Dual band transmitter power amplifier for the LF ( 2190m ) and MF ( 630m ) bands

home made enclosure 25 x 24.5 x 12cm

band module for the 2190m band is shown installed

I've recently finished building another amplifier for my transmitting setup for the 2190m/136KHz and 630m/475KHz bands. It is a switching amplifier design based on Class 'E' topology, using four IRF640N  MOSFETs in parallel. To allow operation on both bands I constructed the output tuning and matching circuit for each band as a removable module; changing the frequency band of operation just requires installing the appropriate module. I have also fitted a RF voltage sensed automatic antenna changeover circuit.
In use the amplifier runs only slightly warm. Each MOSFET is mounted on a separate 4.4degC/W heat-sink, two cooling fans are running and there is ample ventilation. So my work on the thermal aspects of the design was worth the effort. By using 4 MOSFETs in parallel there is very little heat to be dissipated anyway as their combined 'on-resistance' is extremely low. Amplifier efficiency is about 83%.
I was very pleased to get a reception report of my signal on the 630m band from Bantry, south-west Ireland, ( distance 2085 kms ), as well as reports from Greece and Spain.
My future plans are to paint the front panel of the enclosure, and make a hinged top cover.
I can provide the circuit diagram on request by email.

Improvements to the 475KHz amplifier

Note the new, larger heat-sink and its position

Automatic antenna changeover circuit

I've made a couple of changes to the amplifier since I first reported on this project on 22 September 2014.
The mosfet has now been fitted to a larger ( 4.5degC/W ) heat-sink for better cooling, and repositioned to make device replacement much quicker and easier than before, should it again be necessary !
I've also added a circuit for 'rf voltage sensed' automatic antenna changeover switching which is a much more convenient arrangement than a manually operated  coax switch which I was using.

By the way, my 'countries worked' total on the 475KHz / 630m band has now risen to 14.