[Amps] 200-ohm tank and "un-un" in HF amp design

[Jim Thomson]

Date: Mon, 21 Oct 2013 10:32:27 +0200
From: peter chadwick <g8on at fsmail.net>
To: amps at contesting.com
Subject: Re: [Amps] ?200-ohm tank and "un-un" in HF amp design?

>The touted higher harmonic suppression of a PI-L is mainly a myth.....a plane jane PI  is more than ample for harmonic suppression.<

As a Class AB stage has a second harmonic component of plate current only 6dB down on the fundamental, what sort of Q do you run in the pi network to get the extra 37dB of suppression needed to meet the FCC regulations or the extra 44dB needed elsewhere?

73

Peter G3RZP

##  ameritron uses a simple PI net on 40-10m on their bigger amps...and it easily meets the 44 db down FCC requirements.  Ameritron uses a PI-L  on 160+80m..but that’s so a smaller load cap can be used. 
My ants all exhibit sky high swr on their harmonics..including the 3rd...so whats the problem ?   Me,  I could care less  about harmonic suppression  since the amp kills most of it..and  the ants kill the rest.   My  F12  40m yagi barely resonates just a bit below the
12m band...and not at all on 15m.     Optibeam uses linear decoupling stubs on their 40m yagis  so they wont resonate on  15m.      I enter a  Q of 8-10 on  the GM3SEK  spreadsheet  for all my hb amps.  30-36 db of 2nd harmonic suppression 
is plenty  imo..... from any amp.   With 1 kw out....and  36 db down.... that means   1/4 watt on the 2nd harmonic.   .25 watt  into a sky high swr isnt gonna  bother anybody anytime soon.  


Jim   VE7RF