024, 09:15:04 am
content: Quote from: T3sl4co1l on February 04, 2024, 04:20:55 amFor this application (making a DIY 3-port VNA) it won't matter if the 100R terminators are exactly impedance matched. What does matter is that both x/y inputs to the scope are connected with terminators that have the same impedance so the scope can display any imbalance in the balun.To test the balun, I will drive it in reverse. I will be driving the unbalanced balun output with the unbalanced signal generator output. The balanced antenna balun inputs will be connected to the x/y scope inputs. With 2x 50R scope terminators, the signal generator will see 25R. With the new 100R terminators, the signal generator will see parallel 2x 100R = 50R load on the x/y scope inputs. The higher 100R resistance will increase the displayed amplitude of the x/y signals. This simple vna will work as long as the impedance of the x and y scope inputs is the same.Now that I have the tooling to open up this type of sma style attenuator, it is relatively easy to make custom adapters. They are cheap but good, even when left original as attenuators. I should be able to make custom scope terminators impedance matched to a specific scope. The adapters are cheap enough to make a set of matched terminators for each scope. For now, I just need 2x 100R terminators with the same impedance.

author: dazz1
author_rank: Frequent Contributor
timestamp: March 10, 2024, 04:29:48 am
content: HiI has been a while since I posted on this topic.Made some progress on my balanced to unbalanced 3-port VNA. I need a fixture to hold everything in place. This is a piece of Aluminium Clad Composite.At the bent end, two sma connectors and 100R oscilloscope terminators are fitted. These will connect straight into the scope without cables.Along the length of the fixture are holes for a chassis SMA connector connected to the unbalanced output of a signal generator. This is the RF ground reference point.I am going to try toroid ferrites as a balun. It is not an original idea. I can't find any literature on this as a balun. The hypothesis is that it should have a much wider bandwidth than threading a coax as a coil through a larger toroid.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 10, 2024, 05:52:56 am
content: Try "current balun" -- choking the current flow (raising CM impedance) to the point where balance is good enough. It canneverbe perfect, you can't null the current this way -- but it can be good enough, where "good enough" might be 20 to 40dB.Tim

