[Amps] Strange pi network tuning problem

[Guido Tedeschi]

Hi Ron,

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

https://drive.google.com/open?id=1SGQP9o7CZHm7UfXB0Fk31Nce3_TA_0Eo

https://drive.google.com/open?id=1Bp1VLDjcnb5wGVVBCMHjNGkmwvZgQkN4

https://drive.google.com/open?id=1hWDKsd51tpFTfHuoWapJ2M1t1OBAdBjA

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:

https://drive.google.com/open?id=1cWJJ90h-cEa7J54KZOB6NWDmS7HIFM4b

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
> significant.