WHAT IS ANTENNA BANDWIDTH? THE PURPOSE OF ANTENNA BANDWIDTH?
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)/fc x 100
where
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
fU/fL
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 wideband 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).
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)/fc x 100
where
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
fU/fL
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 wideband 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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