How to Split an FM Signal From the Antenna to Receiversby James T Wood
Radio signals from an FM antenna can be split to go to more than one receiver with the use of a coaxial cable splitter. The FM radio spectrum, from 88 to 108MHz, falls between channels 6 and 7 on the television dial, so television cables and even TV antennas can be used to receive FM stations.
Divide and Conquer
Antennas function by creating an electrical signal in response to electromagnetic radiation. The antenna wire transfers that small electrical current to the receiver, which then decodes it. Since the electrical current is relatively weak, splitting it takes nothing more than using a coaxial signal splitter -- a device that can divide the signal into two or more channels. The best results come from having a strong signal and using high-quality components for your cable run, splitters and connectors. The RG-6 coaxial cable is well shielded and will maintain the signal longer over distance and in the face of interference.
You Win Some; You Lose Some
The addition of a signal splitter to an antenna line will result in about a 3-decibel loss of signal from the input to the output. The strength of a radio signal is measured in decibels similar to the way audio volume is measured, since it is a relative scale that relates to the signal received against the background noise surrounding it. A strong signal losing 3 dB of strength won't have much of an effect, but if you are between 25 and 40 miles from the transmitter you may lose signal quality after splitting.
Pump up the Jam
If your signal isn't quite what you wanted it to be, you can use a signal booster to add energy to the signal. A booster reads the incoming, weak signal and amplifies the power to it so that it is a stronger signal at the output. However, if your signal woes are due to too much FM radio, but from other channels, you can use a directional antenna to focus in on the station you want to hear. In cities with tightly packed radio signals the interference from other stations might overwhelm a weaker station if you're using an omni-directional antenna. Switch to a directional antenna and point it at the tower of the station you're interested in. Mounting your antenna on a rotor will allow you to tune in to different stations using your directional rig.
Splitting up is Hard to Do
The point where you split the signal matters most on long cable runs -- of 100 feet or more -- when a power amplifier is used and when there is an outdoor antenna. For outdoor antennas, they must be grounded against lightning strikes and the grounding also helps to reduce some of the extra static. The split must happen after the ground. If you're using an amplifier to boost the signal, you want the amplifier to be in the line before your splitter so both your receivers get the same, increased, signal strength. For long cable runs and especially if you're trying to listen to a weak signal, the amplifier should be as close to the antenna as possible with the split as close to the receivers as possible. At 100MHz -- roughly the middle of the FM range -- RG-6 coaxial cable will lose 2 dB of signal strength for every 100 feet. So the longer your cable run and the more splits you have, the more likely you will be to need an amplifier to boost the strength.