DIY RF EMC Biconic Antenna - Page 1
Forum section: electronics / rf-microwave

author: dazz1
author_rank: Frequent Contributor
timestamp: August 20, 2022, 09:39:09 am
content: HiMy latest DIY project.I am making a biconic antenna for EMC pre-compliance testing. I am actually making two identical antenna so I can calibrate them against each other.With any calibration, a reference is required. I have recently had my restored Wavetek 2025 output calibrated to within 0.1dB. My starting reference will be a hard wired (coax) connection between the signal generator and the spectrum analyser.I will then replace the coax with two identical antenna separated by an exact distance, and orientated the same way relative to each other. I can then calculate the gain compared to a pair of isotropic antenna (which only exist in text books).I simply compare the measured gain of the antenna to calibrate them.Making two antenna is only a small increment in effort. Making two identical antenna will be faster than making dipoles etc. I have spent a lot of time with the detailed design. Much of the time required to make these is in the setup for each machining operation.The approach means I will end up with two identical calibrated antenna. I only need one.The first step is to make the top hat. This is the piece that all of the antenna arms and the centre element connect at.Each arm will be held in place with a ball and spring detent. The arms will snap into place. This will make assembly and storage very easy.The cross section shows the interior detail of the top hat. The antenna arms will slide into the top had and be held in place with the detents.I took a lot of care in the marking out to ensure accurate location of the holes.The setup ensures that every part is identical. I have 24 radial holes to drill and 24 opportunities to screw up. The setup will reduce the risk of error.

author: mag_therm
author_rank: Frequent Contributor
timestamp: August 20, 2022, 04:46:50 pm
content: Hi dazz,To make bicycle spoke hubs on the lathe I made a tool post drill spindle with a small dc motor. The indexing was by a 64 tooth gear on the back of the lathe main spindle with a tooth shaped pawl. That gear allows only 2, 4, 8, 16, 32, 64 holes on pcd.

author: artag
author_rank: Super Contributor
timestamp: August 21, 2022, 11:09:43 am
content: When spacing them an exact distance apart, do you measure to the centreline or to the nearest point, biconics being quite large ?

author: nctnico
author_rank: Super Contributor
timestamp: August 21, 2022, 12:35:29 pm
content: Interesting project!Recently I have been testing with antennas as well for EMC testing (I bought a relatively cheap discone antenna which is also wideband) but I quickly found out that there is too much junk floating around in the air already to make any sensible measurement. IOW: I'm receiving so much noise already that it swamps what I want to measure. I came to the conclusion that I'd need a shielded room in order to have any chance of making a meaningfull measurement. Do you have a shielded chamber?Also: how large is your antenna going to be? I looked at the $500-ish biconical anntenna from Tekbox but at the lower end (30MHz to 100MHz) it has very poor reception due to the relatively small size.

author: coppercone2
author_rank: Super Contributor
timestamp: August 21, 2022, 07:32:24 pm
content: Nice antenna. I have the parts needed for a balun, but I need to make the enclosure (I made a antenna at the wrong impedance, I am not sure I measured it right but it looks like I got like 130 ohms. The balun is for me the hard part of these antennas. I tested them in the basement and it looks like under 1GHz its pretty crowded.Also have you considered a pre-selector for broad band antenna work? They always have a preselector in the RF labs when they use broad band antennas on the compliance analyzer machine (I think those machines are built with a preselector). Narrow band antenna does the work for you with the filtering.. broad band requires extra hardware I guessFor a hobbyist that collapsible design of the antenna you made in is very important. They are rather imposing on the laboratory.

author: dazz1
author_rank: Frequent Contributor
timestamp: August 22, 2022, 04:59:44 am
content: Quote from: mag_therm on August 20, 2022, 04:46:50 pmHiI looked at using the lathe but I needed precise depth control. That was easier to do on a mill than a lathe.

author: dazz1
author_rank: Frequent Contributor
timestamp: August 22, 2022, 05:01:04 am
content: Quote from: artag on August 21, 2022, 11:09:43 amI marked the centre lines and aligned on these. The priority was to achieve 60 degrees between elements.

author: dazz1
author_rank: Frequent Contributor
timestamp: August 22, 2022, 05:13:12 am
content: Quote from: nctnico on August 21, 2022, 12:35:29 pmI don't have a shielded room. For this sort of testing facility, an anechoic chamber is required. They cost $ $ $ $ $.TEM and GTEM cells are more reasonably priced, and can be DIY built, but still lots of $ $ and bulky. I won't be building one.I have access to a RF quiet valley about half an hour from where I live. There is a nearby low capacity cell site, but nothing else. It is about as quite as I will find. I does have mains power.End to end, my bi-conic antenna will be about 1.1m. That matches the dimensions of the original Roberts design. Noting your comments about the Tekbox, I can easily make it larger. The elements will snap into place.

author: dazz1
author_rank: Frequent Contributor
timestamp: August 22, 2022, 05:28:46 am
content: HiToday I worked on drilling the holes that will house the sprung ball detents.The holes have to be drilled to a precise depth. If the hole is too shallow, the antenna element will hit body of the detent. If the hole is too deep, the sprung ball will not adequately grip the antenna element.For each hole, the following operations were required.1. clamp work piece in position.2. spot drill to guide the first drill bit.3. drill 2.5mm hole to allow detent to be pushed out if one ever needs replacing. Also, this hole guides following drills.4. drill 5.5mm hole almost to depth to remove most metal.5. drill a 6mm hole to 9mm depth to guide the d-bit drill.6. drill a 6mm dia, flat bottomed hole for the detent to sit on.6 ops, 6 holes and 4 work pieces. That is a total of 144 operations for just one feature.My setup included a Vee block to locate the work piece x & Y. A new drill inserted backwards into the horizontal holes allowed me to index work piece rotation. This setup held the work piece with a very close tolerance.The flat bottom hole was formed with a D-bit. This is a type of DIY precision drill I have made for another project.

author: dazz1
author_rank: Frequent Contributor
timestamp: August 22, 2022, 05:32:41 am
content: HiI had to precisely drill the flat bottom hole for the detents. First I needed to know how deep the hole needed to be. I used a height gauge to measure the top of the hole that the elements will slide into.I then figured out how deep the detent hole needed to be. The setup I used to drill the horizontal element holes ensured the height of the horizontal holes was within 0.1mm measured tolerance.

author: dazz1
author_rank: Frequent Contributor
timestamp: August 22, 2022, 05:34:20 am
content: HiI made a gauge specifically to check the depth of the holes. I made this so I could use it while work piece was clamped in place.

author: dazz1
author_rank: Frequent Contributor
timestamp: August 22, 2022, 05:35:36 am
content: HiThe last operation was to use the D-bit to form a flat bottom hole.

author: dazz1
author_rank: Frequent Contributor
timestamp: August 22, 2022, 05:37:15 am
content: HiThe work done on the top hat is now finished.The next item is the bottom hat. That is the conical part where all the elements meet.

