FM radio signals are line-of-sight. The FM radio signal does not follow the curvature of the earth, as does the AM radio wave. Early FM stations broadcast in either a horizontal wave or a vertical wave. Current FM broadcasters use a circular polarized antenna configuration that broadcasts both the horizontal and vertical components of the FM radio signal. The original folded dipole FM receiving antenna may not produce the desired results for receiving the modern FM station.
How FM Works
Check your local FM radio stations. Because the signal does not follow the earth's curvature, the Height Above Average Terrain (HAAT) and Effective Radiated Power (ERP) of the station determines the coverage area. You can easily check the coverage area of your local stations with the online radio station locator.
FM Coverage Area
Look at the colors of the coverage area. The red circle shows you the station's primary coverage area. If you live within this area, you will receive the strongest signal and your antenna needs will be minimal. The next circle is purple. This is the distant signal area. A good car radio should provide reception in this area but the signal will probably be weak inside home or office. In this area, you may need a better antenna. The largest circle is blue in color. This is the fringe area. Your car radio should receive a signal but it may drop out behind hills or buildings. If you live in this area you will need a better antenna.
FM Recption Problems
Check the terrain between your location and the FM radio station. This is the second major consideration. Most FM stations transmitters are located as high on a hill or mountain as possible. In level areas, they depend on a large tower to raise their antenna to maximum height. If you have hills, buildings or other obstacles between you and the station your reception will have disruptions and you will need to make an antenna.
What Kind of Antenna Will I Need
In the primary or local coverage area, the antenna that came with your FM radio may provide adequate signal strength. If you have hills or buildings blocking the signal, you will need to make an antenna. In the distant coverage area, a folded dipole antenna which most manufacturers provide with your new FM radio may provide adequate signal strength. You may have to move it to various locations and turn it in different directions to find the strongest signal. A better antenna will be needed to overcome signal obstructions.
Guidelines for Making The Antenna
In the fringe area, you will have to make a better antenna for good reception.To determine the exact length and configuration for an FM radio station antenna, follow the mathematical formula placed on line by WRYR low power FM. A guide to the construction of a practical antenna can be found on the WERU FM radio website. Use this strategy to determine what type of antenna you will need to make to enhance your FM reception. The websites listed in the references will give detailed instructions on make it.
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