# Antennas

The basic antenna is of length and termed a dipole.

When resonant at the design frequency there are voltage and current standing waves. The voltage is high at each end and low at the centre; the current is high at the centre and low at each end. Since impedance is the ratio voltage/current, it will vary along the length of the antenna but be low (about 70) at the centre and high (about 5- 6k) at each end. The electrical length is determined by the relationship .

In practice the length should be about 5% shorter than this. The dipole can be successfully used on odd harmonics of the design frequency.

To improve the directional properties of an antenna additional elements can be added. A reflector and director give a basic beam, but additional directors will improve still further the directional properties and forward gain. The addition of these parasitic elements will reduce the input impedance but there are several methods available to solve this problem. A reflector is normally about 5% longer than the radiating element, and a single director about 5% shorter.

Consideration of the voltage/current standing wave ratio will indicate that an end-fed vertical antenna can be used. Radiation will be omnidirectional but of low angle. Of the loop antennas the cubical quad is perhaps the most popular. The feeder supplies a square of side

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