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Re: [Amps] Strange pi network tuning problem

To:, "" <>
Subject: Re: [Amps] Strange pi network tuning problem
From: Guido Tedeschi <>
Date: Wed, 15 May 2019 23:01:34 +0200
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Hi Ron,

here are the graphs with Return Loss and Z, plus also two markers above and below the frequency of resonance:

I'm suspecting that some component, probably capacitors, have different behavior with and without voltage applied; this could explain the perfect match with VNA and the mismatch transmitting...

Here is also the schematic diagram of the PA with driver:

Thanks and 73

Guido, ik2bcp / k2bcp

Il 15/05/2019 13:40, Ron Youvan ha scritto:
  Hi Guido Tedeschi:

First of all, thanks to everybody for the answers.
  /* snip*/
4) I connected again the VNA, this time directly to TX SO239 with the antenna relay manually closed, and a 2200 ohm resistors between tubes plates and ground: I was able again to tune the PI network for 50 ohm on VNA on 80, 40 and 20m with plate and load capacitors set to "normal" positions, only on 40m the plate is a little outside of the mark.

Here follow some pictures:

  I think your recorded graphs would be more useful if you put RL on the
left hand trace and impedance on the right hand trace.

  Knowing if these two are changing sharply or gradually can be
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