author: dazz1
author_rank: Frequent Contributor
timestamp: March 10, 2024, 07:31:12 am
content: Quote from: T3sl4co1l on March 10, 2024, 05:52:56 amHiThe coax coil through a toroid was he result of research on current baluns. Tests showed the upper frequency performance was limited by the number of turns. Not enough turns = not enough CM attenuation. Too many turns = reduced upper frequency response.So my logic is to unwind the coil to make it zero turns. Balanced currents will completely cancel out and the ferrites won't see a magnetic field. The ferrites won't be excited so only coax loss. The upper frequency response of balanced currents should be defined by the coax cable, not the ferrites.Any unbalanced CM current is going to be seen by the ferrites, and impeded. Any unbalanced current coming from the sig-gen is going to have the choice of a 50ohm cable, or a CM impedance. I think it will look like a RF voltage divider, and I expect slight imbalance.If I find the expected imbalance, I plan to experiment with the tiny Ethernet torroids I rescued from a MoBo. I plan to pass the centre core conductor of the coax through one of those tiny ferrites. The aim being to add just enough impedance to achieve balance. If it works, the trade off will be increased insertion loss of balanced signals.I key question to answer is how many ferrites will it take to provide a reasonable lower frequency limit?In many applications, the long skinny form factor would be an issue. For the Biconic, it would fit nicely inside the tube supporting the antenna.If will be a while before I run any tests because I am waiting for some connectors/adapters from China.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 10, 2024, 12:16:58 pm
content: Quote from: dazz1 on March 10, 2024, 07:31:12 amThat's... not quite how that worksFor one: as long as it's a current balun, there's no cancellation, regardless of number of turns, or cores, period. At best there are nulls due to resonances, at specific frequencies, but not in general. The simplified circuit is, putting an impedance in parallel with one diff port (the CM/shield impedance to GND), and not the other (the core/signal/positive port is untouched otherwise), and therefore it intrinsically and necessarily must be unbalanced, by a margin given by the ratio of port to CM impedance. You could tweak that impedance by loading the positive port with a compensating impedance, but this is only correct when the unbalanced port is common ground; or add balancing transformers to better enforce it, but that would be a voltage balun, or hybrid.For two: turns is the number of loops through the core aperture. The fact that the wire is locally straight, near the core, is irrelevant: it still must close into a loop around the core; or if not, it must be an open circuit! You can't have zero turns, or you don't have cores looped around the conductor at all.(Note the core is itself a single turn of core around the winding! In fact, there are some games that can be played with the winding number of a core, for material with very high permeability, but that doesn't apply here.)QuoteYes -- keyword "impeded". That impedance must be made infinite, for current imbalance to be reduced to zero. Any finite impedance will necessarily result in some loading of the CM-connected (here, negative diff) port.RF may be complex, but it's not black magic. There's no point where we avert our eyes and pray to elder gods; circuit theory doesn't simply go out the window because waves bounce around sometimes! We can -- and must! -- apply basic circuit theory, in situations where it remains applicable (which, here, will be for the chain of cores / TL stub being less than 1/4λ). Draw the equivalent circuit, do the sums, and determine the required core count. Check the datasheet for impedance(freq), beads always(?) provide this.Offhand, I'm guessing seven cores about that size, might be around 100Ω each, and 700Ω out of 50Ω and loading one but not the other port is half the effect or 1/14th, and 14x (volts) is 23dB. If you measure wildly different from 23dB balance at up to, oh, 200, 300MHz or so, I'll be pleasantly surprised.QuoteYes, smaller, thicker cores are handy -- you need to use smaller transmission line though, which may make problems not just in terms of impedance matching but losses (smaller wire = higher loss) as well, and we can't ignore the common mode through the ferrites themselves -- they aren't air, ferrite has a fairly modest dielectric constant actually, and MnZn ferrite has notable resistivity -- so we expect some antenna effect where the strip shown will eventually radiate as a 1/4 wave antenna, if a poor one*. This also limits how much impedance we can put on, even with enough turns on very thick cores.*Probably the effect will be down in the +/-0.1dB sort of range, and may not be visible as a result. One way you could tell, is testing the system as-shown, then bringing a reflector near it -- a sheet of metal parallel with the existing ground plane, some distance above it, but still far enough that it's not affecting near-field too much. (Up close, you can imagine it increasing CM capacitance, or reducing Z_CM as it begins to resemble a ferrite-loaded stripline geometry.) Bonus points for demonstrating the reflection is, well, reflective -- say, pointing the reflector at different angles and observing no effect except for angles close to parallel; but so close in, and for lengths and distances comparable to a wavelength anyway, I suspect the effect would be small. Or, at distances where the reflector is reasonably in the far field, the return gain will be so small (< -40dB say??) that the change in balance or port impedance or whatever will be imperceptible.There's also skin effect in the core material itself; how much depends on material, but this is one reason MnZn isn't so suitable for VHF+ use for example (the skin depth is too shallow and you don't get much impedance, or it even looks capacitive). NiZn ferrite may also be low enough loss that, rather than a skin effect as such, there is some wave propagation through it -- that is, if you consider the amplitude and phase of an incident wave versus depth through the material, you may find it goes through a few cycles, i.e. propagation is dominant, and it looks more like a lossy bulk material, than a poor conductor. (Which makes some of these materials useful at high frequencies, like the application of magnetic bias to obtain relatively massive Faraday rotation, and construct a circulator or isolator.)But some combination of these will likely get you a very good balun; I would imagine better than 40dB balance is possible. You may need to use a stack of cores, like NiZn close to the line, MnZn outside those, and nanocrystalline outside that (allowing operation down to quite low frequencies, 10s kHz or below even).It's doubtful that better than 20dB is really needed here, and the mast / feedline can be grounded to the test chamber at the base, further attenuating CM / feedline currents. 20dB balance error corresponds to about a 0.83dB amplitude error, readily calibrated out.Cheers,Tim

author: dazz1
author_rank: Frequent Contributor
timestamp: March 10, 2024, 08:20:00 pm
content: HiI agree with everything you have said.The coax coil through a toroid proved to have almost good enough performance.Basically I am aiming to improve on that performance by wrapping the toroid around the coax instead of wrapping the coax around the toroid.I don't expect perfection. I am aiming for good enough.This could be a complete failure, but that would be a useful thing to learn. If it does fail, I will fall back to the coax coil through a toroid.

