View Full Version : Portable radio am/fm question
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?
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.
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:
ƒo = (2×pi×/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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