where can I buy a FM Transmitter
An outline of how to make or where to buy FM transmit antennas for low power radios.
Once you've identified the open frequencies, perhaps using these handy FM frequency survey log sheets, and set up the transmitter for the selected location on the dial, the next step is to use a good FM radio antenna to establish an effective connection with potential listeners.
The good news is that there is nothing unique about transmitting in the FM broadcast band. From a radio theory point of view, the goal is to radiate a VHF frequency signal to the location of the listener. All the great research, calculations and configuration done by the amateur radio operators for the FM broadcast bands in the 87.5 MHz-107.9 MHz range.
The most common low power FM broadcast antenna configurations are either omnidirectional or a few directional.
Omnidirectional includes 1/4 Wave Whip, 1/4 Wave Ground Plane, Dipole and 5/8 Wave Vertical, Slim Jim and J-Pole.
Directional antennas include inverted V for better impedance matching and less complex flying V antennas.
Do or buy?
In addition to the nifty information mentioned above, many of them include handy instructions, and there are some additional how-to readings below. Our friends at PCS Electronics created the PCS Electronics Homemade FM Broadcast Antenna Guide and the FM DX Antenna Co has a page on FM Transmission Tips. Bob Grove, publisher of the Surveillance Times magazine, has created a beautiful antenna book, your taxes paid to a consultant to draft Expedient AM for a radio station facing some kind of disaster and FM radio antenna.
If you can't get up to building an antenna and you want to click a buy now button, try just electronic's wide variety of FM low power broadcast antennas or visit FM DX Antennas Ltd and their tunable and fixed frequency 1/4 wave ground plane FM radio transmitter antenna.
When connecting your antennas, the two main issues are signal loss and matching. The importance of these is inversely proportional to the power output. Transmitters with a lot of RF output power do not suffer from signal loss, but matching is critical.
For a very low power transmitter the opposite is true - signal loss is far more important than matching.
Antenna Matching If matching is most important, matching will take place in two places. The first place to match is at the antenna connector. The output impedance of most FM transmitters with external antenna connectors is typically 50 ohms. The impedance of a typical configured FM transmit antenna will almost never reach 50 ohms (Oh#%*+!), so the first matching task is balanced operation.
Enter the balun (see what I'm doing there?). Balun comes from "BAlanced UNbalanced" and is an intermediary that provides the correct impedance for the transmitter and antenna. Baluns and other impedance matching techniques can be built into the antenna or added as a third element between the antenna and transmitter.
The second form of matching is to match the electrical length of the antenna to be resonant with the wavelength being transmitted. There are several online calculators linked above and math as this is a quick rundown here. Emulating the transmitter's energy in waves, if the antenna is resonant then the waves will provide the most energy - to us, which means more range.
For any transmitter with significant output power, an SWR meter will help make the ultimate, fine-tuned adjustment to your antenna components.
No antenna connector?
The last item that comes up every now and then - what about a transmitter without an external antenna connector? These devices are usually sold under Part 15 FCC certification and therefore do not provide any connectivity.
This is a favorite cheat - connect a 2-1/2 foot wire to the negative battery terminal or the power input connection. Making this unbalanced dipole with a near-resonant leg usually moves the range to one or so.
FM low power broadcasters have an easy time getting the antenna up and running, but AM or FM, low power or otherwise, watch out for overhead power lines, seriously, watch out for any power lines and other hazards erecting any kind of antenna Time.