What is half-wave dipole antenna?

Most antennas can be analysed by considering them to be transmission lines whose configurations and physical dimensions have been altered to present easy energy transfer from transmission line to free space. In order to do this effectively, most antennas have physical sizes comparable to their operational wavelengths.

Figure 1.12(a) shows a two wire transmission line, open-circuited at one end and driven by a sinusoidal r.f. generator. Electromagnetic waves will propagate along the line until it reaches the open-circuit end of the line.

At the open-circuit end of the line, the wave will be reflected and travel back towards the sending end. The forward wave and the reflected wave then combine to form a voltage standing wave pattern on the line. The voltage is a maximum at the open end. At a distance of one quarter wavelength from the end, the voltage standing wave is at a minimum because the sending wave and the reflected wave oppose each other.

Suppose now that the wires are folded out from the λ/4 points, as in Figure 1.12(b). The resulting arrangement is called a half-wave dipole antenna. Earlier we said that the electromagnetic fields around the parallel conductors overlap and cancel outside the line.

However, the electromagnetic fields along the two (λ/4) arms of the dipole are now no longer parallel. Hence there is no cancellation of the fields. In fact, the two arms of the dipole now act in series and are additive.

They therefore reinforce each other. Near to the dipole the distribution of fields is complicated but at a distance of more than a few wavelengths electric and magnetic fields emerge in phase and at right angles to each other which propagate as an electromagnetic wave.

Besides being an effective radiator, the dipole antenna is widely used as a VHF and TV receiving antenna. It has a polar diagram which resembles a figure of eight. Maximum sensitivity occurs for a signal arriving broadside on to the antenna. In this direction the ‘gain’ of a dipole is 1.5 times that of an isotropic antenna.

An isotropic antenna is a theoretical antenna that radiates or receives signals uniformly in all directions. The gain is a minimum for signals arriving in the ‘end-fire’ direction. Gain decreases by 3 dB from its maximum value when the received signal is ±39° off the broadside direction.

The maximum gain is therefore 1.5 and the half-power beam-width is 78°. The input impedance of a half-wave dipole antenna is about 72 Ω. It turns out that the input impedance and the radiation resistance of a dipole antenna are about the same.

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