The length of an antenna determines the frequency that it best receives. The most efficient antennas have a length equal to half the wavelength of the wave it is receiving. This allows the induced electrical current in the receiving antenna to resonate at that particular frequency. If the antenna is a simple rod it is most sensitive when its length is one quarter the wavelength.

A loop or coil antenna are used for the low-frequency, long-wavelength signals in the AM band. A half-wavelength straight wire antenna would be over one hundred of meters long. Shorter wires or rods can be used and are more efficient if coils of wire are used to “load” the antenna.

Home FM radio and television antennas are designed to receive a broad range of frequencies, but with less sensitivity. Antennas for the ultra-high frequencies used in digital high definition televisions are very short and can be easily mounted outside on rooftops or on top of television sets.