Dirt-Cheap Ham Radio Antenna Tuner

By PrepperDoc

ham tunerIn the midst of a ham-radio 2-day “bootcamp” license exam course, I wanted to help the group recognize just how easily and cheaply they could make much of their own ham radio station.  People today are so used to BUYING everything, that they have lost touch with their ability to MAKE small pieces of equipment for their own use.   Licensed ham radio operators are unique in that they are specifically authorized to construct their own radio equipment.  I decided to make a simple antenna tuner mostly out of homemade components – literally making the components, not just the circuitry.

While a resonant half-wavelength dipole (468/F(Mhz) = ½ wave dipole in feet) fed with RG-58 50-ohm coaxial cable is probably one of the simplest and most common antennas, many experienced ham radio operators want an antenna that can be tuned to any frequency on any band, and will still present pretty close to a 50-ohm impedance to their precious transmitter’s output stage.   A dipole antenna, or even a single random length wire (with “ground” as the other conductor), as long as it is longer than a small fraction of a wavelength, can be made to work by using an appropriate matching system (“antenna tuner”).   (When the total length of the antenna gets down to 0.1 wavelength or so, there are likely to be inefficiencies because of the low radiation resistance, but that is complicated).  In practice, most people can throw up a 30-100 foot dipole, a dozen or more feet off the ground.  The insulators for the ends & center can be made from commercial insulators, or even segments of PVC pipe with holes drilled.  Electrical pvc conduit is sunlight-resistant.    Instead of using expensive coaxial cable (which can have very high losses (and even arc-over) when used with a non-resonant dipole like this) it is simpler to purchase or even make “ladder line” open (air insulated) parallel wire transmission line.   One way to do this is to cut sunlight-resistant electrical conduit PVC pipe into 5-inch segments, drill a hole just inside each end, and pass two regular old insulated 14-gauge household wires (either stranded or solid) one through each of the holes.  Spacing the PVC pipe spreaders every 12-18 inches or so should keep the two wires nicely separated and bingo, you have made a “ladder-line” transmission line that will have extremely low loss, no matter what is the SWR on the line.   What you care about is not the SWR on the ladder-line, but instead what is presented to the transmitter.  (I’ve written about this before, suggesting a simple L network that can also be homemade if need be. [1] )   If you choose to purchase transmission line, you can use inexpensive 300-ohm “TV ribbon line” for perhaps several dozen watts, or 450-ohm line just as well.

ham tuner diagramThe real education for the group was building the homemade T network.  The T network is an extremely versatile matching system that can generally match almost anything your antenna can do, and make it look just like a resistive 50 ohm load to your transmitter.    The ARRL has a nice write-up that will explain a lot more about it. [2]   In my case, I found some 2” water pipe PVC, drilled a start hole at one end and an end hole at another, and had one of the energetic teens wrap about 60 turns of insulated 14 gauge house wire, tightly wrapped, from the start hole to the end hole, passed the wires through them so it wouldn’t unravel.  (This created 55 microhenries or so of inductance.)    I then use a kitchen knife to cut off the surface insulation across many of the wires, and soldered in “taps” every ½” or so – or about every 3-4 turns.  An alligator clip on a wire from one end of the inductor allows you to short out a variable amount of this inductor.   While a nice roller inductor would give infinite variability, I was trying to show how cheaply you could do things.  If you want maximum variability, at one end put taps every single turn for 5 turns or so and use a second alligator clip from a wire connected to that end of the inductor to allow “fine tuning”.

We had one nice high-voltage air tuning capacitor with 1/16” gaps between the plates, probably 200 pF or more, but I decided to simply MAKE the other one.   Taking a large manilla envelope, I taped aluminum foil to the outside of one side, and then taped aluminum foil to a throw-away advertising magazine that would slide nicely in and out of the envelope.   These two aluminum surfaces, separated by one thickness of the manilla envelope, make a variable capacitor.  Alligator clip leads connected to the aluminum foil.   I attached an “ear” of duct tape so I could slide the inner magazine in and out without having to touch the foil.   It turned out that the capacitance was more than I needed, and I ended up cutting away half the envelope, leaving a surface of about 60 square inches.  I measured the maximum capacitance of about 260 pF.    The breakdown voltage of paper is about 16kV/mm [3], so a .003” thick paper has a breakdown voltage > 1000 volts.      Both variable capacitors could easily have been made by this same technique and then all three components could have been homemade.

