ferrite core set for low impedance differential antenna testing? - Page 1
Forum section: electronics / rf-microwave

author: coppercone2
author_rank: Super Contributor
timestamp: February 06, 2021, 11:49:55 am
content: I want to hook up my loop amplifiers to a VNA and I see that online people have suggested a few different cores for this to make a single ended to differential match but the information is no more then hearsay on unfamilar forums.Does anyone have a good ferrite road map for testing different frequency ranges?I am not necessarily looking for wide-band because these amps are kinda expenisve and they are a pain to make the boards for so I don't mind swapping out a few different baluns for a VNA in order to get more precision. I see the talk is mostly on wideband systems but I am wondering if you go narrowband and put the time into it, can you get better results. My VNA goes up to 300MHz and it has a 50 ohm input impedance.If anyone has some suggestions of ferrite cores suitable for balun for different frequency ranges starting in the low frequency range where the use of magnetic balun makes more sense then an active balun made with amplifiers, if there is such a transition point.I assume the magnetic balun is lower noise so it is preferred in all cases? I know for a broadband antenna a broadband balun makes sense , but for initial testing I assume this might be a detriment.

author: profdc9
author_rank: Frequent Contributor
timestamp: February 07, 2021, 07:19:29 pm
content: I think the best broadband core for use with a low impedance antenna would be a binocular core, something like ahttps://www.fair-rite.com/product/multi-aperture-cores-2843000102/For higher frequencies, 61 mix is preferred (>100 MHz), lower frequencies 43 mix is preferred (5 to 100 MHz), 73 mix is preferred below 5 MHz. If you want a single core that is pretty good for all bands from perhaps 1 MHz to 300 MHz, I would choose 43.The parasitics are going to be due to the stray capacitance between the turns of the balun that reduces its impedance at high frequencies, so as few turns as needed to achieve the necessary impedance is desirable. Binocular cores tend to have the greatest AL (inductance per turn squared) and so minimize the number of turns. Since ferrite cores require more turns to be used at lower frequencies, but lower frequencies are less sensitive to stray capacitance, you might have several cores with different numbers of turns in the balun, with the cores with more turns used at lower frequencies.For low impedances, you may be making a Guanella 4:1 balun, or two cores in series at the 50 ohms port and in parallel at the 12.5 ohm port. The transmission lines in the balun will probably be thin magnet wire twisted together. If the length of the transmission line through the balun is much shorter than the wavelength, then the impedance of the line will not have a great effect. But if you are going to really short wavelengths, you may need to make more specialized low impedance 25 ohm transmission line, for example, two flat conductors (perhaps copper tape) separated by kapton for example that are wrapped around the core.For high impedances, a Guanella 1:4 balun is used. This is pretty much the 75:300 ohm antenna balun used for broadcast television, and one of those may not be a bad choice.

author: coppercone2
author_rank: Super Contributor
timestamp: February 07, 2021, 07:22:31 pm
content: is there any benefit from not using a broad band core and trying to get a few more cores to break it up into different regions ?Sure the windings can change, but what about the cores. There are many options for different frequency ranges but I don't know how many optimally fall into 100KHz-30MHz.

author: A.Z.
author_rank: Super Contributor
timestamp: February 09, 2021, 08:01:42 am
content: Quote from: coppercone2 on February 07, 2021, 07:22:31 pmbetter linearity and lower losses come to mindQuoteFor about 1 to 30 MHz (a bit more in effect) the design shownheremay fit pretty well, notice how the design uses a capacitor to compensate the winding capacitance and obtain more linearity over the whole range, going for such a transformer, you'll probably also want to add a 1:1 choke to the secondary to take care of CM currents; then, willing to go below in frequency (VLF/ELF) you'll probably want another transformer to maximize performance

author: coppercone2
author_rank: Super Contributor
timestamp: February 09, 2021, 09:53:33 am
content: I meant how many materials choices there are that split .1-30MHz into subsections with greater accuracy, that paper had more broad band ferrites. I want to know if this range can be divided up with narrow range devices.I want to get my moneys worth from the 'span' setting on the network analyzer if possible. I.e. is there some wonderful 2-5.5MHz (etc) ferrite. My network analyzer is quite capable IIRC, focused more on speed/assembly line testing then resolution but I have a feeling it might be able to get some more resolution, perhaps by virtue of its stability, over the solutions that are popular now i.e. nanovna, which may be too difficult to use with 'narrow range' while maintaining measurement accuracy. I want to make a series of these for better test results if possible.. maybe call it metrological interest

