View Full Version : Portable radio am/fm question

Cat Patrol
08-19-2002, 08:31 PM
Why do portable radios have the AM antenna on the inside of the radio, instead of on the outside like the FM antenna?

08-19-2002, 08:44 PM
The usual method of construction for an AM antenna is to wind small wire around a powdered-iron rod (also called a ferrite rod). Otherwise, the antenna would be too long for practical purposes. The ferrite rod is fragile, and is mounted inside the case for it's own protection.

Patty O'Furniture
08-19-2002, 09:20 PM
The AM antenna is of such construction as described by n5tp because of the much lower band of frequencies for which it needs to be receptive. AM broadcast is 535KHz-1605KHz, compared to the FM band at 88MHz-108MHz. Now look at the formula for resonant frequency of an LC (inductve-capacitive) circuit, in which the AM antenna coil of wire acts as L:

o = (2pi/LC)-1

That's supposed to be the square root of LC but the symbol font seems to be hosed up for some reason. You can see that the values of L and C must be larger at lower frequencies. A coil of wire wrapped around a ferrite core has much greater inductance than a straight telescoping rod, and therefore works better at lower frequencies.

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