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<p>Dick Rollema wrote:
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<b>To All from PA0SE</b> <b>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.</b><b>The transmitter
is connected to tap on the loading coil, a few turns up from the bottom.</b><b>The
aerial current is clean, due to the selectivity of the aerial circuit.</b> <b>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.</b> <b>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 the</b><b>fundamental frequency is wrong,
causing FET overload.</b><b>What do you think?</b> <b>73, Dick, PA0SE</b></blockquote>
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