author: dazz1
author_rank: Frequent Contributor
timestamp: March 30, 2024, 10:51:45 am
content: HiMy theory works. The concept of winding a toroid around a coax rather than winding a coax through a toroid has performed better than expected in my tests. I will call the concept a linear coax balun because it is straight, not a coil.The test balun achieves a practically constant 2degree phase error and 5% amplitude error from about 5MHz to 300MHz. Insertion loss is near zero.The upper frequency range is limited by my 100MHz scope. I suspect the upper bandwidth will easily exceed 500MHz.The simplicity of my 3-port VNA does not appear to affect the results of the test. I made a balun substitute to prove the 3-port VNA fixture. I tested the simple 3-port VNA with an ideal balun substitute to prove it works.The photo shows the balun substitute laying beside the 3-port VNA fixture with the linear coax balun installed.The balun substitute consists of a coax with a sma connector at one end, and a sma Tee adapter at the other end. A signal from the sig-gen is split 50:50 at the Tee. The branches of the Tee are connected to the X-Y inputs of the scope. Note that only the coax core is connected at the Tee. The coax shield is open. The X-Y inputs see signals that are perfectly in-phase and of equal amplitude. The unbalanced output of the sig-gen is converted to a balanced X-Y input. The coax substitute was used to test the 3-port VNA fixture. It showed that:the displayed amplitude was attenuated above 100MHz (stating the obvious for a 100MHz scope)At over 150MHz, the oscilloscope shows some imbalance and non-linearity. (again no surprises there)The linear coax balun consists of a length of coax threaded through a number of ferrite toroids. The X input is connected to the coax shield, and the Y input is connected to the coax core wire. That is it. The toroids impede any imbalanced current in the coax. The single ended signal from the sig-gen appears at the other end as a balanced signal seen by the oscilloscope.I know toroids over cable have been used in emc forever. What I haven't seen anywhere is toroids over coax used as a wide-band balun. If someone else has come up with the same idea, my search of the literature hasn't found it.The X-Y signals show a small imbalance that is frequency insensitive. I did expect a little imbalance and I did try adding very small toroids to the balun coax core, but saw no significant improvement. I suspect this balun needs trace imbalance to work properly.There is no sign of any self resonance because there is no inter-winding capacitance.Tests to date demonstrate the linear coax balun is easily the best balun tested. It is very simple to make with good performance. It could be described as untra-broadband.

author: shabaz
author_rank: Frequent Contributor
timestamp: March 30, 2024, 03:36:32 pm
content: Glad its working! Regarding:What I haven't seen anywhere is toroids over coax used as a wide-band balun. If someone else has come up with the same idea, my search of the literature hasn't found it.It's quite a common technique, e.g. thisbaluns article(I wrote it a couple of years ago) mentions it (in the section titled Real-World Baluns.

