The PRT-4A is a 2-channel crystal-controlled handheld FM transmitter. The "A" version here transmits a 150 Hz tone that opens thesquelch on receivers so equipped, such as the VRC-12 series, the PRR-9 (XE-9),and PRC-25/77. Power output is 450mW on Channel 1 and 50-250mW on Channel 2. It's checked out and crystalled up on both channels for 51.0 MHz. Unit requires 15VDC, normally supplied by a BA-399/U battery (not included, and usually dead or weak when found). Unit has an internal regulator and may be powered by two 9V batteries in series.