The wavelength of frequencies in the frequency modulated (FM) band (87MHz to 108MHz) are long enough --- about 9 feet to 11 feet --- that full-length FM dipoles aren't practical. In these cases, a half-wave or quarter-wave antenna is a better choice.
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How Antennas Work
Antennas convert radio waves to electricity and vice-versa --- when you're receiving a signal, radio waves in the atmosphere are converted to electricity your radio translates into sound. Antennas work best when they're the same length as the peaks and troughs of the radio wave (also called the wavelength), but they also operate at resonant frequencies of one-half and one-quarter the length of the wave.
A dipole antenna consists of two conductive elements --- like the rabbit ear antennas on old televisions. Because FM waves are about 10 feet long, a half-wave FM dipole will contain about 5 feet of wire. Half-wave dipoles are a good compromise between the space needed by a full-length antenna and the weaker performance of a quarter-wave FM antenna. If space is at a premium, however, even a half-wave antenna may be too large.
Quarter-wave FM dipoles can be half as short as half-waves, meaning antenna lengths as short as a couple of feet. Note that they must also be grounded --- a quarter-wave FM antenna uses the ground to mirror the missing half of the dipole. For home antennas, the ground is the Earth; portable antennas, such as those in cars, may take advantage of metal parts in the automobile to serve as the ground.
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