author: dazz1
author_rank: Frequent Contributor
timestamp: March 30, 2024, 09:31:43 pm
content: Quote from: shabaz on March 30, 2024, 03:36:32 pmYes, I see that you mentioned a coax going through a tunnel of toroids. That exactly describes what I have done. Your comment is the only reference I have seen to this type of balun, so maybe it isn't so common. However, if others have arrived at the same solution, then that is good because it confirms my thoughts and experimental findings.If you have been following my posts, you will know that I have tried a number of different balun types. They all looked good on paper but suffered when exposed to the real world. None of the theoretical papers I have reviewed discuss in detail, or measure, the effects of inter-winding capacitance. For a 1:1 ratio balun, the coax in a ferrite tunnel easily has the best real-world performance. In my case the measured performance appears to be limited by my available instruments. I don't think I have reached the frequency limits of the test fixture or the balun.The attached image shows the phase plot of the balun at 100MHz. The amplitude deviation is 5%. The phase angle deviation is 2 degrees. Insertion loss is practically zero. This balun is not far from being ideal.My target is 350MHz and I have been able to measure to 300MHz on my 100MHz oscilloscope. I think the balun would be good to at least 500MHz but making measurements at that frequency would require Siglent to donate a free scope, and that will only ever happen in my dreams. More realistically, I will ask a favour of a local company with the right gear.At frequencies above 500MHz, the physical dimensions of the test fixture are likely to start affecting the accuracy of the measurements. I would be able to quantify any errors with the balun substitute.In any case, my search for a suitable balun has ended. This will allow me to complete my biconic antenna. One of the nice features of combining a very wide band balun with interchangeable plug-in antenna elements is that I could easily make a range of element sets to change the frequency range of the antenna. For example, I could double the length of the elements and still use the same balun and machined parts. The elements are cheap and easy to make. The machined parts are difficult and expensive (in time) to make.One of the really nice features of this linear type balun is that it will conveniently fit inside the mounting tube that is commonly used on biconic antenna, including mine.This project isn't finished yet. The next phase is to make a final version of the balun and connect it to the antenna. I will then need to measure the performance and characteristics of the completed antenna.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 31, 2024, 04:58:09 am
content: I mean, it's nothing unusual, easily enough derived from principles; and, you mentioned EMC, but if you're kind of excluding EMC references, I dunno, EMC uses a lot of antennas, baluns and other cable-impedance-modifying and coupling structures so it's pretty much the go-to domain to find electromagnetic structures and tricks like this.For example, a typical EMC cable clamp for measuring / injecting signals on cables/harnesses, uses a tall stack of ferrites, alongside a coupling strip that's grounded to plane at one end and couples to the cable at the other end. Thus giving isolation along the cable, coupling to a BNC... it's a clip-on bias tee, pretty nice eh?And I say "easily enough", but those principles aren't exactly the most common information, you can pick it up from an undergrad curriculum I would say, but you're more likely to find it in explicit detail in grad level EE, and if you're not officially EE at all, it can take quite a lot of study and insight to develop these ideas, and many working in electronics never get there at all, it's just not in their wheelhouse (say, those working with audio, equipment repair, digital/embedded, etc.). It's fairly specialized knowledge, and so you have to search it out in particular, it won't come to you on its own, so to speak.If you're confining yourself more to radio references, and ham stuff in particular, or online references even more narrowly, yeah there's a lot of junk information there unfortunately. There are a lot of RF and EMC engineers that are hams,but there are few hams that are RF engineers. It works one way but not the other. So that selection slices the literature the wrong way and you...don't get very good results.The tipoff is, such results -- postings, web pages, informal articles -- rarely cite anything; they should only be used as possibly correct information subject to further proof. When they do make citations, whatever of those references you can find in turn, check that they support the claims, that the claims are reasonable given what theory you know or are able to test, etc. You are searching for proof of correctness of the claims, and of the supporting material in turn, recursively, until you have reduced the problem all the way to well-proven claims and verifiable theory. Burdensome at first, but as you map out available knowledge, you can pin more things as proven, or in what ways they claim things correctly and what others they miss and haven't yet proven as such. This is the challenge of academic rigor, and the primary means of identifying, developing and curating well-proven science.So, you see claims from, say, someone selling an antenna kit -- maybe they claim directionality/bandwidth/size beyond the theoretical (Chu–Harrington) limit, but if they provide no test results (feedline impedance or SWR or etc.; CM current; exact configuration/geometry tested; radiation pattern, at least along axes; calibration where possible; etc.), feel free to ignore their claims, doubt them fully, it's just a bunch of words it doesn't mean anything in relation to anything else until proven as such. And indeed, you will find claims of impossible performance, where the configuration is basically an unbalanced dipole let's say, but it's far wider bandwidth and higher gain than should be possible, and the only possible resolution to that is ground effect and feedline acting as antenna element, and perhaps nearby structures acting as reflectors if it was actually measured for radiation pattern. Or another common "trick", they're claiming SWR is antenna efficiency, or gain even, when this is only true when the antenna is lossless, and they might've simply cooked up a particularly shite configuration with a lot of equivalent resistance so the actual efficiency is awful but the bandwidth is wide and SWR is low. Again, verifiable claims, consistency, does it match theory, are the methods correct, etc. etc.Regarding balun design specifically, you probably don't find many references discussing interwinding capacitance, but the problem can be decomposed into the N-turn impedance of a ferrite bead or other core, which you sometimes see plotted in datasheets, and yes the peak goes to higher Z at lower F as turns goes up as one would expect, and, you can test this independently by rescaling the problem say to magnet wire on a smaller core, it doesn't have to be coax -- and then substitute solid wire for coax and the outside fields / shield/CM impedance don't know the difference -- itcan'tknow that there's a wire inside the shield -- and that's your answer, that's the CM impedance across the balun from shield to shield, an impedance that isn't shared on the signal wire and therefore imbalances the result. And by scaling, I mean the frequency response is proportional to length scale, give or take material invariants (paramagnetic resonance perhaps, but most applications are well below that and ferrite just looks like a bulk lossy permeable material as we usually model it, plus minor wave effects due to core size and shape). So it's not a balun design problem, it's a transformer or inductor design problem, and you will likely find many more results in that more general direction, than for baluns alone.Beyond that, you can do some hand-wavey thoughts about fields within and around the ferrites, because dielectric constant is fairly high so the tubular stack itself looks like a fat wire above a ground plane, and that impedance adds with the total bead impedance; fields and waves means wave mechanics and you could have cavity resonances or radiation or whatever depending on if the stack is in an open or closed configuration (open as shown, or closed in a box / surrounded with shield). Pretty soon, you'd want to measure or simulate these, but likely these are also to a degree of precision unnecessary (e.g. maybe if you were doing a VNA with a precision directional bridge, but for an antenna, who cares), and so we can truncate our efforts depending on accuracy required.So, applying these principles, let's say, this:http://on5au.be/content/a10/trans/41balun-2.pdfNo references (or maybe inlined, I just skimmed), but lots of measurements of basically what you're doing, different orientation (1:4 instead of 1:1+1), lower frequency (but why stop so short, first resonance might be 100s MHz) but we have every expectation it works to much much higher, and the measurements being very flat in the range tested certainly checks out with expectations.Ooh, this is a good one:https://www.nonstopsystems.com/radio/frank_radio_baluns.htmI wonder if this is kind of the holy grail that you've been looking for forever but just never found (probably because Google sucks these days); the references aren't necessarily high-tier journal results, but it looks like a reasonable selection of experimental data that can be checked in turn. (And local copies are provided for easy viewing!)https://www.nonstopsystems.com/radio/pdf-ant/BALUNS2006-ang.pdfThis is one of the references, it seems familiar, I might've seen it a long time ago, but anyway -- showing off cores and some basic geometry, and more or less confirming the manufacturers' curves on these (at least of the EMI bead parts where impedance is most likely to be provided -- you'll have to look them up and confirm), and showing testing methods, and a range of typical applications. Good stuff....Maybe the formatting is a bit gaudy? I mean, it's no \$\LaTeX\$ article.But what do you expect, it's PowerPoint, that's easily excused -- don't let superficial aspects like aesthetics get in the way of some good data. (Now, when it's poorly presented/arranged data, or misleadingly formatted, by all means critique; such aspects go a little deeper in meaning and impact!)Tim

