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I will soon need an antenna impedance match (aka "antenna tuner"), because I'll initially only have a single antenna for my HF rig (needs to cover 80m, 40m, 20m, 15m, and 10m). Obviously, in order to manage SWR, I'll need a matching network, quite likely with a wider capability than the pi network built into my Heathkit SB-102. Even if the SB-102 can manage without help, I'll also need to match my portable antenna to my portable QRP rigs.

One of the core components of any matching network -- L, T, SCS, or pi -- is a "variable" inductor. Home builders seemingly usually use a tapped coil for this, giving discrete increments of inductance and depending on a variable capacitance to finalize the match.

However, variable caps are getting harder to find; they're no longer manufactured in the old "interleaved plates, air spaced" form, and the tiny plastic dielectric ones that are still available can't take much voltage (and are difficult to adjust precisely).

It occurred to me that what's needed is to adjust the ratio of inductance to capacitance, not either one in particular; if one had an inductor with stepless adjustment over a wide range of value, one might be able to use common fixed capacitors, or possibly a switch-selected gang of parallel fixed capacitors.

Now, adjustable inductors have been around for decades; aligning an old superheterodyne receiver involves tweaking up to a couple dozen components, of which roughly half are slug-tuned variable inductors. I recall from studying for my license exam that a ferrite or iron slug will increase inductance when inserted into a coil, while a brass (or presumably copper or aluminum -- conductive but non-magnetic) slug decreases it.

What wasn't covered in the study materials is how widely one can adjust the inductance with slugs. Common variable capacitors out of old radio or TV tuners run from zero to several hundred picoFarad, and a tapped coil can likewise run near zero inductance when tapped down to two or three turns. What sort of range could I get with, say, a tuning slug that's iron on one end, brass on the other?

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  • $\begingroup$ um, "tiny plastic dielectric ones that are still available can't take much voltage" that's not actually true, see this commercially available list of >=1kV-rated variable capacitors $\endgroup$ – Marcus Müller May 12 at 17:22
  • $\begingroup$ @MarcusMüller I looked at that DigiKey link, and they are all trimmer capacitors, not exactly convenient to use in an HF antenna tuner. Not to mention that the ones that have a wide enough capacitance range for use in an HF tuner are \$200 to well over \$300. $\endgroup$ – Mike Waters May 12 at 18:05
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    $\begingroup$ fair point! I tend to forget that the goal wouldn't be achieving an acceptable matching for the whole band (so, you'd use a trimmer for each band of interest and trim that once), but an exact matching for the frequency of interest. My bad! $\endgroup$ – Marcus Müller May 12 at 19:10
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What wasn't covered in the study materials is how widely one can adjust the inductance with slugs. Common variable capacitors out of old radio or TV tuners run from zero to several hundred picoFarad, and a tapped coil can likewise run near zero inductance when tapped down to two or three turns. What sort of range could I get with, say, a tuning slug that's iron on one end, brass on the other?

The inductivity of a coil is very much dominated by the magnetic permeability of its core – use a core with a twice as high a permeability, get (pretty much) twice the inductivity. A ferrite core can have a permeability a couple thousand times higher than that of air. So, by inserting a core into an otherwise air-core coil, you could achieve that factor of variability.

Problem: Cores tend to saturate in strong fields. You'll have to dimension the core such that saturation does not occur at the powers you plan to use. That can be large, challenging and hence expensive!

