A compact transmitter power amplifier for 475KHz

Internal view: Right - input circuit, Left - output circuit and fan

Ex-pc power supply cases are ideal for small projects

With the previous amplifier, ( see 14 February ), I contacted a couple of countries on the 475KHz/630m band. I have now replaced that amplifier with a more efficient Class 'E' design. It uses a type IRF640N mosfet and 13.8Vdc supply to produce an output power of 50 watts with an efficiency of about 81%.
I built the amplifier in an old pc power supply case, measuring just 165 x 90 x 85mm, retaining only the original fan.
During the evenings I have been transmitting a beacon signal on 478.5KHz using my DDS frequency synthesiser as the drive signal source, and been getting reception reports from across Europe. So far Essex in England at 1380Kms is the farthest that reception of my signal has been confirmed. If the amplifier remains healthy during these tests I shall attempt to increase the output power to about 140 watts, and start making some more two-way contacts with other radio amateurs on the band.

19.10.2014. Till now my 50 watt beacon transmission has been received the farthest in NW England at 1602kms, and my 'countries-worked' total has risen to 4; Poland, Germany, Finland and France. So as planned, I have increased the output power of the amplifier to 140 watts by connecting it to a 24Vdc supply. The efficiency has improved to 86%.