author: dazz1
author_rank: Frequent Contributor
timestamp: March 31, 2024, 09:05:15 am
content: HiMy research did not find the documents you referenced, but I did find similar, which I soon discarded, or discarded after experimental results were unsatisfactory. The main reasons I discarded papers were:1. They could not be readily adapted to provide a 1:1 impedance ratio.2. My experimentation revealed that coils on toroids did not have the band width and performance I was seeking,3. They used technology unsuitable for the frequency range of interest, especially if they were working in the GHz range,4. No measured data or other evidence to backup the claims,The critical information I was seeking to define the performance of a balun I never found. I wanted to see measured data for:1. The phase difference of the balanced lines (ideally 180 degrees),2. Amplitude difference between the balanced lines ( ideally zero),3. Insertion loss (ideally zero).Those 3 parameters essentially define the quality and performance of a balun.The most useful paper you referenced is the last one, the gaudy power point presentation. It has useful info on measuring ferrite characteristics I have not seen elsewhere.Page 24 shows the exact balun configuration I ended with, but no measured data relevant to my application.Page 53 also shows ferrites over coax used as a balun, but again without measured data.I know there is a lot I don't know about RF, but I try to fill the knowledge void with facts and evidence. My old Professor used to say "It isn't true if it isn't measured." I made my own truth by experimentation. Part of that experimentation included designing, building, and validating custom fixtures to make the measurements to provide the data and evidence I was seeking. So yes there are plenty of papers out there on baluns, but in my experience most would, and did, end up in my discarded bin.

