Antennas can find use in systems that require narrow or large bandwidths depending on the intended application. Bandwidth is a measure of the frequency range over which a parameter, such as impedance, remains within a given tolerance.

Dipoles, for example, by their nature are very narrow band. For narrow-band antennas, the percent bandwidth can be written as

( fU − fL ) × 100/ fc

fL = lowest useable frequency
fU = highest useable frequency
fC = center design frequency
In the case of a broadband antenna it is more convenient to express bandwidth as

One can arbitrarily define an antenna to be broadband if the impedance, for instance, does not change significantly over one octave ( fU / fL = 2).

The design of a broadband antenna relies in part on the concept of a frequency-independent antenna. This is an idealized concept, but understanding of the theory can lead to practical applications. Broadband antennas are of the helical, biconical, spiral, and log-periodic types.

Frequency independent antenna concepts are discussed later in this chapter. Some newer concepts employing the idea of fractals are also discussed for a new class of wide band antennas.

Narrow-band antennas can be made to operate over several frequency bands by adding resonant circuits in series with the antenna wire. Such traps allow a dipole to be used at several spot frequencies, but the dipole still has a narrow band around the central operating frequency in each band.

Another technique for increasing the bandwidth of narrow-band antennas is to add parasitic elements, such as is done in the case of the open-sleeve antenna (Hall, 1992).

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