ADF

Basically, you dial up a selected radio beacon. If you're in range, you'll hear the Morse Code identification letters for the beacon, and a needle on your gyrocompass will swing over to point at the direction the beacon is from your aircraft.

ADF beacons are now a bit archaic in an era of VOR/DME, Inertial Navigation, and, even more recently, GPS. They're still supported, however - every proper avionics suite is equipped to recieve ADF, the beacons are still published on FAA navigation charts, and instrument approaches using ADF beacons still exist and are occasionally practised.

As a side benefit, you can pick up AM Radio stations on this equipment. So, if you can find something to listen to on AM, you can listen to it in your headphones as you home in on the transmitter...