author: shabaz
author_rank: Frequent Contributor
timestamp: March 31, 2024, 03:40:26 pm
content: I think most EE undergrad courses won't cover any of this, it's too vocational, so unfortunately, unless one comes across it in their line of work, some extra study is needed by engineers and non-engineer hams alike, and that includes trying to read academic papers, reading the decent online information (granted a lot will be wrong or omit information, but recommendations from others helps to weed out the bad links) even if some of it is hard to initially follow (some content is extremely narrow-focussed so it's natural it may be hard to follow if one isn't an expert in that specific field), and finding books on the topic at hand.For what it's worth, if you're an amateur radio enthusiast, there's a very popular book by Jerry Sevick called 'Transmission Line Transformers' that is easy to read; it's practical with examples. It is not worth the new price on Amazon; there are second-hand copies from time to time at low cost. But the original Guanella paper is easy to read, too (the book contains it from memory). Also, the Chris Bowick book 'RF Circuit Design' is super-easy to read (although it is brief, which could be considered a positive thing) and covers quite a few ferrite and inductor topics with clarity, and non-domain-experts will easily follow. Again a used copy is worth getting, since the new price has almost tripled over the past few years.

author: dazz1
author_rank: Frequent Contributor
timestamp: March 31, 2024, 08:31:04 pm
content: HiI am a pro EE and its been nearly 40 years since I last did an EE undergrad course. My career path has included a lot of RF stuff, but not R&D. Courses and books explain the theory, but little of the practical problems with application. I am not a licensed ham radio guy. When I started this project, I new what a balun is, how it worked and why I needed one. I had never made one before. I started with a review of the literature and of commercial baluns. Practically all of the papers ended up in being discarded. I had to invent my own wheel.At the risk of making a huge generalization, the main problem I have seen with amateur radio info is the typical lack of validated measured data. What they say "works" but no evidence of how well it works. Exceptions apply. The commercial antenna manufacturers don't give any measured data on the baluns they use. The modern academic papers tend to be undergrad or at GHz frequencies.The other problem I hit is instrumentation. One example, I needed a VNA to measure the performance of my balun. The VNA needed to have a 50ohm unbalanced port and a 100ohm balanced port. I found a good HP reference on VNA measurements. In order to measure balanced signals with a 50R VNA port, the manual said a calibrated balun is required (HP part numbers given). Even if I had all of that, I still needed a fixture and a means to validate the test method and setup. That lead to a mini-project to create a 3 port VNA and test fixture with a signal generator and an oscilloscope. This may be a common method but again I don't know of any similar setup, pro or amateur. There is no known course or paper on this topic that I could find. My undergrad courses and later experience gave me an understanding of the theory, what I needed to do, and how to do it. Most importantly, it worked.So for me this has been a self made journey of learning that I have recorded on this forum for the benefit of others who may be interested. This added discussion is healthy because it draws out more relevant info.