author: vk4ffab
author_rank: Frequent Contributor
timestamp: February 11, 2021, 03:20:39 am
content: Quote from: coppercone2 on February 09, 2021, 09:53:33 amSeems kind of a redundant thing to do because a single mix will give a known frequency response over a much wider frequency range. Seehttps://www.fair-rite.com/materials/for the details of what they actually make.I should also mention, that the suppression frequency range and the wideband transformer range of each mix type are 2 different things. SEE:https://palomar-engineers.com/ferrite-products/ferrite-cores/ferrite-mix-selectionfor more details.

author: coppercone2
author_rank: Super Contributor
timestamp: February 11, 2021, 06:48:51 am
content: do people maybe make ferrites tailored to a narrower range in small batches/single design? i.e. can you get a custom ferrite mixture that would theoretically be better, I assume this HAS to be a thing for SMPSU design since that field is still evolving ?Or does the chemistry/crystalography just not allow you to 'bound it' in frequency to increase performance in a easy way?I do wonder what the ferrite manufacturers tend to come up with when someone has a special request.There were discussions on this forum about making your own ferrites but most of the problem comes from die making and pressure.. most of the machines for the assembly processes are some how obtainable but for me the pressure-setting die making part makes me not want to deal with it (kiln work is more interesting, but you need some fancy powder mixers and cold-sintering stuff.. you cannot just mix it up and set it in the oven like a cup cake unfortunately).However this is the interesting part, normally the idea behind making your own ferrite was to get the geometry perfect for the application, but the problem is of course you need a tool and die shop to make you the stuff for the press every time. But, I do wonder if you just had a small series of tooling made.. what can you do with chemistry/composition alone without constantly changing the geometry/shape of the parts to pretty much mimic standard core sizes with custom compositions, if you managed to figure out the assembly process. Obviously experimental use only because I would expect LOTS of fracturing problems and stuff.

author: vk4ffab
author_rank: Frequent Contributor
timestamp: February 11, 2021, 09:13:28 pm
content: Quote from: coppercone2 on February 11, 2021, 06:48:51 amThere is no one that I know of that does small batch runs. But, you could go on ebay and buy a bag of iron powder, zinc powder and magnesium powder and use epoxy as a binder to make your own test pieces for experimentation. You do not need to make cores, but just rods will do the trick. They will be somewhat fragile, but it would allow you to experiment in this space and come up with a mix that suits a specific application. You only need a VNA and LCR meter and you can characterize the mixes you make. Maybe a set of accurate scales would be useful also. Mix your flavor, add some binder and pack it into PVC tube and tamp it down, epoxy does not stick to PVC and you can then push it out.Food for thought.

author: coppercone2
author_rank: Super Contributor
timestamp: February 11, 2021, 10:24:35 pm
content: I read the real ones are pressed under 50-500 tons and then baked like porcelain to get reasonable magnetic propertiesI do wonder if the prototype made with epoxy will have predictable properties to one that is made properly with reasonable indication of performance, though the last thing I want to do is deal with any kind of presses, molds, dies or money

author: vk4ffab
author_rank: Frequent Contributor
timestamp: February 12, 2021, 04:13:33 am
content: Quote from: coppercone2 on February 11, 2021, 10:24:35 pmthats something you will have to try and find out. I have seen hams crush up commercial magnetic core materials and pack them into slug form to use in projects, so I guess its not out of the real of possibility. You can get all the materials off ebay for under $50 and give it a go and report back on your findings.

