Dear VLF,
At http://www.iup.uni-heidelberg.de/schaefer_vlf/12V_300W_VLF_PA.pdf
you can get the circuit of my 300W PA i used at all the VLF experiments.
The circuit is made in my well known simple hand drawing ;-)
Many stns (at least 10) have recorded my 8,97 kHz signal so you know
that the circuit is working. The efficiency is about 100%, the heat sink
becomes not even hand warm during a 30 min 300W carrier. The output
power is at least 300W CW, maybe more is possible. I haven't tested that
yet.
One hint: The VFO has to run before you push PTT. Otherwise one FET is
switching the whole time and the fuse will melt! There are several
options that can be added to the circuit (e.g. current limiting,
overvoltage protection, temperature regulated fan, SWR bridge...) but i
wanted to keep it as easy as possible. There is an optional fiber optic
cable PTT input to totally decouple the PC from the PA. This also works
very sufficient.
This circuit uses an ferrite output transformer of unknown material. But
it is not critical. Just try what you have, maybe with a power supply
with current limiting. In that circuit, the current without a connected
load is less than 150 mA (this makes it very suitable for /p operation
as well). If you try another ferrite core, you have to check for proper
primary turns number. If the number of turns is to less, the core will
go into saturation (with or without a load, so check it without a load,
just the primary winding). This can be seen either by watching the input
DC current or by watching the Drain-Source voltage. If it is in
saturation, the rectangular edges/slope (falling slope) becomes round.
Something between 2x 3...5 turns may be a good value.
In my configuration the PA works from 8...100 kHz. increasing or
decreasing the operating frequency is another way to check if the
primary turns are optimal. I can give more detailed infos, if wanted in
a concrete case.
The circuit handles reactive power without problems! This is the
opposite to some audio amps ;-) Another advantage is, that the output is
isolated from the input.
The PA works at 12V (11...14V) and can be used with batteries of course.
It needs the douple output frequency at the input, means you have to
apply 17,94 kHz to get a 8,97 kHz output frequency. The input frequency
should be rectangular with about 12Vp.
Have fun and make the band alive ;-)
73, Stefan/DK7FC
|
