[Lf] diplexers for136

[Andre' Kesteloot]

Dave Sergeant wrote:

> >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
> dsergeant at btinternet.com
> dsergeant at iee.org
> http://www.dsergeant.btinternet.co.uk