[Amps] Input VSWR of common grid amplifier

[jtml at losalamos.com]

In 2009-2012 I designed some very large tetrode amplifiers that used 
grounded grid configuration. Not tied directly to ground, but grounded 
for RF via cavity circuits that have built in blocking capacitors to 
allow normal bias voltages. Like their amateur siblings ("tubes with 
handles" as R. Measures used to say), the input match is highly 
dependent on the electron beam, specifically the level of DC cathode 
current. I have a cascade of amplifiers, one 150 kW tetrode driving the 
final stage of several MW. Being pulsed, the grid bias is switched from 
cutoff to conduction just before RF drive is pulsed on, to avoid 100 kW 
of pulsed plate dissipation in the final (about 12 kW average power). It 
took a while before my design team figured out best way to protect 
things, as if the final power supply tripped offline, the IPA would be 
driving into a horrible mismatch. So lots of fast electronics are 
involved, fast logic, that shuts off the RF drive and so forth. Also, 
timing of the conduction bias pulses had to be right, so that the RF 
drive didn't come up too many microseconds before the beam was there. 
Tying to sweep the input match of these amplifiers while they are 
pulsing is not easy. I tested them with a Hp8753 VNA, at 10 mW power, 
and sweep across the bandpass of the amplifier cavity. As Bias is 
cranked up, and a few amperes of quiescent (but pulsed) plate current 
flows, the input return loss goes from < 1 dB to 10 dB or more and a 
definite dip occurs. It is a chopped waveform due to pulsing, as I don't 
have the ability to run these things in CW key down. I bring this up as 
it parallels what we hams have to deal with using GG amplifiers. Not 
having cavity circuits for HF, we use L and C and try and get some Q, 
without narrowbanding the thing - broadbanding the input tuning at least 
a ham bands width.

We have to drive the thing with the normal level, and get the current up 
to know the match. Lots of compromises can be made, and those amplifiers 
that require a length of cable, have input matches that were fine for 
tube exciters but these days not so good for solid state drivers. Some 
of the problem is the cyclic fluctuation of the beam from cutoff to 
conduction across each cycle of RF voltage, as others have suggested.

For HF ham stuff I have the trick now. I can use a Delta Electronics 
OIB-2 operating impedance bridge. This thing can take 1 kW through 
power, and will read the input Z of an antenna, line or amplifier, while 
driven. It reads the R and X component to 5%, and from this some 
suitable matching can be developed to get it to 50 ohms. I now have two 
of them, so one will be made available FS.

73
John
K5PRO