Jim Moritz M0BMU has designed this 500kHz TX
for use with the special research permits being issued by Ofcom in the UK. The
low effective radiated power specified (100mW) means that few will need a TX
with more than 10W output. I'm sure that the design could be adapted to give
more output if required.
Here is Jim's note about the
I drew up a circuit diagram of my new
500kHz transmitter design in case anyone is interested. It uses a "half-bridge"
voltage-switching class D PA with complementary N and P channel mosfets. The
idea of this is that the push-pull design minimises 2nd harmonic levels at
around 1MHz, and the complementary mosfets simplify the gate drive requirements
somewhat. It uses cheap IRF520 and IRF9520 devices, and produces a maximum
output of about 11W with 13.8V DC supply. It can easily run at different power
levels by changing the number of turns on the output transformer. I included DC
current limiting in the keying circuit, which allows the PA to operate into
open, short, and reactive loads without apparent distress. It has a VFO based
on a 4MHz ceramic resonator and divide-by-8 counter. This covered the whole
band with several resonators I tried, and drifts by a few hertz per hour, which
is satisfactory for most purposes. The circuit diagram is the basic CW
transmitter for now. I have also added various other bits and pieces such as
variable output power, FSK for DFCW, and a RF voltage/current/phase output
metering system for easy antenna tuning, but will have to draw these up later.
Cheers, Jim Moritz.
Click here to view
the diagram or right-click to save it.