author: coppercone2
author_rank: Super Contributor
timestamp: April 01, 2024, 04:57:17 am
content: Ehh for the balun I got 2x 50 ohm RF resistors with 3GHz performance and soldered them between banana jacks in a bakelite piece (it has irregular spacing on the HP adapter for 100 ohm).I consider it good enough because IMO usually you are going for cable impedance, and that is probobly gonna depend on how you lay it out and stuff.Since that standard is on banana jacks, it probobly is not that good. I was very annoyed by the price of the 100 ohm standards.but 3Ghz 1% through hole resistors on the 10Mhz banana jack fixture is probobly gonna be pretty damn close. I got like 200 of them so I can pick the best values at DC. Still cheaper then the HP standard, and I have a bunch of cool through hole RF resistors left over.I think the prices people are asking for differential stuff on ebay is unfairYou need another manufactures transformers to get past 10Mhz IIRC for the HP vna fixtures also.Unless we are thinking of somethign different, HP did not appear to sell a calibrated balun, they sold a 100Ohm banana jack insert calibration standard to calibrate their balun or north hills brand transformers.

author: uer166
author_rank: Super Contributor
timestamp: April 01, 2024, 08:28:38 pm
content: Did you get a chance to test the fixture with an actual biconical or at least a dipole? I have a similar interest (and reading the same book but 1st ed.)

author: dazz1
author_rank: Frequent Contributor
timestamp: April 01, 2024, 09:16:47 pm
content: Quote from: uer166 on April 01, 2024, 08:28:38 pmHiNot yet.If you read my posts wayyyy back, my aim was to separately develop and quantify the performance of the balun before mating it to an antenna. I have arrived at the view that the balun is THE critical component of a Biconic antenna, but the least discussed. Also, I have made two identical antenna specifically so I can accurately measure and calibrate their response.I now have a balun with a flat response across a very wide frequency band width. I will be able to accurately characterise the antenna performance.I think there is still potential to improve the linear ferrite over coax balun in two areas:1. Further increase the bandwidth by using ferrite beads of different materials along the length of the balun. I have not tried this.2. Add components to reduce the phase deviation (2 degrees) and amplitude deviation (5%) of the balanced output. I tried adding very small ferrite toroids over the centre core wire of the balanced output to little effect. Further experimentation is required.What I have is "good enough" for my application, but I can see the potential to do better. Any additional effort to improve the balun would qualify as over-engineering.

author: uer166
author_rank: Super Contributor
timestamp: April 02, 2024, 08:07:56 pm
content: Quote from: dazz1 on April 01, 2024, 09:16:47 pmWhen you calibrate the setup against a standard, I'm sure this is easily cal-ed out. For my (similar) use case, these numbers are more than enough, though for a different band (500-1.5G).

author: dazz1
author_rank: Frequent Contributor
timestamp: April 03, 2024, 05:49:54 am
content: Quote from: uer166 on April 02, 2024, 08:07:56 pmHiI made a balun with coax and a SMA Tee. It outputs equal amplitude and 0 phase difference. This is my cal standard as shown alongside the fixture (fitted with the actual linear ferrite balun) in the photo.The other photo shows the A and B channels of the linear balun at 25MHz. If you stand far enough from the computer screen, the two sine waves look identical. Given that the waveform from the centre core is slightly greater amplitude, I think there is the possibility of reducing the phase and amplitude deviation and improving balance.My review of papers revealed that balun imbalance is a significant error, causing beam pattern distortion that degrades measurement accuracy. The deviation measured in my balun is probably within the noise. I am thinking of looking for a simple method of reducing deviation. Over engineering is fun.

author: dazz1
author_rank: Frequent Contributor
timestamp: April 04, 2024, 08:40:51 am
content: HiI have produced a video about what I call the linear ferrite balun, because no one else seems to have given it a name. You can see the video here:https://youtu.be/Bc4mT_oOxRQThe video assumes limited RF knowledge, including those that will never own a proper vna. Some of you might find the video a bit slow but I could have easily made it an hour long. It does demonstrate the diy 3-port vna including the fixture, the calibration substitute balun and the measured performance of the balun out to 500MHz using a 25year old 100MHz scope.I don't think the video will hit the top 10 on YouTube this week, but it may be of interest to some. Remember to watch right to the end because not much happens.