author: OwO
author_rank: Super Contributor
timestamp: February 12, 2021, 02:00:49 pm
content: TL;DR: the best performing common mode choke you can build (highest impedance over wide frequency range) is simply winding coax around a ferrite rod 10-20cm long, not a toroid or binocular core.The explanation is very simple. Imagine an air coil inductor. It's not an inductor, it's a helical waveguide, which behaves like a transmission line not very different from a microstrip. It acts like an inductor only because the line has very high Z0 thanks to the magnetic field coupling, and in fact the higher the Z0 the more it approaches an ideal inductor.What does adding a ferrite do? It changes the permeability of the area inside the coil, which increases Z0 and slows wave speed. It also adds loss. The permeability is frequency dependent and for most ferrite materials it drops to around 10-20 by 1GHz, but usually it becomes purely imaginary (lossy) by 10MHz or so. What if there is enough loss and a long enough coil that the wave decays to negligible levels by the time it travels from one end to the other? Then the impedance looking in is Z0 at any frequency and no longer depends on the coil being electrically small. This is the highest common mode choke impedance you can achieve over a wide bandwidth.If you loop the ferrite around to make a toroid, then one end of the transmission line couples into the other end, which is completely undesirable when the wavelength starts to be comparable to the size of the ferrite, and don't think that's wavelength in free space: remember the ferrite's permeability significantly slows wave velocity, so a small toroid can in fact be electrically large at as low as 100MHz.Obviously you have to wind coax around the rod and not some wire pair, and another gotcha is the turns need to be evenly spaced, otherwise you get Z0 steps which creates reflections that can give you resonances and decrease input impedance.

author: coppercone2
author_rank: Super Contributor
timestamp: February 13, 2021, 07:01:11 am
content: what if you use a torroid but you don't wrap it all the way around just half way? (I guess high leakage inductance winding). I.e. like a e core with a single turnAnd how about shielded slug inductors? those will depend on the mechanical precision of the partsI have found an odd inductor before, a ring with the windings concentrated on one side. it must have been made on purpose by a scientist for some reason (junk salvage after a lab cleanup i.e. 70's parts). So to put my question short, what happens if you don't mind using oversized magnetics

author: vk4ffab
author_rank: Frequent Contributor
timestamp: February 14, 2021, 04:42:25 am
content: Quote from: coppercone2 on February 13, 2021, 07:01:11 amSometimes its just for convenience. Say you are trying to wind 2uH on a core that will give 1uH per turn, so you have a core with 2 turns on it. There is no way to spread 2 turns over the entire core so you just wrap them as best you can. Open up any auto tuner by LDG and take a look, the first 4 cores in many are wound just like that.Winding cores and core sizing is just a matter of balancing the current through the core, wire size and number of turns you can physically fit on a core. No rocket surgery required. We all tend to over think things, with transformers its more about ensuring the best coupling between windings to reduce losses.In some things I have built i needed to transform 50 to 1500z, thats 3T to 23T. There is no practical way to wind 3 turns over the entire core, or twist windings together, so you just make do as best you can and accept the small losses due to imperfect coupling between the windings. Could use a bigger core, more windings and more board space up until the point that becomes cost effective or the induced capacitance becomes a problem or the number of windings becomes problematic at the frequency of operation. Cant think of the formula off the top of my head, but you should be able to find it quite easy on google if needed.

author: coppercone2
author_rank: Super Contributor
timestamp: February 14, 2021, 02:47:34 pm
content: but how does that fit into the waveguide propagation thing since the coil is not looped around but the flux isi don't see a helical waveguide looping in on itself coupling input to output anymore, so long you use 1/4 of the core or so you are basically on the side of a square, so is that not a better inductor since you don't have a giant flux in air but you are also not looping a helix waveguide? where is the trade off between containment and recoupling? or just putting a shell on it (shielded rod inductor that is shielded with a can made of the same material as the ferrite)..

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 14, 2021, 06:27:56 pm
content: The problem then is, you don't get much winding length. And you still have the wires crowded together inside the toroid opening, more proximity than for a longer solenoid.Tim

