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Name: Pinar
Status: Student
Grade: Other
Location: Outside U.S.
Country: Turkey
Date: Winter 2009-2010


Question:
Why is an FM antenna half the wavelength?



Replies:
Pinar

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 full-wave antenna. 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:

http://en.wikipedia.org/wiki/Antenna_%28radio%29

and here is another web site on radio signal propagation in general:

http://www.cybercollege.com/frtv/frtv017.htm

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 :)

Sincere regards,
Mike Stewart


Hi Pinar,

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 an important 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 multiples 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 antenna.

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.

Regards,

Bob Wilson



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