The core is an Arduino microcontroller board driving an Adafruit Si5351 frequency synthesizer module. The Arduino actually controls the Si5351 frequency output to generate the WSPR signal. The output of the Si5351 goes to a FET driver that drives an IRF510 general purpose FET running class D. Most of the similar designs on the Internet drove the FET with CMOS logic chips. The key to efficiency and reliability with switched FETs at high frequency is a solid gate drive. I felt the gate drive circuitry on most of the published designs were a bit soft. I bought a tube of the IRF510s figuring I would blow up a lot of them while getting the circuit going. I’m still using the first one! Not bad for a $1 output final transistor!
The remaining inductors and capacitors transform the FET output to 50 ohms and filter out the harmonics to meet the FCC -43dBc regulations.
The Arduino software was found on Github and written by NT7S. I made some changes. The original software only covered the HF bands so I had to make changes so the Si5351 operates at 475 KHz. The original version used a GPS module to control transmit timing. I changed it to accept an external signal from a PC running WSJT. The PC USB-serial port RTS line is normally used to drive the transmitter PTT. This signal is used by the Arduino to start a new transmission sequence.
Beacon information like call, grid square, and power output are hard coded in the program.
The current antenna is an inverted L. The vertical part is about 40’ high and the horizontal part goes about 45’. A full sized inverted L for this band would be about 510’ long. It obviously needs a lot of loading. The ground system is lacking as well and the antenna is very inefficient. I estimate I have an ERP of about 100 mW.