BANDWIDTH OF LINEAR ANTENNAS

The bandwidth of an antenna is the frequency range over which the performance of the antennas meets a specific re­quirement, such as the gain does not fall below 3 dB of the maximum, or the mid-frequency value. There is no unique characterization of bandwidth since the different properties such as input impedance, radiation pattern, polarization and gain of an antenna vary in entirely different ways in a fre­quency range. The definition meets the requirement of spe­cific application. Nevertheless, the bandwidth is defined in three different ways: (1) Half power bandwidth is the fre­quency range within which the gain does not fall by more than 3 dB. (2) The percentage bandwidth normally defined for narrow band antenna is defined as the bandwidth (the difference between upper and lower frequencies of operation) divided by the center frequency and then multiplied by 100, and (3) for wideband or frequency independent antennas it is the ratio of higher and lower frequencies of operation.

The thin linear antenna that we have dealt with in this article is based on the assumption that the radius is small compared with the wavelength of operation. This necessarily means that the current on the wire is linear and has no tan­gential component. In a practical situation an antenna has to operate in a frequency band. This assumption is no longer valid and the thin linear antenna becomes frequency sensi­tive. Therefore, something needs to be done to the thin an­tenna so that it develops the capability of handling a wide frequency band.

The ways of achieving wider bandwidth will be to use elec­trically thick dipoles or to coat thin metallic wires with lossy

BANDWIDTH OF LINEAR ANTENNAS

E

J=

О

Dipole length l4

(a)

terial. They are given by

dielectrics. A thick dipole and a dielectric coated dipole with their centrally located feeding source are shown in Fig. 20(a) and (b), respectively. The effect of coating the thin linear an­tenna with a layer of electrically and magnetically lossless and lossy material is discussed in two papers in (28) and (29) and summarized in (1). The analytical technique used is a moment method solution. Two parameters P and Q (28,29) involving the electrical and magnetic parameters, inner and outer radii, are found to be of interest in moment method so­lutions and help in designing antenna characteristics using coating of electrically and magnetically lossless and lossy ma-

Q = (Mr — 1) In

(63b)

where a = radius of the thin wire, b — a = thickness of the coating, er = the relative permittivity of the coating or the medium in which the thin wire is embedded, = the relative permeability of the coating or the medium in which the thin wire is embedded.

BANDWIDTH OF LINEAR ANTENNAS

Dipole length L4

Figure 20. Comparison of measured and calculated admittance for a coated dipole of length L = 8 in.; 2a = 0.025 in.; 2b = 0.146 in.; є2 = 2.3e0. The figures show the bandwidth of antennas: (a) conduc­tance and (b) susceptance of an insulated dipole vs. length in wave­lengths.

It turns out that the effects of increasing the real part of both P and Q are to increase the peak value of input imped­ance, to increase the electrical length which lowers the reso­nant frequency and to reduce the bandwidth of operation. The effects of increasing the imaginary part of P and Q are to decrease the peak value of input impedance, to decrease the electrical length which increases the resonant frequency, and to increase the bandwidth. This means a proper choice of a lossy dielectric with maximum imaginary parts of P and Q and minimum real parts of P and Q can achieve an optimum bandwidth. But this will be at the cost of antenna efficiency because of the lossy coating.

Updated: 02.04.2014 — 02:33