author: dazz1
author_rank: Frequent Contributor
timestamp: August 22, 2022, 07:50:36 am
content: HiThe next part to make is what I will call the bottom hat. It is the part where all of the elements meet.It is going to be tricky because of the shape and the lack of obvious references.Most people making something with electronics likely start by turning on their soldering iron.When I do electronics, I start by turning on my lathes and milling machines.

author: nctnico
author_rank: Super Contributor
timestamp: August 22, 2022, 11:54:40 am
content: Quote from: dazz1 on August 22, 2022, 05:13:12 amThe bigger ones from Tekbox (in the US $1k ballpark) is about 1.8m.If I get motivated enough, I might try to make a shielded tent that fits in my office. The fabric can be found on Aliexpress. A plastic frame from thin PVC tubing shouldn't be hard to make.

author: artag
author_rank: Super Contributor
timestamp: August 22, 2022, 02:55:03 pm
content: Quote from: dazz1 on August 22, 2022, 05:01:04 amThat's in construction though, right ?I'm wondering what happens when you come to measure the loss in a system with two aerials. They are a fixed distance apart and that's used to determine the signal lost due to inverse square law.When making that measurement, the centreline might be an obvious point to use, but the nearest points of the large elements will be somewhat closer. I guess the ideal is to space them widely enough that the error is small, but that might give you rather high loss.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 22, 2022, 03:35:00 pm
content: Hi dazz1,I have been down this path. There are some pictures of my antenna here:https://www.eevblog.com/forum/testgear/diy-biconical-antenna-30-300mhz/msg2261550/#msg2261550Regards,Jay_Diddy_B

author: dazz1
author_rank: Frequent Contributor
timestamp: August 25, 2022, 06:41:02 am
content: HiI have started to make the piece that all the elements meet at.It is a complex geometric problem to align the tools to end up cutting in the right places. Lots of opportunities to screw up.

author: dazz1
author_rank: Frequent Contributor
timestamp: August 25, 2022, 06:45:20 am
content: Quote from: nctnico on August 22, 2022, 11:54:40 amIt won't be too difficult for me to have multiple sets of elements. I would also need matching balans.

author: dazz1
author_rank: Frequent Contributor
timestamp: August 25, 2022, 06:54:22 am
content: Quote from: artag on August 22, 2022, 02:55:03 pmMeasure centre line to centre line.The method I plan to use requires that the antenna are identical. Any difference in performance or reflections, calibration etc will degrade the accuracy of the measurements.

author: tautech
author_rank: Super Contributor
timestamp: August 25, 2022, 07:08:19 am
content: Dazz, have you had to invest in a new collet chuck ?Don’t remember seeing such a shiny beast when I dropped by.Looks a cool project. Do you have a reflection bridge for your SA to characterise these antenna ?

author: dazz1
author_rank: Frequent Contributor
timestamp: August 25, 2022, 09:11:57 am
content: Quote from: tautech on August 25, 2022, 07:08:19 amHiI made the collet chuck quite a while ago but only for the small Denford lathe.Yes, I have a bridge to characterise the antenna but what I really want to know is the gain.If I start with a cable between the Tracking Generator (TG) and the spectrum analyser, I can normalise to that. Zero error.If I then split the cable half way along, I can reference the isotropic gain to the cable.If I place the two antenna, separated by the far field (lets say 5m), then I can compare their gain against and isotropic antenna.I will have two identical antenna, with unknown gain. Two knowns and two unknowns. I just need to halve the measured gain between them. If I measure 10dB between the two antenna, then each antenna must have 5dB gain.

author: tautech
author_rank: Super Contributor
timestamp: August 25, 2022, 09:32:39 am
content: Yes understood that from the outset however the bridge will help you impedance match them and determine their bandwidth.I thought of you when she indoors said she was coming down your ways to the NZ Quilting symposium in October and I am tempted to tag along and go visiting customers and you while she's in the various lectures.I'd be bringing my SVA1032X of course.No promises but we'll see how things pan out.

author: dazz1
author_rank: Frequent Contributor
timestamp: August 25, 2022, 09:38:25 am
content: Quote from: tautech on August 25, 2022, 09:32:39 am

author: tautech
author_rank: Super Contributor
timestamp: August 25, 2022, 10:29:09 am
content: Quote from: dazz1 on August 25, 2022, 09:38:25 amThe only catch is you have just over 5 weeks to have it ready for testing !Could even go all hardcore and bring down a 4 port SNA5004A VNA !Would you even have enough N type cables ?

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 25, 2022, 07:08:20 pm
content: Dazz1,Did you know that the MIL461A std has all the drawings needed to build the antenna?The document is attached to this message:https://www.eevblog.com/forum/testgear/diy-biconical-antenna-30-300mhz/msg2257035/#msg2257035There are some difference between your design and the drawings in the MIL standard:The holes are at 30 degree angle.The holes are not flat bottomed. The flat bottom will not change the antenna performance. Only the diameter of the holes and the diameter of the rods have tight tolerances, but I think this is for mechanical not electrical reasons.Your design has the holes at 90 degrees to the axis:The antenna factor is also published in the MIL std:If you build the antenna it should be very similar to the published antenna factors.Regards,Jay_Diddy_B

author: coppercone2
author_rank: Super Contributor
timestamp: August 29, 2022, 12:08:47 am
content: I have been meaning to make that antenna you posted for a long time with aluminum castings.

author: wilhe_jo
author_rank: Regular Contributor
timestamp: August 30, 2022, 07:26:35 am
content: Maybe one should mention that the MIL standard is MIL-STD-461A. The newer editions miss these drawings. Btw: if someone has the original one (without the A), I'd be glad to have a look at it...The biconical is IMHO not too good of a choice outside some chamber. You can find broad band TV Yagi antennas which work reasonably well for this purpose and allow for some tuning (front/back gain) to reduce some peaks at least for some 10-20dB.However, if you have lots of Broadcast stations around, that's still a huge problem.Btw, the 2 antenna calibration procedure is in mil-std as well...73upps, the std-number was already mentioned... never mind

author: dazz1
author_rank: Frequent Contributor
timestamp: September 05, 2022, 11:02:25 am
content: Quote from: Jay_Diddy_B on August 25, 2022, 07:08:20 pmHiMy design has 6 elements at 60 degree spacing.The original Mil-Spec design used a lot of pipe fittings. They would be expensive and plain ugly. I believe that good engineering should look good. It's an art.

author: dazz1
author_rank: Frequent Contributor
timestamp: September 05, 2022, 11:03:53 am
content: Quote from: wilhe_jo on August 30, 2022, 07:26:35 amI never claimed that the two antenna calibration was my idea. I only claim that I plan to use that method.

