If I owned an amateur radio license I would be looking at something like the “WSPR-TX Mini” for 4 main reasons:
- It is cheap enough that you could buy one for each band of interest.
- It has a fully open source firmware and PC software (GPLv3) , so if I did not like something, I have the option to modify it to make it exactly the way I want.
- The overall hardware is simple enough to understand, that is not to say that it is basic - they provide a full schematic (but no Gerber files, nor a full Bill Of Materials). A cheap 10 ppm 26MHz crystal is used as the reference, which does sound bad, but they measure the absolute frequency of the reference crystal and stored that in hertz on the EEPROM of the Arduino at the factory. There is a GPS module (For Maidenhead Grid Location and obtaining the correct time, both of which are needed for WSPR transmissions). A Si5351A ( I2C configurable clock generator with VCXO ) is modulated to generate the 4 FSK over a bandwidth of nearly 6Hz ( 4 x 12000 / 8192) required for WSPR, controlled over I2C by the Arduino. And the brains controlling it all is an ATmega328 with the standard Arduino programming interface.
- Once configured the device can run standalone, you could even connect a battery and make it mobile.
The design is simple enough that with any cheap GPS module, any Arduino, a breadboard, a “msop to dip” board with the 10-pin Si5351A soldered to it, and an appropriate filter for the WSPR band selected you could at least test the concept. If you were going to build your own board you could even modify the code and add physical relays to swap between multiple filter banks with something like an I2C to GPIO chip.
The one thing that may be a problem from some people is that it does not come with a band pass filter installed, you need to wind your own ferrite toroidal inductors and solder those three inductors along with four capacitors to the board.
That is the direction I would look for WSPR TX, those frequencies ( 135.7–137.8 kHz and 472–479 kHz) are just too high for an audiophile sound card, and too low for most wideband SDR devices. But since WSPR is only 6Hz wide, maybe there is a better option for TX, anyone ?