[Lf] LF: Poor waveform at output transmitter]

[Andre' Kesteloot]

Dick Rollema wrote:

> To All from PA0SE Like at many stations  the output signal
> (both voltage and current)  of my transmitter looks
> terrible (4 x IRF520 parallel/pushpull class B). On a
> dummy load it is very similar to a sine wave.The
> transmitter is connected to tap on the loading coil, a few
> turns up from the bottom.The aerial current is  clean, due
> to the selectivity of the aerial circuit. The voltage at
> and current through the FET's stays within the rated
> limits as long as I can resist the urge to increase the
> supply voltage from 30 to 40V  (aerial current rising from
> 2.0A [1.6A when it rains] to 2.5A). At 40V the situation
> becomes critical and  detuning the aerial system causes a
> FET (always one) to act like a crowbar. Probably the
> maximum permissible current or voltage is exceeded so I
> have to blame myself. As many amateurs seem to suffer from
> FET blow-ups due to fatal voltage and/or current peaks
> caused by proper TX loading only at the fundamental
> frequency I wonder whether it would be wise to insert a
> diplexer between TX output and tuning/loading system; a
> combination of a lowpass filter feeding the aerial circuit
> and a highpass filter feeding a resistor that absorbs the
> power in the harmonics. That might save  FET's.  It won't
> help of course when the loading at thefundamental
> frequency is wrong, causing FET overload.What do you
> think? 73, Dick, PA0SE
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