author: dazz1
author_rank: Frequent Contributor
timestamp: September 05, 2022, 11:13:03 am
content: HiManufacture of the bi-conic has hit a pause for a while. I have wandered off the path.I had a few options to configure my milling machine to make the top/bottom hats. I chose to fit a table with angle adjustment. When I fitted the table, I found that it clashed with the Digital Read Out encoder. To fix that I needed to remove a gearbox cover. When I did that I found 45 year old Soviet era grease. Not a pretty sight. So now I am stripping down the milling machine for deep maintenance to clean and re-lube.I also purchased a Philips PM3072 scope that doesn't work. The power supply has a fault. There may be more.I will get back to the biconic.

author: coppercone2
author_rank: Super Contributor
timestamp: September 05, 2022, 05:15:30 pm
content: lol thats how I feel about my equipment too, I found a dust bunny in a key and ended up taking a entire machine apart, washing it, and baking it. Thats electronics, but mechanical stuff too, like I end up taking crimp tools apart and shit like that because there is usually some crap with old grease, dirt, etc, in side. I hate that in my tool box. sometimes its a little risky and expensive when I screw up but its the right thing to do.And I mean stuff like that is totally rational too because giant machines are hard to move and stuff like that, and you might find a cracked gear or whatever. And sometimes you have energy for a project but not for the maintenance work, and having something break down because of prior inaction is a deal breaker, especially if its something crazy thats deep in the bowels of a machine, talk about change of plot.

author: dazz1
author_rank: Frequent Contributor
timestamp: June 08, 2023, 09:18:29 am
content: HiWell after a long pause and a period of completing deep maintenance on the milling machine, I have restarted work on the Biconic antenna.I refitted the digital read-out scales and removed as much Soviet era black grease as I could.I am currently working on the fitting that sockets the ends of the elements. This is a conical shaped object with 12 holes drilled for the elements and sprung ball indents. Each hole requires 4 operations to complete. These include:1. centre drill to guide the 1st drill bit.2. drill a pilot hole3. drill a full size hole to depth.4. chamfer the hole edges.Each fitting has 12 holes and I need 4 of these fittings for two antenna. For a total of 96 holes that requires about 380 operations just for the holes for those fittings.I will still have lots of things to make when the fittings are made, including the antenna elements.

author: coppercone2
author_rank: Super Contributor
timestamp: June 08, 2023, 07:11:15 pm
content: just a fyi the black grease might be molybedium disulfide and the best thing might be fresh black grease, or it could be oxidized garbage, at least according to AVE.Do I think that molybedium disulfide is the best thing? Not sure unless its like on the power transmission for mining equipment or something with high volume titanium hogging or whatever

author: coppercone2
author_rank: Super Contributor
timestamp: June 08, 2023, 07:33:37 pm
content: Also, it might not be a bad idea to alodine that stuff. You can't get the electrical one easily but the other one might still help an antenna over time.Ultrasonic clean and dip coat them in a bucket if you want to do that, its the easiest wayat least, if they are being threaded.. if you are welding or soldering that then you should be way betteryour project made me think of this, I would want to try thishttps://www.sra-solder.com/al26-181-75-aluminum-solder-paste-250-gram-jar

author: dazz1
author_rank: Frequent Contributor
timestamp: June 20, 2023, 12:56:18 am
content: HiAnother task completed on the path to making a matched pair of Biconic antennae.I have finished the 4x fittings that socket the elements at the apex of the cones.These took days to make. All of the holes had to be drilled to close tolerances in position and depth to avoid joining up with other holes.These fittings look remarkably similar to the UFOs from the Jerry Anderson classic U.F.O. For those who have never heard of Jerry Anderson and the important mission of S.H.A.D.O, it is only necessary to know you led a deprived childhood from which you will never recover.The next task is to make the elements. You might be thinking this would be easy, but there would be no challenge or fun if it was. I need to cut the elements to length and machine the indents for the sprung loaded balls before I start any bends. No chance to make corrections later. The elements are 6mm aluminium rod. I have printed off a full scale side elevation of the elements to make sure the elements are all the same.

author: dazz1
author_rank: Frequent Contributor
timestamp: June 20, 2023, 01:09:14 am
content: Quote from: coppercone2 on June 08, 2023, 07:33:37 pmI don't have space to store fully assembled Biconics. Mine will snap together so I can break them down to store and transport in a flat case. They will never be exposed to the weather. So while solder or threaded joints might be close to perfection, I think spring loaded ball detents will be good enough.

author: coppercone2
author_rank: Super Contributor
timestamp: June 20, 2023, 03:16:48 am
content: hmm i wonder if nasa has umbrella deploy types of this

author: Kean
author_rank: Unknown
timestamp: June 20, 2023, 03:46:49 am
content: Quote from: dazz1 on June 20, 2023, 12:56:18 amNice work!

author: dazz1
author_rank: Frequent Contributor
timestamp: June 23, 2023, 05:18:32 am
content: HiI have started making the elements.I have previously made the 90 degree bar bender. I now needed a tool to form a large radius curve (535mm radius) on the short leg of the L. There are lots of DIY slip roll machines on the internet. The tool I made isn't like any of them.I used a row of nails in a scrap piece of plywood to bend the element around.There is a lot of spring-back so I did a test bend with a arbitrary radius, compared the rod with the desired radius, then calculated the radius tool I needed to finish at 535mm. It turns out I need to bend the rod around a 305mm radius curve to end up with the desired radius.The large formed radius will create spherical ends to the antenna. Will this radically improve performance ?? Probably nil observable effect, but it will look cool.Easier to bend the rod than to drill angled holes in the top fittings.

author: coppercone2
author_rank: Super Contributor
timestamp: June 23, 2023, 11:12:45 pm
content: it will also probobly be stronger if you drop it

author: dazz1
author_rank: Frequent Contributor
timestamp: June 24, 2023, 02:08:25 am
content: Quote from: coppercone2 on June 23, 2023, 11:12:45 pmThis is the Internet. I am looking at flat-earth theory for inspiration to invent some pretend science to justify a spherical ended antenna.

author: coppercone2
author_rank: Super Contributor
timestamp: June 24, 2023, 02:31:21 am
content: that is a very legitimate design I see it in alot of papers. But it will also be more robust if you drop it like a old BMW car.I have had antenna tri pods tip over before. They like tipping over.

author: dazz1
author_rank: Frequent Contributor
timestamp: June 24, 2023, 05:47:53 am
content: Quote from: coppercone2 on June 24, 2023, 02:31:21 amYes I leaned on those papers when designing my antenna, but I haven't actually seen a physical antenna with spheroid ends.One option I have is to make another set of elements to try different shapes, including smaller/larger/conical ends. The elements will be cheap and easy to make.

author: dazz1
author_rank: Frequent Contributor
timestamp: June 27, 2023, 10:33:46 am
content: HiAnother small step for mankind with progress on the elements.The attachments show the jig to make the 90 degree bends.The antenna calibration process will depend on both antenna being identical. So the jigs will go a long way to achieve that.

author: dazz1
author_rank: Frequent Contributor
timestamp: June 28, 2023, 09:25:34 am
content: HiOK, after spending way too much time making jigs for the elements, it took very little time to bend up the sets of elements.I just plugged-in the elements into the top/lower fittings for a photo op. I need to fit the sprung ball indents. For that I need to make another jig to file indents into the ends of the elements.

