While standard FM antennas are designed for picking up stations operating at a particular frequency range, this isn't always sufficient for amateur "ham" radio broadcasting and reception that operates at different frequencies. Calculating the optimum antenna length is possible with a simple formula, though you must take into account the way the antenna is designed.
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Confirm the frequency range within which you want to operate. This will depend on your location, the allocated frequencies in your area (see Resources), and the terms of any license under which you are allowed to broadcast.
Calculate the middle of the frequency range within which you will operate. That is, take the upper frequency, subtract the lower frequency, and divide by two. The result is the megahertz figure you'll use to calculate antenna length.
Divide 468 by the megahertz frequency figure. The result is the total optimum antenna length in feet. (0.083 feet = one inch, 0.166 feet = two inches, and so on.)
Divide the optimum antenna length by two if you are using a dipole antenna, which simply means "two poles." The result is the optimum length for each of the two poles. (Alternatively, just divide 234 by the frequency.)