Site analytics indicate many are searching for FM antenna information. Here is a compendium of how to make or where to buy FM transmitting antennas for low power radio. After identifying an open frequency, perhaps using these handy FM Frequency Survey Log Sheets and setting the transmitter for the chosen spot on the dial, the next step is to make an effective connection with potential listeners with a good FM broadcast antenna.
The good news is that there is nothing unique about transmitting in the FM broadcast band. From a radio theory perspective, the goal is to radiate a signal in VHF frequencies to where listeners are. All of the great research, calculations and configurations developed by ham radio operators apply to the FM broadcast band in the 87.5 MHz - 107.9 MHz range.
The most common low power FM broadcast antenna configurations are either omni-directional or somewhat directional.
Omni-directional include 1/4 wave whip, 1/4 wave ground plane, Dipole and 5/8 wave vertical, the Slim Jim and the J-Pole.
Directional antennas include the better-impedance-matching Inverted V and less complicated Flying V antenna.
Make or buy?
Along with the pithy info noted above, much of which include handy instructables, some additional how-to readings follow below. Our friends at FM DX Antenna Co offer an FM transmission tips page. Bob Grove, publisher of Monitoring Times magazine, created a nice Antenna Book and your tax dollars paid a consultant to draft Expedient AM and FM Broadcast Antennas for radio stations facing some kind of disaster.
If you're not up to building an antenna and you'd like to click a buy now button, visit FM DX Antenna Co. and their tunable and fixed-frequency 1/4-wave ground plane FM broadcast transmitter antennas.Connecting your Antenna
Two primary concerns when connecting your antenna are signal loss and matching. The importance of these varies inversely related to power output. A transmitter with lots of RF output power will not suffer from signal loss but matching will be critical.
For a very low power transmitter the opposite is true - signal loss is vastly more important than matching.
Antenna Matching
If matching is most important then matching will happen in two places. The first place to match is at the antenna connector. The output impedance of most FM transmitters with an external antenna connector is most often 50 Ohms. The impedance of a typical configuration FM transmitting antenna is almost never 50 Ohms (Oh #%*+!), so this first matching task is a balancing act.
Enter the balun (see what I did there?). Balun comes from "BALanced UNbalanced" and serves an intermediary that presents the correct impedance to the transmitter and antenna. Baluns and other impedance matching techniques can be either built in to 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 the be resonant with the wavelength being transmitted. There are several online calculators in the links noted above and a quick rundown of the math for that is here. Analog transmitter energy travels in waves and if the antenna is resonant then the waves will deliver the most energy - to us that means more range. For transmitters of any significant power output, an SWR meter will be helpful in making final, fine tuning adjustments to your antenna assembly.
No Antenna Connector?
One last item that comes up from time to time - what about transmitters that don't have an external antenna connector? These units are often sold as FCC certified for Part 15 and as such, don't provide any connection.
Here is a favorite cheat - connect 2-1/2 feet of wire to the negative battery terminal or power input connection. Making this unbalanced dipole with one approximately resonant leg that will usually move the range out to a block or so. For more information of hacking those little Part 15 units, please see an earlier post entitled The Mod Squad.
FM low power broadcasters have a little easier time getting an antenna up and running but AM or FM, low power or otherwise, WATCH OUT FOR OVERHEAD POWER LINES, seriously, PLEASE WATCH OUT FOR ANY POWER LINES and other hazards when erecting any kind of antenna - I need every blog visitor I have!
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