author: dazz1
author_rank: Frequent Contributor
timestamp: June 30, 2023, 05:33:24 am
content: HiSomething I haven't yet nailed down is the choice of balun.I don't yet know the frequency range of the antenna, nor do I know the characteristics of the antenna.

author: tautech
author_rank: Super Contributor
timestamp: June 30, 2023, 06:50:33 am
content: Quote from: dazz1 on June 30, 2023, 05:33:24 amYou can get a fair idea with your Siglent analyzer and a RLB.

author: dazz1
author_rank: Frequent Contributor
timestamp: June 30, 2023, 11:02:07 am
content: Quote from: tautech on June 30, 2023, 06:50:33 amIdeally I would like to measure the balun and antenna separately.The references indicate my antenna should be ~50ohms. What I could do is fit one side of the bi-conic antenna to a sheet of aluminium to make it a mono-conic. It would be something like a mono-pole antenna on a ground plane. The aluminium sheet would act like a rf mirror. I could then drive this directly with an unbalanced signal from the sig-gen. The problem is I don't have a large enough sheet of aluminium.Roberts invented his balun for the biconic antenna, so I think I will try that first.

author: nctnico
author_rank: Super Contributor
timestamp: June 30, 2023, 03:29:56 pm
content: Quote from: dazz1 on June 30, 2023, 05:33:24 amAn easy way is to simply connect a coax cable directly with a ferrite clamp around the cable near the antenna feed point. That way the shield of the cable does not become part of the antenna.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 01, 2023, 08:40:15 am
content: Quote from: nctnico on June 30, 2023, 03:29:56 pmI think what I will do is make a Roberts balun driving a 50ohm load to characterize the balun.Then connect the antenna. I need to finish the antenna first.A VNA would be really useful, but I don't have one. I do have a Siglent spectrum analyser with a reasonable DIY RF bridge.

author: tautech
author_rank: Super Contributor
timestamp: July 01, 2023, 09:11:48 am
content: If your VSWR/Return Loss option is active you can do return loss and SWRActivating Markers will bring up real measurements.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 01, 2023, 09:23:09 am
content: HiI have all software options enabled so I can do VSWR/Return Loss OK.Attached is an image recording the rf bridge return loss with a DUT = 50 ohm. If the bridge and 50ohm loads were perfect, the reflected RF would be zero. My DIY Aliexpress bridge can get down to -44dB, which is good enough for me.Many of these bridges have a factory fault. See this link:Mine did not have this fault.It would still be nice to have a VNA.

author: tautech
author_rank: Super Contributor
timestamp: July 01, 2023, 09:34:57 am
content: Quote from: dazz1 on July 01, 2023, 09:23:09 amSorry no plans to visit anytime soon and currently I'd be little use anyways as all VNA's I owned are now sold....even the one I had arrive to replace my demo SVA1032X.Only a new SVA1015X left that I could cut you a deal on Dazz if it's of interest.Enjoying your fine work.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 01, 2023, 09:50:42 am
content: Quote from: tautech on July 01, 2023, 09:34:57 amAs much as I would like to take up your offer, I just couldn't justify the $$ for the use it would get. Even the SSA is an extravagance for me.

author: nctnico
author_rank: Super Contributor
timestamp: July 01, 2023, 10:15:47 pm
content: Problem is that VSWR doesn't tell you the whole story when an antenna that is used to receive signals. It only shows how well an antenna is matched to the receiver / transmitter but doesn't necessarily means power is radiated / received most efficiently. Measuring real antenna performance in an absolute way is an art in itself.For EMC antenna testing, a comb generator with a known output level at the frequencies it is producing, is a much better way to determine its performance.Something like this one from Tekbox:https://emcsupplies.com/products/tekbox-radiating-comb-generator-tbcg1

author: dazz1
author_rank: Frequent Contributor
timestamp: July 02, 2023, 09:33:25 am
content: Quote from: nctnico on July 01, 2023, 10:15:47 pmThat is why I am making two identical antenna. I can normalise the SSA with a cable, then compare the signal sent via the two antenna. That doesn't tell me anything about matching or impedance. An SSA won't give me a Smith chart to stare at.My ancient sig-gen is calibrated to within 0.1dB, and it has a GPIB interface. I can synthesize a comb generator by stepping across the frequency range to produce an output that looks like a comb generator. Better still, I can just sweep the sig-gen across the freq range.It would all be a lot easier with the VNA I don't have.

author: tautech
author_rank: Super Contributor
timestamp: July 02, 2023, 10:10:57 am
content: Quote from: dazz1 on July 02, 2023, 09:33:25 amYet just VSWR and Return Loss measurements are valuable and even getting SWR low for the frequency/s of interest can obtain good performance and provide enough info to get each close to matching.They are very similar to the inverse of the other.

author: nctnico
author_rank: Super Contributor
timestamp: July 02, 2023, 10:14:59 am
content: Quote from: dazz1 on July 02, 2023, 09:33:25 amI don't think that will work to calibrate your antenna.The problem with using a signal generator and a random antenna is that you don't know how well the antenna emits the signal coming from your signal generator. The same for using 2 identical antennas. You don't know how much power from the generator ends up being transmitted, you don't know how much energy that is being transmitted gets received and you don't know how much of the received power ends up in your receiver. Too many unknowns, too many variables. Bottom line: using a comb generator with a known output level takes all of those unknowns away.To show the effect of using an antenna to antenna setup, I refer to the test I did to compare a self made log-period antenna to a self made bowtie antenna. I used a 27MHz antenna to transmit the signal from my network analyser:https://www.eevblog.com/forum/rf-microwave/log-period-antenna-for-emc-measurements/msg4785857/#msg4785857You can clearly see the signal level (purple and orange traces) jumping up & down as the transmitter antenna goes in&out of resonance. As my test is relative, the absolute levels didn't matter to me. I'm going to get myself a comb generator (the one from Tekbox or another one with a knows output) to calibrate my log-period antenna and TEM cell. From the result I can create a correction table that I can use together with the receiver. That way the mismatch between the antenna and receiver is also calibrated away.You could opt to add a balun but you'd need to make relative measurements as many points to see if the balun gives an improvement or not. Keep in mind that for a receiving antenna the mismatch doesn't matter much because the cable is terminated at the receiver and thus you won't get reflections. It works similar to a low-Z probe.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 02, 2023, 10:29:25 am
content: HiThe method only works with two identical antenna, and it is not my method. It is described in one of the MIL-STD461 versions. Probably other references as well.I can apply a known signal level from my sig-gen. I normalize that via a direct cable connection to my SSA. This effectively calibrates the SSA against the sig gen.Setting the antenna a known measured distance apart referenced against calculated signal between two (theoretical) isotropic antenna.If the antenna are identical, then gain of each actual antenna will be the same.

