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Friday, October 3, 2008

Flea Market Find -- Pi Net Tuner -- With a Mystery

Last Sunday's "Flea at Trastevere" (I'm the only one who calls it that) was a huge success for me. Minutes after arriving, amidst all the junk, Billy and I found this homebrew Pi network antenna tuner. The guy who sold it to me told me he had built it himself. The workmanship is top-notch. The quality of the parts is superb. There's a built in SWR meter and a 10 db attenuator.

When I started drawing out the schematic I noticed something weird: As the front panel indicates, it is a Pi -network, but for some reason the builder has the input variable cap floating. The physical connection to the chassis is via insulators. The other variable cap has the standard mechanical and electrical connection to chassis ground. That input cap needs to be grounded too, correct? Why would this builder -- who obviously knows his way around a soldering iron -- go to the trouble of allowing that input cap to float?


  1. With the input cap insulated from ground you have an "L" network that would match high impedance antennas. Turn the tuner around backwards and it matches low impedance antennas. Look to see if there is a means of grounding the cap to make it a pi network for mid range impedances. 73, Gene - K8EE

  2. Bill, are you certain it's insulated? Maybe the builder simply used insulators as spacers.

  3. Yea, it is really floating. It looks like at some point he had a wire to either ground it or to put it in series with the coils that form the top of the pi.

    As currently configured the input cap isn't doing anything -- the rotor plates are connected to the input terminal, but with the stator plates floating, you don't get any variation in C. So now it is -- as Gene notes -- an L network.

    Too bad there is no room for another switch for the kind of option Gene suggests.

    Still kind of puzzling. If he started out building a pi network, then disovered he really needed an L, why not just take the input (or output) cap out of the circuit? Why go to the trouble of floating it? Bill

  4. Maybe plan "A" was to put the C in series with the L in a L match so that he had finer control of the L value.

  5. I had a couple more thoughts on this. The first was that the builder might not have been able to find a cap that was as low as he wanted and might have wanted to put a C in series, but then he would have put that at the hot end.

    The second thought was that he might have wanted to deliberately reduce the min cap value - either because the min was too high for his application, or just to make it less sensitive to tune. Again the first thought was that he would put the C in the hot end, but then if he wanted to switch it in/out he would also have a hot switch and maybe he did not want that.

    So my new guess is that the plan was to be able to switch a C in/out of series with the input cap to reduce it's value.

  6. There's an interactive 360deg panoramic view of a stall in the market at

    Unfortunately not the radio junk stall

    Neil GM8EUG

  7. Maybe that otherwise excellent ATU was for sale because the builder forgot to put in the grounding switch for C1, and since he could not make it work, he put it up for sale.


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