FM Half Wave Antenna
Location: Outside U.S.
Date: Winter 2009-2010
Why is an FM antenna half the wavelength?
Because the half-wave antenna is mounted on a ground plane
And the ground plane "reflects" the bottom half of an antenna to make it a
For instance, an FM radio wave is on average 3 meters long.
So on your car, your radio antenna is about 1/2 m long but it is installed
on the metal body of the car that serves as the ground plane.
Read about resonance on antennas at:
and here is another web site on radio signal propagation in general:
Keep asking questions, because that is the only way you find answers, and
that way you get a little bit smarter every day and when you get as old as I
am you know just about everything about everything :)
First, I should point out that not all "FM antennas" (that is, antennas that
operate at frequencies FM stations transmit on) are so-called half-
wavelength antennas. Secondly, many types of antennas operating at other
frequencies are also often half-wavelength antennas as well.
The reason why some antennas are designed to be half-wavelength in
length is because this is the minimum length a free-standing antenna can
be, in order to be electrically resonant. Designing the antenna length to be
electrically resonant at its transmitting or receiving frequency is
way to vastly improve its performance. Think of a guitar string,
fixed at both
ends. It vibrates as a half-wavelength, and will produce a sound only at one
frequency... its resonant frequency.
A free standing antenna will be resonant if its length is made to be
of half a wavelength. So antennas can be half a wavelength long, 1
wavelength long, 1.5 wavelengths long and so on. A half wavelength is most
often chosen because it is the shortest resonant length for a free-standing
Other common antenna lengths are based on the quarter-wavelength. A
free-standing quarter wavelength antenna is not self resonant. But, if it
stands upright with its bottom end attached to a "ground plane" (usually a
plate of metal), something interesting happens. The ground plane acts as
an electrical mirror, and produces an electrical reflection of the quarter
wavelength antenna -a sort of ghost image mirrored below the ground plane-
resulting in it performing like a full half-wavelength resonant antenna. Many
cordless phones use this principle, where the antenna sticking up is a
quarter wavelength, and the metal inside the phone acts as a crude ground
plane to produce an electrical image of the mission quarter wavelength.
All the above works for FM frequencies as well as all other frequencies too.
Note that at lower frequencies, such as with AM radio, the wavelength is so
long, that resonant lengths are not normally employed. For example, an AM
radio station transmitting at 1000kHz has a half-wavelength of 150 meters,
which makes a half-wavelength antenna rather impractical! So at lower
frequencies, resonant antenna lengths are not practical and other design
principles are used.
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Update: June 2012