author: nctnico
author_rank: Super Contributor
timestamp: July 02, 2023, 10:47:12 am
content: But you still don't know what gets lost between generator - transmit antenna and receive antenna - receiver. For that you'd need to measure the SWR at your test frequencies and compensate. A while ago I read a report from R&S where they use 2 different antennas in 4 combinations in order to retrieve antenna gain versus frequency.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 02, 2023, 09:40:15 pm
content: Quote from: nctnico on July 02, 2023, 10:47:12 amI have a rf bridge so I can measure SWR. If the sig-gen sends 10dBm to the antenna and 3dBm is reflected, then I know that 7dBm is being transmitted into the ether. Both the Roberts balun and the antenna are entirely reactive (no power lost to resistance). The same applies to the signal path from the antenna to the receiver (spectrum analyzer).The unknown I am trying to find is the gain vs frequency of my antenna. In an ideal world, I could mount an isotropic antenna (gain 0dB) on one end and my antenna on the other (at a known distance separation), and measure gain directly. No one will sell me an isotropic antenna, not even R&S, so the next best alternative is to make the connection between Rx and Tx with a cable (gain = 0dB, no separation loss). That provides a reproducible reference.So the problem is then that I don't have a calibrated antenna as a substitute for an isotropic antenna. If I mount two identical antenna (known separation) with identical gain vs freq, then I can simply measure and calculate the gain.If the matched antenna pair measure a gain of 10dB, then each antenna must have 5dB gain. That is it. I can then produce a measured, verifiable gain/freq curve for my antenna.So I don't need a VNA to do any of that. It would be useful to measure the impedance of the antenna so I can match the antenna to the balun.I do plan to use a 3rd antenna (any old coat hanger would do) specifically to look for any difference in the gains of my individual antenna. If they are identical, the gain/freq curves will also be identical. I am putting a lot of effort into ensuring the two antenna are identical. I am working to far tighter tolerances than are necessary to build a biconic antenna.So a VNA would be nice and I want one, I just don't need it for a one-off antenna project.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 04, 2023, 12:25:30 am
content: HiAnother ants step forward with the antenna.I glued in the spring detents with LocTite.The photo show bits of element inserted to apply pressure to the spring ball detents to keep them in-place while the LocTite sets. If they ever need replacing there are holes in the back of the bracket to push out the detents.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 09, 2023, 07:31:32 am
content: After waiting for the LockTite to set, I assembled the antenna.It just snaps together so easy peasy to assemble/break down. No threaded elements to turn into place.The centre element is made slightly too long so it is clamped in place by the 6 elements pulling down on it.All the parts will fit flat in a cheap Chinese gun case.I still need to make the loop shorting bar that will also snap in-place.I also need to figure out what balun to use, and then make it. That is not a simple thing to do.

author: dazz1
author_rank: Frequent Contributor
timestamp: August 04, 2023, 04:28:25 am
content: HiThe next step is to make the shorting bars and brackets. These damp oscillations within the loops formed by the 6 elements.This could be easy or it could be complicated. I chose complicated.I have extended the snap-together theme to the shorting bars.The brackets will connect the shorting bar to one element and the central bar. They will be held in place with the sprung ball detents.The centre bar is offset so the ball is over-centre to keep it in place in the slot. The shorting bar has a flat that the ball rides in. This will allow the distance between the brackets to vary a few millimeters without having to stress anything.Simple really. Now I just have to make them.

author: dazz1
author_rank: Frequent Contributor
timestamp: August 29, 2023, 12:04:23 am
content: HiI have started to make the over engineered, overly complex brackets for the biconic shorting bars.I am only making 4 brackets, enough for one antenna, as prototypes. If they meet expectations, I will make another 4 for the 2nd antenna.The picture shows a setup for drilling holes in a precise location and depth.

author: coppercone2
author_rank: Super Contributor
timestamp: August 29, 2023, 12:41:45 am
content: the real antenna costs thousands of dollars so I am sure you should feel bad about not using coat hangers to short it outits OKyou won't get people off your back unless you figure out how to do it with bailing wire. but then someone will tell you that your wasting bailing wire because you don't need an antenna.

author: dazz1
author_rank: Frequent Contributor
timestamp: August 29, 2023, 03:50:31 am
content: HiA rational person would pay the thousands for a tested, proven, calibrated antenna, but I never pretended to be rational.

author: coppercone2
author_rank: Super Contributor
timestamp: August 29, 2023, 05:01:46 am
content: ehhhhhh I bet it will be pretty close. That kind of stuff is when someone thinks a business deal or law suit problems. I bet its more brand name then anything (vouching). cover your ass by over spending so to speak. like board room upper management paranoia.RF sales + federal authorities + lawyers + statements = insanity with a walletIts kind of funny with RF because it all changes if you just move it physically. Something that stays in place is way more justified for high cost/precision/checking (i.e.power company connectors)

author: tautech
author_rank: Super Contributor
timestamp: August 29, 2023, 05:02:51 am
content: Quote from: dazz1 on August 29, 2023, 03:50:31 amYet you are in person....or we're both nutters !

author: dazz1
author_rank: Frequent Contributor
timestamp: August 29, 2023, 07:48:57 am
content: HiThe thing I like about RF is that I can start with raw materials I have in my workshop and turn them into something that does something.I can't do that with regular electronics where I have to buy components and assemble them to do something that probably needs programming.You can't buy snap-together EMC antenna (at least I haven't found any) like I am making. That's the other thing I like about RF. I can make something not quite like anything that anyone else has made before.

author: dazz1
author_rank: Frequent Contributor
timestamp: September 10, 2023, 07:44:36 am
content: HiAnother ant sized step forward with the completion of the shorting bars, with the over engineered and overly complex brackets. I would have preferred something that looked a little more organic in shape but I'd really need a CNC mill for that.These shorting bars attenuate the resonance that happens inside the antenna.The next thing to tackle is the housing that supports the antenna. I also need to make and test the baluns.

author: tautech
author_rank: Super Contributor
timestamp: September 10, 2023, 07:47:48 am
content: When you you use the shorting bars Dazz ?

author: dazz1
author_rank: Frequent Contributor
timestamp: September 10, 2023, 07:55:58 am
content: Quote from: tautech on September 10, 2023, 07:47:48 amAll the time. They are only required because the conic is approximated by the 6 elements. These elements form loops inside the antenna. The shorting bar reduces this unwanted resonance.