author: dazz1
author_rank: Frequent Contributor
timestamp: April 05, 2024, 10:02:21 am
content: HiI thought it might be useful and relevant to compare the linear ferrite balun with a more conventional toroid based balun.I made a video here:https://youtu.be/T0NOIWpfvf8that sweeps the frequency from 1MHz to the maximum limit of the toroid balun.The test results prove that the linear ferrite balun is significantly better than a more conventional coil over a toroid. I think the key difference is that the toroid is affected by inter-winding capacitance that changes the relationship between the X and Y inputs.The image shows a toroid based balun being tested a while ago. Previous measurements recorded and bandwidth of 20MHz to about 300MHz. That is confirmed to looking at the test data from the 3 port vna.

author: dazz1
author_rank: Frequent Contributor
timestamp: May 30, 2024, 11:52:11 am
content: HiI have just found a video on a fixture to test common mode current on a balun.The principle of operation is the same as the fixture I made for my spectrum analyser. That is to divert the signal to the coax shield.This fixture has just been released as a commercial product here:https://electronics.halibut.com/product/common-mode-current-choke-test-rig/There are no specs but based on the video, the user manual, and observation of the internal photographs, it would appear the bandwidth is limited to the HF band. That is fine if that meets the requirements.It does not surprise me that someone else has developed a common mode test fixture applying a similar design principle, because standard affordable test equipment doesn't do it. It necessary to create something to go between the DUT and the test equipment.Just for comparison, my test fixture is shown attached. The chart plots the insertion loss of the test fixture with the calibration link in place up to 500MHz. This was done by comparing the fixture with calibration link fitted, referenced against a simple coax cable. I think I tested it to about 1GHz but only recorded the plot to 500MHz.There are probably more versions of common mode test setups and fixtures somewhere out there. If you know of any other examples, post the links here. It is interesting to see what others are doing.

author: dazz1
author_rank: Frequent Contributor
timestamp: June 16, 2024, 07:47:54 am
content: HiI have finished the final version of the balun and assembled two completed antennae you can see at this link:https://www.eevblog.com/forum/rf-microwave/diy-rf-emc-biconic-antenna/msg5543799/#msg5543799The photos show spacers made to protect and locate each ferrite in the steel tube that the balun is installed within.The second photo shows the completed balun. This type of balun is an ideal form factor to fit in the tube.The steel tube is left floating, unconnected to anything. This is not typical practice with metal components, but I decided that there was too much risk that the steel tube would act as a tuned stub if connected to the coax shield at the BNC connector. If I am wrong, then the 3D printed BNC connector housing includes a feature to add a washer that will connect the BNC Connector and the steel tube.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 16, 2024, 03:45:14 pm
content: It's a stub whether you want it or notSpecifically, it's a double-open stub, so, half-wave multiples are relevant.But it's also ferrite loaded, so it should be well terminated, and grounding one side or the other will be fine. Which of course suggests the unbal side.Probably, the balance is slightly better with it floating, but as that puts some voltage on the tube, it will also affect the radiation pattern or feedline current a bit. (Probably negligibly.)Tim

author: dazz1
author_rank: Frequent Contributor
timestamp: June 16, 2024, 11:58:08 pm
content: Quote from: T3sl4co1l on June 16, 2024, 03:45:14 pmPrior to making the balun assembly I did some testing specifically to look at effects of a steel tube. Originally I planned using plastic or phenolic tube, but too difficult and too expensive to source. I just happen to have some 22mm OD steel tube on hand. Experimental results showed the steel had no effect, grounded or floating.I still have options. The 3D prints are designed to fit over a 22mm OD tube, regardless of the ID bore, so I can change the tube for any other material with the same 22mm OD. Leaving the tube floating significantly reduces the coupling with the balun/coax. So yes it is still a tuned stub, but lightly coupled.The bi-conic antennae are still prototypes. I can still make changes if I find problems with further testing.