Class C transmit amplifier for the 630m MF band

I continue to try different amplifiers in order to improve my station for weak signal digital modes, such as JT9 and WSPR, on the 475KHz, 630m band.

unboxed prototype amplifier with space left for antenna relay

The amplifier posted here is a medium power, and physically small, two IRF640N mosfet class C design. I designed the amplifier so that the 13degC/W heatsinks would provide adequate cooling for up to 50 watts output power when run from a 54Vdc power supply, while still retaining the compact size. With 2.5W drive power at the input, I measured 33W output power, ( gain 11.2dB ).  This was deliberate because at that power level, the optimum mosfet drain load is almost 50 Ohm; so I avoided the need for any output matching circuit. It will be interesting to see what I can achieve on digital modes now.

antenna change-over circuit added (lower-right)

Using it for the first time yesterday evening, my WSPR beacon transmission was received at a record distance 1655kms in France, and I was rewarded with a JT9 digital mode contact with Scotland at a similar distance.

 finished amplifier in home-made enclosure

 

3 February 2023

I received a request from a radio amateur in the USA ( Jeff, callsign WB8RJY ) for the circuit diagram, as he wanted to construct the amplifier. A few days later Jeff sent me several pictures of his completed amplifier, and a summary of his WSPR activity using the amplifier detailing some very long distance reception reports.

amplifier built by WB8RJY based on my design

 

 

 

 

 

 

10 watt amplifier for MF phasing exciter

Internal view, cover off - amplifier built inside old pc psu box

Bias circuit bottom left, antenna changeover circuit top right

In my last post, ( 2nd November ), I described the phasing exciter I made especially for data mode communications on the 630m 475KHz MF band. I have been using it for beacon transmissions and 2-way contacts. I tweeted news of one such contact using JT9 mode. All this has been achieved with an output power of just 100mW. For consistent results, however, particularly when propagation conditions are not favourable, ( e.g., long periods of fading ), more power would be beneficial. So I have built a single-ended class AB rf amplifier rated for 10W max output which is connected externally between the phasing exciter, low pass filter and antenna.
Once again I have used a metal box from an old pc power supply, complete with cooling fan, for the enclosure. A type 2SC3039 power transistor is fitted to a 5K/W heatsink directly in front of the fan. I have included a variable bias circuit, based on a 2SC2958 transistor; actually it's the same transistor type as the driver in the exciter as I had spares. I have used this to set the current for my chosen no signal operating point for the 2SC3039 for class AB operation.
The amplifier has made a huge improvement to the reception range of my WSPR signal; so far up to 1632kms.