From Dave G3YMC
I am always a bit puzzled at how low pass filters work with a real antenna
load, which is only 50ohms or anything near it at the resonant frequency of
the antenna. At other frequencies it will present a highly reactive load,
which not only makes the theoretical response just that (theoretical) but
also reflects high voltages back to the pa. At one time I was also
considering a lpf/hpf arrangement similar to Alan's but didn't persue the
idea - it will indeed safely sink harmonic responses, but will possibly make
close in reflected signals worse by upsetting the input impedance of the
lpf.
When I was going through a phase of blowing up FETs, I found this was indeed
caused by low frequency instability in the pa, but although I tried various
things to tame the input side of the FET I felt the real problem was of
highly reactive loads being fed back to the drain circuit from antenna
mismatches - and with my loop I don't need to go very far off resonance to
get those. Most blowups occurred when I accidently transmitted a few
hundred Hz off the 1-1 swr frequency (and occurred of course
instantaneously,
glad I have foldback limiting on my power supply!). I found the only way to
prevent these instabilities was to put impedance in the supply side of the
output network, in my case a small ferrite ring with 3 turns on it in series
with the output coupling transformer killed them and I have had no blowups
since - but at a power loss as this ferrite gets rather hot.. See the
circuits on my website if this doesn't make sense.
I am of course using a single FET at a relatively low 35W. Obviously when
you have multiple FETs and a few hundred watts you have even less scope for
accidents. But it really does seem that the output side is where to
concentrate, not at the input which has been suggested. I have in fact
always used dc coupling direct from transistor drivers (based on ideas of
Finbar in the earlier source book) - I have an improved version using a
Scmidtt trigger for my next tx, that is not finished yet but promises a more
stable drive less dependent on the input level from my FT101ZD.
Cheers Dave G3YMC
[email protected]
[email protected]
http://www.dsergeant.btinternet.co.uk
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