[Amps] Passive grid stability calculation for a tetrode?

[TexasRF at aol.com]

Hi All, a late input here: I have been wondering what the feedback  
power/voltage does to the input vswr. Seems there would be two standing waves  
present: one from the input circuit mismatch (if any) and the feedback voltage  
of hopefully a smaller magnitude.
 
The resulting combined vswr would depend on the relative phase of the two  
standing waves, right?
 
It seems that there would be some good benefits to adding a neutralizing  
circuit even for a passive grid design.
 
All of my tetrode experience has been with cathode driven designs. I have a 
 TH347 PA on 1296 MHz that has a fully adjustable input matching circuit. 
No  amount of tweaking will make the input vswr much better than about 2:1. I 
always  assumed it was because of the unneutralized feed back from anode to 
cathode. By  tuning the driver and input match for maximum PA power output, 
it all seems to  work well so it has been left as is.
 
73,
Gerald K5GW
 
 
 
 
In a message dated 11/15/2013 12:17:40 P.M. Central Standard Time,  
g8on at fsmail.net writes:

Actually  Bill, the formula was derived using tuned circuits, where Rp = 
QXLp and Rg =  QXLg. As you move off resonance, the impedance and gain 
naturally drops.  However, it is worth keeping the value of H around 1 to 1.25  
maximum.

The input impedance should be figured as the swamp resistor in  shunt with 
121pF, although as that is a lower impedance, it will actually  increase 
stability. Whether or not the driver can stand the SWR (and be  stable!) is 
another matter.....

73

Peter  G3RZP
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