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Author Topic: Homebrew tuner power limits  (Read 1658 times)

Posts: 5215

« on: January 07, 2013, 11:37:29 AM »

I've got a homebrew link-coupled tuner I built back when I just had a 100W out rig, but I built thinking that I'd get an amp someday.

I'm thinking the voltage-limited component, is the big-ass Johnson dual (balanced) variable cap with a 4500V rating that is across the output coil.

I also have an old (1930's?) air variable in series with the input coil, probably 1000 pF with 0.045 spacing. Usually I just keep it fully meshed so I'm not sure I actually need it. (Maybe if I didn't use taps on the output coil to hook the antenna up, I would need it.) But the designs in the 40's and 50's handbook had it there, so I copied them :-)

The link-coupled coils are wound from 14 Ga copper wire and have 0.25" spacing. I doubt they would be the limit on power, after extended use at 500-600W RTTY in the contest this weekend, the coil was completely cool as a cucumber.

I use alligator clips to hook the ladder line to taps on the output coil. The input and output coil are mounted on a polycarbonate frame with banana plugs and jacks.

It's far from an "automatic tuner" but I can do a band change (unhook alligator clips; swap in a different coil set; hook up alligator clips; tune) in less than about 30 seconds.

This works just fine at the 100W (barefoot) and 600W (Ameritron AL-811H) level. But if (hypothetically) I upgraded to a big-boy amp, 1500W out, would I be pushing things too far? Is the limit I'm likely to hit, capacitor voltage? I'm thinking that on the high bands, that the tank formed by the output coil and Johnson variable might have a high Q factor, which would multiply up the standing voltage?

My one tuner flash-over I did have, was an experimental link-coupled tuner I built for 160M. I think the output coil tank was way too high Q, because I was able to get the air variable across it to arc over at 100W level, and the tuning had to be adjusted as I moved around on 160M. Of course that was just a small receiving cap. Since then I built an L-match using the same air variable for 160M that works just fine at a much lower Q (and has a much wider bandwidth as a result!). So based on this experience I mostly think in terms of capacitor voltage. Are there other limits I should be thinking of, if I scale up to 1500W? Maybe ladder line feedthrough the wall, or standoff insulator spacings for ladder line?

Tim N3QE

Posts: 1757

« Reply #1 on: January 07, 2013, 02:00:15 PM »

The coil can have high voltage on it but the voltage drops are spread across the length of the coil so the interwinding voltage isn't all that high.

You are correct in assessing the limits being the capacitor plate spacing (it's voltage rating)

73 de N4CR, Phil

Never believe an atom. They make up everything.

Posts: 5688

« Reply #2 on: January 07, 2013, 02:29:56 PM »

Also consider that a tuner that is close to resonance takes less beating than one that is way out there somewhere.  I learned as a young and near broke all the time ham that my old Johnson Matchbox would not arc 'n spark if I first tuned it to near resonance at the frequency of interest, then applied lower power and tuned it up, increased the power and tuned it again, then hittin' it with all that my building power and budget could afford. 

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