author: piotekp90
author_rank: Contributor
timestamp: January 02, 2024, 01:52:17 pm
content: I recently opened up my Tekbox TBMA1 antenna and this is what it looks like inside:

author: dazz1
author_rank: Frequent Contributor
timestamp: January 02, 2024, 07:50:51 pm
content: Quote from: piotekp90 on January 02, 2024, 01:52:17 pmThat is interesting. It appears to be similar to the Ethernet matching network I rescued from a dead motherboard. When I tested it, the performance was decidedly mediocre so I started looking for another solution.You can see the results of my R&D here:https://www.eevblog.com/forum/rf-microwave/correctly-calculating-impedance-of-a-biconical-antenna-and-impedance-matching/My results do not necessarily apply to the Tekbox solution. That could only be confirmed/denied by testing the Tekbox balun.I took a completely different path. One of the key problems I had with a twisted pair design is the difficulty in sourcing wire with the required and known insulator properties. This is needed to obtain the correct impedance. The problem of impedance matching a twisted pair was compounded by on-line twisted pair impedance calculators that produce the wrong answers. That is why I switched to using coax cable.Coax requires physically large toroids. The combination has a measured -20dB bandwidth of 20MHz to 300MHz. I think that is close to the limit achievable with coax/toroid approach. I am about to change tack to look at a different coax/ferrite topology. First I need some parts.

author: Kean
author_rank: Unknown
timestamp: January 03, 2024, 12:08:42 pm
content: Quote from: piotekp90 on January 02, 2024, 01:52:17 pmThis got me curious, so I took the cover off my Aaronia Bicolog 30100.https://aaronia.com/en/emv-antenne-bicolog-30100Unfortunately there isn't much to see without some difficult desoldering which I'm worried could be destructive.I can't even see the type of conductor used through the ferrites, but they are somewhat compliant.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 03, 2024, 12:55:55 pm
content: Probably the same basic design:The twisted pairs are Zo = 100R, two in parallel is 50R at the port. Notice the far ends are grounded oppositely, making one an inverting 1:1 autoformer, and the other a straight pass-thru (delay). The latter doesn't need any choking but I had the binoc cores so I figured why not. The former drops the full input voltage so needs choking accordingly. The output in series is therefore 200R, a reasonable match to the, in this case wire frame bowtie, but bicons are similar of course.Tim

author: dazz1
author_rank: Frequent Contributor
timestamp: February 11, 2024, 08:30:44 pm
content: HiI have formed the view that emc antenna performance is largely defined by the balun, which is also the item with the least info, test data results and possibly the weakest link in antenna design.

author: dazz1
author_rank: Frequent Contributor
timestamp: May 29, 2024, 10:31:23 am
content: HiIt has been a while since I posted on this thread, but I have been working on developing a wide band balun for the bi-conic antenna. It has been a journey.In this post I am displaying (a.k.a showing off) a prototype mount for the two halves of the bi-conic plus bar. There is minimal contact between the antenna metal and the plastic to reduce capacitance. The opening at the Tee end is to allow access for a spanner to get to the M6 nut that will hold the conics in place.The attached photos are development prototypes used to refine the design. The mount will accept a tube of light steel. That tube will also hold what I call the linear ferrite balun. The tube will be clamped to a non-conducting tripod. Too expensive to buy so I will make one.I anticipate some may call out saying that it should be a shielded metal enclosure. That 3D printing the mount in plastic is a mistake. To that I would say, no it isn't. There will be less than 2cm conductor exposed between the apex of each conic, and the shelter of the RF coax cable feed. No significant opportunity for RF to leak in. Certainly not enough to justify the time and cost of a metal enclosure.

author: dazz1
author_rank: Frequent Contributor
timestamp: June 02, 2024, 05:24:39 am
content: HiNow I have decided on the type of balun I will use (see this thread for the story of that saga:https://www.eevblog.com/forum/rf-microwave/correctly-calculating-impedance-of-a-biconical-antenna-and-impedance-matching/new/?topicseen#new), I have made the central enclosure. I have now got to the point of assembling the antenna so that it looks like a biconic as shown in the picture.So now I am waiting to receive a cable assembly from Aliexpress to fit the balun and connector. Then I need to make a non-conductive tripod.

author: dazz1
author_rank: Frequent Contributor
timestamp: June 16, 2024, 05:18:57 am
content: HiI have finished the pair of bi-conic antennae.The balun is fitted in a steel tube that doubles as the way to clamp to a stand.The ferrites are separated by spacers to stop the ferrites touching each other end to end. The ferrite is very brittle.The spacers also keep the ferrites away from the steel tube.The colour of the steel tube changed because I needed a slightly larger diameter to fit the hex nut on the rear of the panel mounted BNC connector.Now that I have finished the antennae construction, I need to test and calibrate the antenna. The problem with testing it is that if it doesn't work, I will have two of the largest paper weights I have ever made.

author: dazz1
author_rank: Frequent Contributor
timestamp: June 17, 2024, 11:53:30 am
content: HiJust done some preliminary testing of the Biconic antenna by measuring return loss.The attached images show the test results.Notes along the bottom of the images describe the tests.These tests were done under far from ideal conditions but experimentation indicates the tests are repeatable, and not greatly affected by objects in the near vicinity.The tests included measuring the return loss of a 50ohm test loads. The aim of the test being to quantify the limits of the test setup. A 50 ohm load should have infinite return loss. My DIY balanced bridge is not perfectly balanced. I found that reflections on the cable between the bal. bridge and the antenna connector had a significant effect.Initial testing indicates a bandwidth of around 1.5GHz, substantially greater than the 100MHz-300MHz I have seen on commercially available bi-conic antenna. Early tests of baluns indicates that about 300MHz is the limit of common wound ferrite cored baluns.Even allowing for a wide margin of error, the measured results are looking positive. More testing is required.

author: uer166
author_rank: Super Contributor
timestamp: June 18, 2024, 01:35:13 am
content: Of great value would be to somehow know the field (V/m) to output (dbuV) factor output, since it's for EMC after all. I'm not sure how you'd be able to characterize this outside of a full anechoic chamber lab with a known reference antenna though. Return loss measurements don't include antenna efficiency loss, which was always vexing for me for RF experiments.

author: dazz1
author_rank: Frequent Contributor
timestamp: June 18, 2024, 03:32:28 am
content: Quote from: uer166 on June 18, 2024, 01:35:13 amHiI am getting there. These initial tests are intended to answer questions like, "does it work?" and "how good/bad is my instrumentation setup?". I have already learned some important things.I never expected the antenna to work anywhere near the GHz range. My target was around 500MHz, but I thought that was ambitious given the highest frequency for this size/style commercial biconic I have seen is 350MHz. I should probably be looking at changing the connector from BNC to sma or N-type.The SWR appears to be typical for a biconic but the length of the cable between the balanced bridge and the antenna makes a significant difference to the measured frequency response. I think cable resonance is responsible for the periodic response seen in the plots.From the start, the plan was to calibrate the field to output. I don't have calibrated dipoles so I have built 2 identical antenna. Both with unknown gain, but both are identical. I will be able to measure the total gain, then halve it to get the antenna gain. The procedure is included in version of MIL-STD 416.I don't have an anechoic chamber but I do have access to a substitute for an OATS. It is a deep valley about 30 minutes outside of a significant population. It has power available. Before I go there, I need to refine the test setup. The antennae are still prototypes so I will make changes if I need to.

