RF question on Am and FM antennas for a stereo

[rtarbell]

On the back of my stereo, there are jacks for an AM loop antenna, and an FM antenna.

Why is it that the FM antenna wire plugs into one jack on the back of the stereo, but both ends of the AM antenna wire need to be plugged in to two jacks on the back of the stereo?

[Kent Smith]

The answer is due to the different wavelengths of the bands.

FM, centered at 98 MHz, has a wavelength of about 12 ft. A simple 3 ft wire provides an optimal 1/4 wave antenna. When connected at one end, this antenna is a "monopole", and works against the ground area provided by the metal case and circuit board. (This makes it a complete dipole)

At around 1 MHz, The AM band has a wavelength of around 1200 ft. A good antenna would be overly large. A loop can solve this problem with the help of many turns of wire, and even including a ferrite rod to help increase inductance. To be a loop, the wire must be connected at both ends.

It is possible to connect a loop of wire as a monopole, but the impedance will be so low that it would not be very effective.

Have Fun,
Kent, the antenna guy

[AndrewT]

Hi Kent,
you should write us an aerial Wiki.

The FM antenna has a two pole connection, but it's wrapped up in a coax termination.