author: vk4ffab
author_rank: Frequent Contributor
timestamp: February 14, 2021, 09:32:04 pm
content: Quote from: coppercone2 on February 14, 2021, 02:47:34 pmAt HF and below where we use toriods for inductors in tuned circuits and transformers none of this really matters. There is theory and there is practical reality. VHF and up these kinds of things and the theory becomes really important. But a frequency with a wavelength of 10's of meters long in a trace or wire thats fractions of its wavelength has no appreciable iterations and affects. Its why people can build things like this and they work well. I mean, every basic rule of thumb and design rule has been broken here and yet, someone has build a radio that works well.Peter, VK3YE who posts here occasionally has build many rats nets like this that not only work, but perform extremely well. What this says to me is that at HF the theory is extremely forgiving, to the point where much of it can be ignored. Much like waveguide theory with cored inductors can be ignored at HF and below.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 14, 2021, 11:58:15 pm
content: Waveguide theory is certainly applicable at HF. Large RFCs for PAs are a great example -- if not so much at the low impedances of SS amps, then with ye olde toobs. Which I guess are still popular among hams? So it's relevant!It affects power electronics as well. The worst current transformer I've ever bought is a, Triad CST206-1A I think it was. Laughably rated to 200kHz, it has a sharp and uncontrollable resonance just over 300kHz!The reason the resonance is uncontrollable, is because it's the 1λ resonant mode of the wire and core. The core I'm not sure what it is, perhaps powdered iron, I don't think it's actually ferrite -- in any case it seems between the relatively high conductivity of the core (exacerbating winding capacitance), and given the relatively high number of turns: the resonant frequency is quite low, the resonant impedance is high, and the Q is high.We only have three-ish ways to control it externally: 1. increase the terminal impedance; 2. link the primary winding perpendicular to the terminals; 3. attempt to suppress low-order resonant modes.1. We can't afford to increase terminal impedance, because the burden resistor has to be whatever it does, some ohms. Increasing it would cost frequency response and saturation. Increasing it at AC helps a little (i.e., with an R || L in series), but still cuts into saturation. And the amount of increase is untenable: it needs to be hundreds of ohms, even low kohms, to dampen the resonant mode. In short, for any practical "burden" load impedance, there's almost no impact. (At the terminals, the resonant mode manifests as a series resonant LC.)2. We can actually observe the response vary with angle of the primary current link, with a null near 90° -- fascinating, but impossible to manufacture...What's happening here is, there are two perpendicular modes superimposed -- one where the terminals are a node, the other, antinode. At a voltage node / current antinode, the disturbance is maximal. So, rotating it 90 degrees excites the other mode instead. This still isn't great, because the two modes couple slightly (visible as a slower-changing envelope on the resonance).3. We can attempt to short out resonant modes, by linking opposite antinodes together. Instead of one 'U'-shaped wire link for the primary current to flow through, maybe we place several around the CT, at magic angles, all wired in parallel.A pair at 0 and 180° to the terminals, cancels out the current anti-nodes for that respective mode, as well as all other odd modes. If we add links at 90 and 270°, we knock out the 1λ cross mode, 2λ parallel mode, and etc.In the extreme, we can simply put a toroidal shell over the winding -- slitted at the outer periphery so that current is forced to flow through the transformer. (Or a shell slitted at the inner annulus, which cannot carry current through the center by itself, but provides shielding for any wire placed there -- this is how your typical nice Pearson CT is constructed and used.) In this case, resonance reduces to a microstrip transmission line mode, effectively the wire above the shield which acts as ground plane. This is much faster than the core-loaded mode (i.e., speed of light in the dielectric, not in the core), and easier to dampen -- simply place a termination resistor at the far end, of a reasonable impedance i.e. ballpark 100 ohms.But for the CST206, we can't do much of this because -- again besides the practicality of manufacturing this -- its plastic shell casing is so large, we can only couple to resonant modes so closely.I forget if I ever tried testing it with solid foil tape over the enclosure, I don't think so? But I wouldn't expect it to help all that much, because of this.So, what to do, to get good CTs? Use fewer turns, and use a slitted toroidal shell if possible.This of course applies to RFCs on toroids, too -- though in that case, I think by the time you're using enough wire to matter, you'll have lapped several layers on. Which frustrates the helical resonator modes, but more to the point, the inter-layer capacitance dominates. For single layers, you're likely using little enough wire, and the permeability is low enough (<100 say), that the resonant mode will be quite high, not too many times lower than the wire's electrical length.Or, these things again apply at very high frequencies, of course -- a good example might be the twin-lead "data line" common mode choke. These are usually wound bifilar on a ferrite bar with end caps, and a top-cap of ferrite to close the magnetic path between the end caps. The CM impedance is simply the impedance of the winding, a helical waveguide-in-ferrite mode. Which is pretty well absorbed at those frequencies, so a lumped RLC equivalent is pretty close. The differential mode however, has several peaks and troughs -- this should be read not so much as nasty DM impedance, but rather as the transmission line that it is.Cool fact, those transformers often saturate in the mid 10s to low 100s of mA -- it seems the magnetic path isn't as closed as it could be. They're not bad isolation transformers, if you need very little power, low isolation voltage (<100V?) and a small size and cost.Tim