I wired the commercial variable capacitor as my input capacitor, the inductor to ground, and the homemade paper capacitor as the output capacitor in the traditional T-network format. (See accompanying schematic).    If you are going to use up to 100 watts of RF, I would suggest using 16 gauge or better wiring (lamp wire works).   IMPORTANT:  My quick-built example has the components spread way out and very long wires between them:  it works on 80 meters, but you should make the wiring much shorter than my quickie-built example, and the components closer together, if you want to have this work well on 7MHz or above.

Such a simple T network should be able to easily drive an end-fed random length dipole, with the ground wire of the T network sent to an earth ground.   Commercially built T networks use a balun (“balance to unbalanced”) toroidial transformer to provide two completely symmetrical connections to a ladder line transmission line.   I have such a commercially built system (ancient, and procured through Ebay) but I also found that I could skip the balun transformer and simply wire the ladder line to the two outputs of my homemade T network and it works!   Use standard coax wiring from your HF transmitter to your SWR meter, and then from the SWR meter on to the input of this T network.

It takes trial and error to find the best adjustment points for each band for your T network.   The first time can be a bit frustrating, so write down or mark the approximate positions after you find them for each ham band.   The ARRL article [1] provides a strategy that is likely to more quickly reach an optimum solution – there are actually an infinite number of combinations that work with a reasonable SWR presented to your transmitter, but you prefer to arrive at one that causes the least circulating currents through that homemade inductor, so that you have the lowest heating effects and losses.  Typically, this occurs with maximum output capacitance.

REFERENCES

[1] PrepperDoc. Inexpensive and Simple Mono- or Multi-Band HF Ham Radio Antennas.

https://survivalblog.com/inexpensive-and-simple-mono-or-multi-band-hf-ham-radio-antennas-by-prepperdoc/

[2] Griffith, AS.  Getting the Most Out of Your T-Network Antenna Tuner.   Accessible at:   http://www.arrl.org/files/file/Technology/tis/info/pdf/9501046.pdf

[3] Breakdown voltage of paper:  http://hypertextbook.com/facts/2007/VashtiPrasad.shtml

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Comments

  1. Great Info PrepperDoc; This is one of the many things I need to spend more time on. I remember many years ago helping dad with his antenna, cutting trees for a tower, de-barking them, etc. He never saw need to get us involved w/ his HAM hobby though it was with him everywhere he went world-wide for work & still remember his call sign: W7UY, went by the handle: Tex. Mores the pity. Maybe we just showed more interest in that new fangled invention: TV, to our own detriment!

    • My dad was a commercial industral electrician. The only thing that I know about electricity is that it can mess me up more then I can it. The ham radio articles make me want to learn something about it.

      • You might watch the six-hour ARRL technician training video that is on YouTube, done by the kings.

      • @Axelsteve; It pays to know a little about everything, I would say, maybe a little more about something that could put you down permanently. My dad did gov’t contracts as an “operating engineer” during the ’50s thru the ’70s from Europe to Viet Nam setting up satellite tracking stations, listening posts, communications systems, etc. and passing them off to the military & others. Met someone a few years back that worked at one of the listening stations in Turkey that my dad put into service. Its a small world. Maybe that’s how I ended up in the communications industry and electronics for a while.

  2. Good info, my station is completely homebrewed from scratch. My only cost was solder. Parts of it have been in various publications at one time or another.

    • That is quite unusual and very impressive! Congratulations, can you tell us a little bit more about it?

  3. We are still behind in our communications preps. I have some equipment but need more and training.

    It’s always something….

    • PrepperDoc says:

      For local neighbors & security patrols: the Baofeng UV5R has some impressive features that would make it a first choice, since you could employ some levels of security with it.

      For outside world news: I am partial to older vacuum tube HF gear (Heathkit particularly) since they are SO CHEAP and are basically EMP resistant from the get go.

      • OhioPrepper says:

        PrepperDoc,

        First of all, this is an excellent article, showing someone that they can do meaningful homebrew equipment additions to their station, with some simple things like PVC tubing, standard insulated wire, and some aluminum foil, and not just learn to be another appliance operator.. I remember building a Tesla coil back in the high school physics lab, with a Neon sign transformer, some hand wound coils, a high voltage capacitor made from sheets of glass and aluminum foial, and a spark gap. When you understand the basic physics, all things are possible. You also reminded me that I have some air variable capacitors and a roller inductor in a box somewhere, and that perhaps it’s time to dig it out and start a build.