author: uer166
author_rank: Super Contributor
timestamp: June 18, 2024, 06:08:09 am
content: Quote from: dazz1 on June 18, 2024, 03:32:28 amIf you have the specific clause/appendix number handy, would be good to know. It's a big standard!

author: dazz1
author_rank: Frequent Contributor
timestamp: June 18, 2024, 07:42:55 am
content: Quote from: uer166 on June 18, 2024, 06:08:09 amHiI will have to find the right section in the right version.Basically, it requires a (tracking) sig-gen connected to one antenna, and a spectrum analyser connected to a 2nd identical antenna. The antennae are place a known distance apart.The test begins by substituting an antenna pair with a cable. The zero loss reference.With the antenna pair connected and separated, the total signal path loss is then: G1 + air path loss + G2where G1 + G2 = 2G = antenna gain at each frequency.The flaw with the test method is the standing waves along the Tx/Rx coax cables, and their lengths, will affect the results. I am thinking I need to fit a 50ohm attenuator at the antenna to suppress the effects of the SWR.

author: tautech
author_rank: Super Contributor
timestamp: June 18, 2024, 08:03:47 am
content: Getting a low SWR across a wide frequency range is very difficult IME.I need arrange a road trip down your ways with a few goodies in the wagon.Do you have any long N type cables Dazz ?I might have some a few metres long I can leave with you.

author: dazz1
author_rank: Frequent Contributor
timestamp: June 18, 2024, 08:20:51 am
content: Quote from: tautech on June 18, 2024, 08:03:47 amI think it is a given that such a wide band antenna is going to have a significant SWR so it is something I will have to deal with. It should be noted that the angle of the cones sets the impedance. My biconics are designed to be 50ohm, but that is clearly not perfect.I haven't yet brought/built everything I need to make use of an EMC antenna. I have been incrementally creeping along that path. The initial testing indicates the antennae work so the next step is to design/make non-conductive tripods.I haven't yet purchased the cables I will need to do actual EMC testing.

author: tautech
author_rank: Super Contributor
timestamp: June 18, 2024, 08:33:45 am
content: Quote from: dazz1 on June 18, 2024, 03:32:28 amYet none of your screenshots show a Marker set anywhere near 500 MHz.

author: dazz1
author_rank: Frequent Contributor
timestamp: June 18, 2024, 08:44:09 am
content: Quote from: tautech on June 18, 2024, 08:33:45 amNot a lot happening at 500MHz.I suspect a lot of what I am seeing in the plots is cable resonance in the test setup. The 50ohm test loads don't have cables and don't have a periodic frequency response. I need to do some more testing.

author: tautech
author_rank: Super Contributor
timestamp: June 18, 2024, 08:54:30 am
content: Quote from: dazz1 on June 18, 2024, 08:44:09 amIf that is your target frequency more tweaks are required.Never lose sight that an antenna is a transformer, 50 Ohms > 277 Ohms and SWR of 1.

author: dazz1
author_rank: Frequent Contributor
timestamp: June 19, 2024, 12:55:07 am
content: Quote from: uer166 on June 18, 2024, 06:08:09 amThe double antenna calibration procedure is contained within MIL-STD-461A dated 1 August 1968, section 5.2.7.The important equation is attached:

author: dazz1
author_rank: Frequent Contributor
timestamp: June 24, 2024, 11:06:33 am
content: HiDid some more testing of the biconic antenna. The balun is enclosed in a steel tube that I will use to clamp the antenna to a stand.These tests measured the return loss of the antenna up to 1GHz of the complete antenna with and without the steel tube. The aim of the test was to determine if the steel tube within which the balun is installed affected the frequency response.The steel tube is 0.45m long, so if it was going to couple with the balun and resonate, there would be a peak/dip at around 333MHz. There is a little blip at about 330MHz with the steel tube, but nothing that really stands out. The test setup is far from perfect. The antenna response changes with my movement near the antenna so no expectation of a perfect match between the two plots.None of the commercial Biconic antenae of this size and style advertise a frequency response greater than 350MHz but this antenna and balun combination are good up to at least 1GHz.If the steel tube is shown to be a problem, I always have the option of changing it to something else.

author: dazz1
author_rank: Frequent Contributor
timestamp: June 29, 2024, 10:31:51 am
content: HiDid some more testing tonight.Standard biconic antenna include an element down the centre, and a shorting element connecting the corner of an element to the centre element.I tested the effect of removing these additional elements.The testing focused on the ~80MHz resonant frequency. The natural loop frequency of a finite cone.Note that the test setup is awful. No anechoic chamber, no OATS, lots of nearby conductors etc. This testing was qualitative so don't read too much into the data.I did 4 tests across a 200MHz frequency range measuring return loss:The standard biconic config with all elements in place.Removing the shorting elements. On my antenna, these just clip into place.No.2 and removing the centre element that is along the axis of the antenna.No. 3 and me standing `2m away. I noticed that if I stood about about 2m (1.9M = half wavelength at 80MHz), the return loss jumped significantly.Conclusions are that the shorting and centre elements help reduce the natural resonant frequency of a finite sized biconic antenna.The enhanced return loss when the antenna is 1/2 wavelengths from a conductor are relevant to 2x antenna calibration. If the antenna are placed at about 2m apart, the frequency response will be significantly affected around 80MHz.

author: tautech
author_rank: Super Contributor
timestamp: June 29, 2024, 11:11:11 am
content: Dazz, I don't remember if with your analyzer you can select a Marker with a Continuous valley Peak ?They can be very useful when testing antennae.Marker 2 can be set to the desired resonance to see the difference.

author: dazz1
author_rank: Frequent Contributor
timestamp: June 30, 2024, 01:51:31 am
content: HiI haven't looked too hard at the Marker functions but I will.Today I tested the return loss with a somewhat better setup. I call it my home OATS. I supported the antenna on my EMC rated clothes basket (means the basket is now ten times more $ valuable $), and then put that on top of my front fence. I had some crappy RG58 coax to connect to my balanced bridge and spectrum analyser. I don't have enough cable to do a back to back antenna calibration. I did have enough to measure the return loss.In this fence top position, the antenna was a reasonable distance from the nearest reflectors. That included aluminium window frames, internal wall house wiring, steel roof etc. My home OATS is a long way from a real OATS.I recorded the return loss of the antenna out to 1GHz when mounted on the EMC rated basket. The unedited plot includes lots of periodic response that I interpret as reflections along the cable. With a non-zero VSWR, reflections along the coax cable are reasonable to expect. My DIY balanced bridge has proved to be useful but somewhat untrustworthy as the frequency rises. The first plot shows lots of peaks and dips at about 850MHz. I can't be certain if this is the antenna, antenna coax reflections, or reflections between the spectrum analyser and the balanced bridge.The second plot includes a hand drawn red line that I think represents the actual return loss of the antenna. If this is reasonably accurate then the antenna VSWR is reasonably flat from 100MHz to about 600MHz. With compensation, it looks to be usable up to 1GHz. I haven't tested beyond 1GHz because I wouldn't trust my balanced bridge. I am looking at making a new and better DIY balanced RF bridge shown here:http://www.dicks-website.eu/return%20loss%20bridge_part1/part1.htmlThe plan is to make it so it fits directly to the Siglent Spectrum Analyser.So the results from testing in my front yard OATS are positive. I suspect the results would be even better with improved sensors.I still plan to do two antenna calibration and I also want to measure the beam pattern. The balun had 5% imbalance and I want to know by how much this distorts the beam.In the mean time I have started to design a non-conductive antenna stand/tripod.