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    $\begingroup$ Simply build /calculate an air-core coil, and do a rough estimate of the permeability of the core you're planning to insert. Take that as the factor over the air-core inductivity! In such a coil system, the vast majority of the field energy will be contained within the high-permeability core, so that the surrounding material can be disregarded. $\endgroup$ – Marcus Müller May 13 at 11:23
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    $\begingroup$ @MikeWaters thanks for the kind words, but if I ever start being hung up on my degree, please nudge me carefully, with a paddle, or a small rock, or a slightly larger rock, or something so that I'd be getting off my high horse :) The amount of experience in this SE site is why I'm coming back here – and something I'm clearly in lack of! $\endgroup$ – Marcus Müller May 21 at 20:58
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    $\begingroup$ @MikeWaters thank you! I can only respond in kind: It's a pleasure to work and talk with you here, and you duly earned my respect. $\endgroup$ – Marcus Müller May 21 at 21:23
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    $\begingroup$ So, and that is a big thing for this answer: @ZeissIkon I was wrong about using random bits of steel as experimental cores. Get yourself some ferrite rod :) $\endgroup$ – Marcus Müller May 22 at 7:22
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    $\begingroup$ Okay, ferrite it is. $\endgroup$ – Zeiss Ikon May 22 at 11:00
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...variable caps are ... no longer manufactured in the old "interleaved plates, air spaced" form, and the tiny plastic dielectric ones that are still available can't take much voltage (and are difficult to adjust precisely).

Thankfully, Oren Elliott is a surprisingly affordable source of brand new air-variable capacitors. I have used them successfully in several projects.

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  • $\begingroup$ Yipe! 50-65 dollars each in singles isn't "surprisingly affordable" -- at least to me! $\endgroup$ – Zeiss Ikon May 15 at 11:12
  • $\begingroup$ In my experience, this has always been the going rate for such units. A kit manufacturer used to sell them for $75. $\endgroup$ – Brian K1LI May 15 at 12:28
  • $\begingroup$ Maybe it's just another sign I'm getting old. I remember being able to buy a new radio with one of these in it (okay, likely the light, low-voltage version) for $20 or so. $\endgroup$ – Zeiss Ikon May 15 at 13:25
  • $\begingroup$ Any case, seems like another good reason to come up with a homebuilt way to vary the inductance instead of being tied to a $60+ varicap for each antenna matcher... $\endgroup$ – Zeiss Ikon May 15 at 13:26
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Fair-Rite makes ferrite rods which would be suitable for HF applications. It should be possible to create or repurpose a screw-operated mechanism to move the rod into and out of a cylindrical coil.

Preferably, the material will have steady permeability and low loss over the frequency range of interest. Loss is proportional to the ratio of the real and imaginary components of permeability, $\frac{\mu'}{\mu''}$.

The properties of Material 61 are probably best, because:

  1. $\mu'$ doesn't begin to drop off until past 30MHz
  2. $\mu''$ is relatively low and doesn't rise significantly until 20MHz

enter image description here

The relative permeability of a wound rod depends on the ratio of the rod's length to its diameter:

enter image description here

So you will have to make some preliminary calculations before deciding on turns count for rods using Material 61.

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Variable caps are getting harder to find; they're no longer manufactured in the old "interleaved plates, air spaced" form.

They are not hard to find. You can buy those if you know where to look. I have managed to find more than enough for my uses. You just have to look elsewhere than those selling new components.

I have quite a stock of them (and even rotary inductors!) that I purchased very inexpensively from hamfests.

Electronics and radio surplus companies often have them. And have you looked on eBay? I've seen some real bargains from time to time!

Some places that sell surplus radio parts are Surplus Sales of Nebraska, RF Parts, and Fair Radio Sales. And those are only a few of them.


In lieu of rotary (or ferrite-core) inductors, you could use a fixed air-wound coil with taps. One of the homebrew tuners at the base of my inverted-L uses that method (photos below); the other uses just two variable capacitors in an L-network (Omega match).

Air Dux coil with alligator clip to select taps

The red alligator clip shorts out the unused portion of this old Barker & Williamson Air Dux coil that I bought from eBay. If you can't find one, you can wind one yourself.

Air Dux coil with alligator clip to select taps

This is inside the tuner that I made to match my 160m inverted-L on 80 and 40 meters.

