FM Transmission Tips
The FM antenna system is the most important part of your FM broadcast setup. This includes the FM antenna, antenna mount, and FM transmission coax. It is very common for much planning and resources to go into the FM transmitter and FM broadcast studio but not much in the FM transmitter antenna. The FM transmitter and FM studio will suffer greatly if the FM broadcast transmitter antenna system is not up to par. Currently two FM transmitter antennas offer the very best money spent on the FM transmitter antenna part. One is the quarter-wave FM transmitter antenna and the other is the tunable FM quarter-wave transmitter antenna. The only real difference in the two FM antennas is the non-tunable quarter-wave FM broadcast transmitter antenna handles 100 watts and works only on a small segment of the FM broadcast band. Choose this FM transmitter antenna for a low-power transmitter that only works on a small portion of the FM broadcast band or on only one FM frequency. The tunable quarter-wave FM broadcast transmitter antenna will handle up to 150 watts and is freely tunable across the entire FM band.
is the most important factor for range in the FM Band. Without question it is the most cost-effective and important way to get your FM signal out. At minimum, get the FM broadcast antenna above the roofline of the closest building and even better above the tree-line. If using inside a theater, try to get it up above the backstage lights, though mounting it on a light stand should be more than adequate. If using at a ballpark, try to get it mounted above the press box to completely cover the stadium and parking lot. Any FM transmitter antenna will work best when mounted away from any other objects such as trees or buildings and as high as possible.
Coaxial Cable can easily make or break FM antenna system. FM signal can actually be lost considerably if you use the wrong coax. Use the largest coax you can and make it the shortest run. Andrew Hard-line has very little loss at 100 MHZ with a 100’ run. That cable is very difficult to deal with and probably more than you need, but it is an option. However, Andrew LDF-50A or B is a flexible hard-line and commonly available. LMR400 or LMR-400 is another highly recommended cable. If your run gets over about 25 feet, please consider using the larger coax that is mentioned. Less than 25 feet is not as critical though don’t use anything smaller than RG213 but for the very shortest run (<10’). It is possible to use a large cable for the long part of the coax run and then make a short run of smaller cable to make it easy to use with your FM transmitter. One easy way to make the coax run shorter is to use a long audio cable from your audio source to the transmitter and place the transmitter as close as possible to the antenna. Audio cable runs experience little, if any, loss of signal over long runs whereas the antenna coax is the weak point in the chain.
your FM broadcast antenna to a pole. Radio Shack offers 3 mounting poles that are very reasonably priced and work great. RS also offers mounting hardware. For more portable operations a DJ light stand or microphone stand can be a great way to get your antenna up easily. They are commonly as tall as 13 feet and run for around $50 or even much less for shorter ones. A windsock pole is another great way to mount your FM transmitter antenna. They get pretty tall though a 22 foot one is light enough and tall enough to give great results in a portable or movable setup. Indoors, a microphone stand is perfect as they go as tall as 5 feet or slightly more and cost less than $20.
FM Antenna Tuning will complete preparation for your FM broadcast.
Your FM broadcast transmitter antenna package from FM DX Antenna Company includes a chart to show you what length to adjust the vertical element of your FM antenna. This is a simple way to tune your FM broadcast antenna, but for the best tuning you need an SWR meter. You will need to borrow or buy one that covers the FM broadcast band. First adjust the vertical element of the FM transmitter antenna to the desired frequency and then use the SWR meter to tune the FM antenna precisely for the FM frequency you choose to transmit. This allows the entire FM transmission system to function at peak with less heat and even some additional gain plus other advantages.
Consider and plan your FM antenna transmission system carefully as it will be the difference between a wimpy FM signal and a clear solid FM broadcast.