author: tautech
author_rank: Super Contributor
timestamp: June 30, 2024, 08:12:27 am
content: That's a fine 84 MHz antenna.Rather than need to get the red pen out, engaging some Averaging might help.

author: dazz1
author_rank: Frequent Contributor
timestamp: June 30, 2024, 09:06:34 am
content: HiI will try the averaging feature, but my understanding is that averaging smooths noisy time variant signals.For comparison, attached is the VSWR of a commercial biconic antenna. Note the chart for this only goes to 300MHz.I think it would be reasonable to say that my biconic antenna displays significantly better performance over a much wider frequency range.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 05, 2024, 04:44:06 am
content: HiAlong time ago I purchased a PCB 400MHz-1000MHz Log Periodic antenna by WA5VJB.There is no balun for this antenna. Knowing what I now know about baluns, I have low expectations about the performance of this antenna.I have used the Siglent spectrum analyser and DIY RF balanced bridge. Same setup as for the biconic antenna.I did three tests. All were scanned to 1GHz. There was a reasonable attempt to keep the antenna in the clear, but the setup is not an anechoic chamber.The descriptions are in order of the attached images below.2. Same as the first test except I made a fist around the coax at the antenna end. Any imbalance would cause currents on the outside of the coax. Wrapping a fist around the cable will interfere with these unbalanced currents. Changes in the frequency response are consistent with unbalanced currents.3. Same as the first test except I used a 20cm cable between the bridge and the antenna. The aim being to show that the test setup has a significant effect on the measured frequency response. If you compare this plot with the first, you will see quite different results.1. Control test. This was a reference test with 1m cable between the bridge and the antenna.My conclusions from these simple tests are:Without a balun, this antenna is unbalanced. Wow. I expect the beam pattern is highly distorted. Any external signals will be picked up by the coax. The antenna performance will be less than ordinary.The setup has a strong influence on the test results. Specifically the cable length between bridge and the antenna have a significant influence on the measured frequency response of the antenna.It isn't practical to have zero cable lengths.I could have reduced the influence the coax cable by inserting a 10dB attenuator at the antenna, but that would have limited effect given the unbalance of the antenna signal. I didn't even bother to do that test.Given what I have learned with the biconic antenna, I plan to make a ballun. The simplest is to solder the coax outer shield to the centre conductor on the PCB.If I make a linear ferrite ballun, I will try using small diameter coax with matching tiny toroid ferrites.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 06, 2024, 10:49:58 am
content: HiToday I did a dress rehearsal of a back to back antenna calibration as per MIL-STD-461A. The setup is far from ideal. My cables are too short to run correctly. There are lots of reflectors nearby to corrupt the results but I did get some results.I tested the antenna with different separations: 1m, 1.5m and 2.1m. If the setup was perfect, each plot would be identical, but offset by some value of dB. At 1m, there appears to be a significant proximity effect between the antenna.The plots and the setup are attached. This is not the AF plot. This is just the loss between the two antenna relative to a solid link.The Siglent has the option of saving .CSV files so I can play with numbers to get the AF chart.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 08, 2024, 09:57:39 am
content: HiI have been looking at applying the method of measuring and calculating Antenna Factor based on MIL-STD-461A. I have struck a few issues largely related to the technology of the time. For a start, the equation uses wavelength λ. Not the most convenient measure for modern test equipment. My Siglent SSA only measure frequency.Next, the equation specifies the transmit and receive signals in units of voltage. I had the SSA measuring power in dBm.I have attempted to modernize the MIL-STD-461A equation for AF by rearranging the equation to use frequency instead of wavelength and dBm instead of a voltage ratio. My workings are shown in the attached page. It has been a very long time since I had to manually manipulate a log equation so I may have muddled it up.I have looked at more modern guidance on measuring and calculating AF, but many of the articles appear to be written by EMC test houses who provide enough information to convince the reader it requires a fully certified OATS (i.e theirs) to calibrate antenna. Now it may be the case that it really does require a calibrated OATS to calibrate an antenna, but I don't have one of those. I also don't have copies of the mega expensive standards that contain the test instructions.What is worse is that different standards measure AF in different ways (test setups). They do not result in the same values of AF. So AF is not an absolute set of values. None of this applied when MIL-STD-461A was written so I will use that for now. It should be good enough for pre-compliance testing.:No one detected the mistakes in my workings, so I had to find and fix them myself.I have updated the document that shows the conversion of the MIL-STD-461A equation into a form usable with modern test equipment.I have also attached a spreadsheet I used to check my workings.This spreadsheet starts with a number of randomly selected variables (editable).These are applied to the MIL-STD-461A equation for AF.As I worked through the process of converting the equation, I calculated the answer.This was repeated until I reached the finished equation.The initial equation, the intermediate equations, and the final equation all calculate the same answer so I am reasonably confident the final equation is correct.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 10, 2024, 08:45:59 am
content: HiAttached is a plot of antenna factor at 3 different ranges between the antenna pair. This chart was produced using the corrected equations from the previous post.In an ideal world, the 3 curves plotted should lay on top of each other.I suspect some of the issues are related to cables to the antenna. The cables are too short to route properly.The test did act as a dress rehearsal of the methods to be applied to the real test.

author: dazz1
author_rank: Frequent Contributor
timestamp: September 07, 2024, 05:53:51 am
content: Rob TAUTECH showed up at my house today bringing toys.One of those toys was a Siglent VNA that I wish I owned.We tried it out on my biconic antennae pair. The results were encouraging. The results shown in the attached plots are described below:PNG1 shows the return loss of the antennae pair, offset by 5dB. When both plots of the antenna responses were directly compared, the plots were the same. The periodic dips are due to cable resonance. To make it visually easier to compare the plots, one was offset by 5dB. This is the result I was hoping for because I applied very tight tolerances to the physical dimensions of the antenna. The plots prove that the antennae are interchangeable.PNG5 shows the fully calibrated return loss out to 1GHz. Aside from the first resonant dip at 84MHz (a physical characteristic of all biconic antenna) the antenna is very well behaved all the way out to 1GHz. There are no significant resonances or unexpected responses. To achieve this result, the VNA had to be calibrated at the antenna connector to remove cable resonant responses. Without taking out the cable effects, the antenna response looks terrible because of the resonance.These measurements with a proper VNA are closely aligned to my earlier DIY test results. These test results with a real VNA provide evidence of the good performance of the biconic antenna and linear ferrite balun.