An antenna analyser, for amateur radio.
In the bad old days, a SWR (Standing Wave Ratio) meter could only measure the SWR of a radio antenna one frequency at a time. And you had to use your radio to do it, negating any possibility of portable field work if you had a large radio.
But with the advent of modern microcontrollers, displays, and digitally synthesised frequency generation, it is now possible to make a portable self contained unit, capable of measuring SWR over a continuous range of frequencies.
This homebrew unit is based on the design by K6BEZ, using an Arduino Duemilenova to control a AD9851 DDS Signal Generator, reading the signals back in through an opamp, and sending the calculated results to a 2.8 inch TFT screen.
I also added a piezo beeper for audio feedback when the buttons are pushed, and a red light on top that glows when the unit is in measurement mode.
The software was written for the Arduino to run several modes, and to cover all of the ham radio bands plus the entire shortwave spectrum.