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137kHz Transmit Transverter

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In my recent tests on 137.5kHz WSPR I have been using the transverter below which gives around 30-40W output depending on the PA supply voltage. Using my 20m spaced earth-electrode antenna, with nothing in the air at all, I achieve an EIRP of around 30uW, which has been enough to get regular and consistent reports on WSPR2 from 250km (G8HUH).

I have used a 10MHz crystal here, but a lower local oscillator frequency would be slightly better from a stability viewpoint when using narrowband modes like QRSS30 or WSPR15. One choice is to use a low cost 1.843kHz crystal mixing with the driver transmitter's output at 1.979kHz.

When a stable external LO source is possible e.g. a GPS disciplined oscillator, remove C7 and feed the LO into the double balanced mixer. Although an SBL-1 mixer was used an alternative, available from eBay is the ADE-1. As balance is not super critical, a home made double balanced mixer would also be possible.

For the PA I just used a couple of small TO220 heatsinks screwed onto the PA flange in the prototype. With optimal PA matching into 50 ohms there is little heat from the PA device. A larger heatsink could prevent overheating when the antenna is mismatched. My output network was optimised for my slightly higher than 50 ohms output load presented by my earth electrode antenna.

Future upgrades included adding a RX preamp and switching circuitry.