        I’ve built more than a few Heathkit radios in the past, and still own an old R-390 receiver. For those unfamiliar with the R-390, it is a 19 inch, rack mountable, vacuum tube type, digitally selectable, mechanically tuned radio receiver that covers all of the HF frequencies, with some additional impressive advanced features. The model R-390 uses pretty standard military nomenclature, with the ‘R’ indicating receiver, and the 390 being a relative indication of weight (LOL). The only downside to tube type equipment is that it requires considerably more power than the newer solid state systems.

        As for the Baofeng UV5R, I own several and a UV82; but, since they are nearly ubiquitous, I’m not sure how you would use any of their features to employ any additional level of security; however, if you have some suggestions, I’m listening.

        • PrepperDoc says:

          Well, there wouldn’t really be much security, you could add tone selective squelch so it would be harder for people to do a man in the middle attack on you, but you couldn’t really prevent people from hearing what you’re saying.

          You can however use these VHF FM walkie-talkies to pass audio based digital signal such as PSK 31. And although you can’t do it now, in an emergency you could send encrypted files. That would be secure.

          • OhioPrepper says:

            PrepperDoc,
            Good point on the audio modulated digital modes like PSK 31 and the host of others available with FlDigi. We’ve been using those modes to pass practice emergency traffic on our ARES and RACES nets, and they work well. My question was more to determine if I had missed something about the use of the radios; but, I guess not.
            73’s,
            OP

  4. TexasScout says:

    I can’t see this working on much power without you getting RF everywhere in the shack. “Worked ALL TV Sets, AM/FM radios, and Stereos” would be right around the corner.

    AD5TD

    • PrepperDoc says:

      Laughter!!
      Yes, I did this mainly to show how easily it could be done for the group I was teaching. If you wanted to do it for real, you would use shorter wires, and you could even shield the entire thing (even in a cardboard box covered with aluminum foil) — still dirt cheap.

      In my (limited) experience, when you are on the 2nd floor and doing the “end fed long wire” vs “ground” (and your ground is 40 feet away) — yes, you can have RF everywhere.

      However, I currently am using a tuner (enclosed, normal components) to feed a homemade balanced open ladder line to a non resonant antenna just outside my 2nd floor window, and when I get to about 500-600 watts input, a cheap phone in the next room gets some RF — there is no “rf in the shack” however…. works just great. And since the tuner is NOT getting hot — the efficiency is pretty high, far higher than coax would be, from what I read. Works amazingly well.

      But you’d want to shorten those wires and consider enclosing the unit. This was for show.

      • OhioPrepper says:

        PrepperDoc,
        You stated, “when you are on the 2nd floor and doing the “end fed long wire” vs “ground” (and your ground is 40 feet away) — yes, you can have RF everywhere.”
        I concur, but there are a few things you can do to help this. I use coax from the transceiver to a combination 4:1 / 0:1 Unun (like a Balun but made for unbalanced to unbalanced) to either an off center fed (OCF) dipole (4:1) or a long wire (9:1), This is fed from a second floor office/lab/man cave/radio shack, with a relatively long ground path, of about 15-20 feet. Connecting a 1:1 Balum type transformer keeps the RF off of the cable, and seems to eliminate the problem. The 1:1 Balun can easily be constructed with some coax and PVC pipe, with several how to articles on the web. Just search for amateur radio 1:1 balun

        • Hey, thanks very much! You have obviously more experience than I do with random length antennas and I really appreciate your chipping in this incredibly helpful information. I will look into those homemade baluns—-my current Heathkit (cheap, eBay ) has a toroidal Balun built in and works just great. No RF in my shack.

  5. PrepperDoc says:

    All kidding aside though, lest we leave a misimpression I should correct some suggestions here: the fact is that antenna tuner such as this will not increase your risk of interfering with neighbors TVs.

    An antenna tuner simply cannot create harmonics. It is a linear device, with no active components. The thing that radiates far more energy is your antenna! The matching networks in your transmitter should be designed to get rid of all harmonics that could illegitimately interfere with your neighbors’ television.

    The 2nd kind of interference that occurs is when consumer devices are overloaded by valid out of Band signals. In this case it is the TV at fault, not the ham, and it doesn’t matter what you do, once you reach a certain power level the faulty device is going to have problems. The solution for this is a high pass filter on the TV input. Whether you use an antenna tuner isn’t going to make any difference.

  6. Great article, you tube does have several short videos which would help beginners. The ARRL manual is a must have for someone beginning their journey toward a technician class license.

  7. It seems that everything is simple, but not everyone can do this!

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