On the air digital radio transmission experiment

This is the latest prototype HF transceiver that I've built. It uses two IRF510 mosfet transistors with two gate driver ICs to push them into class D. I can transmit morse code, digital FSK modes, and BPSK modes at about eight watts from a 12v supply. I designed the circuit, laid out the PCB, and milled the board on my CNC mill myself.

The onboard microcontroller provides a terminal based interface available over its serial port. By connecting, I can issue commands to the transmitter, including the transmission of messages. Here, the "fh" command specifies transmitting using Hellschreiber, a ~90 year-old digital encoding that sends text data over audio or radio links.

Using a web-accessible radio receiver in Corinne, UT (http://www.sdrutah.org/), I can see how my signal is received 1,400 miles away! The narrow vertical blip is the hellschreiber transmission shown in the previous image. It's faint, but readable.

Using FLDigi software, I can decode the message received from the Corinne, UT receiver. Hellschreiber transmits a series of pixels (Rudolph Hell is considered by some an independent inventor of the pixel!); it's sort of a "digital fax" mode. The signal fades in and out, but with repetition and multiple transmissions, the whole message can be read.

The transmitter can also send messages with morse code. I transmitted my callsign a few times and got picked up by a number of monitoring stations in the Reverse Beacon Network. With a few dollars, some physics education, patience, help from some mentors, and iterative development, getting 8 watts around the country is really fun!