Power Amplifier for the 472KHz/630m Band

🔘This construction project, which I've only just completed, is a transmitter 🗼 power amplifier for the ∿ 472-479KHz 630m medium-wave amateur band. It is a Class C amplifier design, meaning it is a non-linear amplifier.

 Front, on the left, with input socket & indicator LEDs (click to zoom) 

Features of the design include two field effect transistors ( MOSFET's ) connected in parallel, 'RF' sensed automatic antenna changeover on transmit, antenna changeover delay optimised for the data modes I use, -20dB power coupler ( for connecting an oscilloscope, diode probe or power meter ), DC power LED (green), transmit mode LED (yellow), 50 Ohm output impedance. An integral 'L'-Match network matches the MOSFET's load resistance to the required 50 Ohms output impedance of the amplifier.

Underside, cover removed, to show internal layout

Regarding building the amplifier, a two piece 'U'-section aluminium box was acquired for the case, though I replaced one half of the original box with perforated mesh sheet for improved ventilation. A substantial heat-sink has avoided the need for any forced-air ( 🪭fan ) cooling🆒. The coil former ( yellow ), toroid core clip ( blue ) and some capacitor holders ( grey & blue ) were custom made on a 3D printer. The amplifier is physically compact; overall dimensions📏being ( l x w x h ) 230 x 100 x 136mm. The 🏋weight is 1.273 kilograms.

The amplifier produces about 95 watts output when used with a 54Vdc power supply. Approximately 2.5W input is required; i.e.,  gain is 15.8dB.🔘

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