In 2009-2012 I completed the design and testing of a very large tetrode
amplifier that ran in grounded grid configuration. Grids 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 was highly dependent on the electron beam, specifically the
level of DC cathode current. I have this 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 we 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 fast electronics are
involved, in FPGA 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.
Sweeping the input match of these amplifiers while they are pulsing is
not easy. I used a Hp8753 VNA, at 10 mW power, and sweeped 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 excessive
narrowbanding the thing - broaden the input tuning at least a ham bands
width.
You drive the thing with the normal level, and get the current up to
measure 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. Much
of the problem has been alluded to here, 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 use a Delta Electronics OIB-2
operating impedance bridge. Same thing that broadcaster engineers use to
measure their antenna Z under power, but the HF version. 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
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