Frequency modulation (FM) is a form of transmitting sound over a small or large distance. Most radio stations use an FM transmission to broadcast their programming to listeners. Short-distance or microbroadcasting FM transmission is used to allow personal MP3 devices to play music through a traditional radio. Calculating the general distance of an FM transmission is relatively easy to do if you know the wattage of the FM broadcast and the height of the antenna.
Calculate the FM transmission range of an omnidirectional antenna at 60 to 70 feet in height. A logarithmic formula is used to calculate the loss of a signal strength over a set distance. The basic formula is 10n * log 10 (d) + C, where d is the distance of the transmission, n is the path loss exponent and C is a constant. For a broadcast of 6 watts, the transmission range will be 3 miles. At 15 watts, the range increases to 5 miles, at 40 watts to 10 miles and at 100 watts to 15 miles. The general rule of thumb is it will take four times the power to double the transmission distance.
Increase the transmission range of the FM antenna without increasing the power by raising the antenna. Raise the height 10 to 15 yards to significantly increase your broadcast distance. The higher you are able to mount your antenna, the further it will reach.
Take into account the terrain, interference and the type of FM receiver being used. On flat terrain, your transmission will reach farther, as there will be less obstructions. Mountainous terrain can severely limit your broadcast range. If you are broadcasting in or near a big city, interference from more powerful FM transmitters could limit how far your transmission will reach. An FM receiver that uses an outside antenna as opposed to an indoor antenna will have an easier time picking up your broadcast signal.