This UHF transmitter is designed for low power applications as remote for garage doors and operating systems, wireless alarm. This UHF FM transmitter with only one transistor which works on frequencies reserved to low power airwaves signaling.
Transmitters frequency is FM modulated by an audio or digital signal applied in the common point of D1 or D2, through R3. The transmitter schematic can e modified to produce AM modulation by eliminating D1, D2, R2, R3 and R6, and short circuinting A and B points.
Frequency is determined by a resonator, FL1. Can be used the following types:
UHF FM transmitter description
T1 transistor is mounted on the front of printed circuit board, as indicated by the dotted line on the drawing board. Also, on the front face of the PCB should be placed either a short connection (silver wire) that connects positive supply bar and inductance L1 made directly on the PCB and parallel to it. Wire position depends on the frequency transmission. Lower frequencies require higher inductance, so connect it farther to PCB edge.
To find the optimum position start by adjusting trimer C3, about the middle range of variation and mount thread connection about half the route that represents inductance. Watch the signal received from the transmitter and adjust until C3 is obtained peak. If you can not find most, try to move the thread to be connected transistor (lower inductance) or to the edge of plate (higher inductance). Optimum position is that the C3 allows obtaining a maximum signal when it is fixed on middle race.
Because we are dealing with a UHF transmitter, it is self understood that all components terminals should be as short as possible. Fm broadcaster should be closed in a plastic so it can radiate.