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
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
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