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  • $\begingroup$ @ZeissIkon Your comment was Mike, the point of this question is that air-spaced variable caps aren't as easy to find or as cheap as they used to be, and both tapped coil and fixed coil need them. – Zeiss Ikon May 13 at 14:26 deleted by Zeiss Ikon May 13 at 14:27 $\endgroup$ – Mike Waters May 16 at 18:51
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    $\begingroup$ Ah, now I remember. I asked question A ("slug tuning"), and you answered question B.("air variable caps availability"). $\endgroup$ – Zeiss Ikon May 16 at 18:58
  • $\begingroup$ I'm well aware of tapped coils to vary inductance (I mentioned them in the question). I was looking for an alternative to the "stepwise" change in inductance in order to avoid air variable caps. $\endgroup$ – Zeiss Ikon May 16 at 19:06
  • $\begingroup$ @ZeissIkon Good luck with that, because the values of both the capacitors and the inductor interact. Adjust the inductor, and you'll almost certainly have to change the value of the capacitors. $\endgroup$ – Mike Waters May 16 at 23:04
  • $\begingroup$ So, essentially, you're saying when we can no longer get air variable caps (or if we don't have access to them, say because working in Antarctica or some such) we can't match antenna impedance other than with wire cutters? $\endgroup$ – Zeiss Ikon May 17 at 11:01
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For a variable inductor you can make two coils with a fairly large diameters. One diameter a bit larger than the other. Place the smaller one inside the larger one. If you make the length of the inner coil fairly small you can rotate it so the coils will have the same or the opposite winding direction. This way you can get a large tuning range for the inductance. Obviously, if you design for a very small minimum capacitance, Q will be poor since the series R will be much larger compared to a normal coil. Get inspiration from here: http://axotron.eu/photo/2013/2013-08-18_Rundradiomuseet/slides/DSC_1812.html This one is to tune a 150 kW transmitter. Something similar, but much smaller....

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  • $\begingroup$ This is a variable coupling air-core transformer; they're used as ticklers for regenerative receivers, and as antenna couplers (especially for crystal radios), but I don't recall reading that this changes the induction of either coil -- just how well energy passes from one to the other. $\endgroup$ – Zeiss Ikon May 15 at 11:08
  • $\begingroup$ @ZeissIkon Trust me, Leif SM5BSZ knows what he's talking about. :-) You're getting some great advice on hamSE; but respectfully, you don't know that yet. ;-) $\endgroup$ – Mike Waters May 16 at 23:21
  • $\begingroup$ So, he's suggesting the two coils be wired in series, so they add in phase, and subtract when reversed? Might have said so,... $\endgroup$ – Zeiss Ikon May 17 at 11:00
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Is it practical to use a slug-tuned variable inductor in an antenna matcher (aka “antenna tuner”)?

As far as I have been able to determine after countless exhaustive searches, I believe that the answer is no.

While trying to minimize costs is certainly an admirable goal, I think that you should abandon this idea for a proven design. This could include tuners using only variable capacitors, or tuners using tapped or continously-variable rotary inductors.

No company makes antenna tuners with variable inductors using this method. And neither has any ham homebrewer. The inductance range is simply too narrow to be practical across a multiple bands, regardless of the tuner circuit design.

You might ask this on one of the eHam.net forums where Glenn W9IQ (and other experts not yet on hamSE) has historically spent most of his time.

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  • $\begingroup$ @ZeissIcon Your profile seems to indicate that you are humble and realize that you need to learn from others more experienced than yourself: $\endgroup$ – Mike Waters May 22 at 23:10
  • $\begingroup$ I'm KX4QP -- just got my call sign in March 2019. Learning, learning. Passing the Extra test doesn't mean you know everything about radio. That is admirable! Just don't forget just how many more years of experience and technical knowledge that others here on hamSE (who are trying to help you) have. Some have advanced university degrees, and have collectively forgotten more that you or I know. Don't be like me, an old curmudgeon that is sometimes so set in his ways that he occasionally forgets that others might be able to teach him something new and useful! ;-) $\endgroup$ – Mike Waters May 22 at 23:10

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