For many years the radio direction finding system sold by Jim Walston was the gold standard retrieval system for free flight planes. A small transmitter attached to the plane sent a series of beeps and the pilot found the bearing to the plane using a directional antenna on a UHF receiver. Walston no longer supplies these systems, but they are still available according to an NFFS forum. Apparently, Walston got his systems from a supplier to Falconry hobbyists ( radiotracking.com). Systems are also available from wildlifematerials.com. Falconry transmitters weighing as little as 4 grams are available from merlinsystems.com.
The transmitters are tuned to channels about 10 KHz wide in a HAM Radio band near 220 MHz. Walston systems used a WMI TRX-3S receiver with a frequency range of 148 to 222 MHz, which can be tuned to 3 channels.

A list of members tracking frequencies and channels is on the next page.