lar to the plane containing the driven elements. To achieve this all the elements must be fed in phase with one another. This is achieved using a phasing harness. In the simplest case the dipoles are spaced as shown. An electrical half wavelength of feeder is used between each of the dipoles and to keep the phase at the fed point the same a phase reversal is introduced. The directional properties of a broadside array depend on the number of elements used, and whether the elements themselves have gain. The Yagi is certainly the most popular form of directional antenna. More correctly named the Yagi-Uda after its Japanese inventors it was first outlined in a paper presented in 1928. Nowadays the antenna is widely 102 Newnes Guide to Radio and Communications Technology used where a directional array is required, being almost the only type of antenna used for UHF television reception. The basic antenna consists of a central boom with the elements mounted to it at right angles as shown. The antenna consists of the main driven element to which the feeder is connected, and parasitic elements either side. These parasitic elements are not directly connected to the feeder but operate by picking up and re-radiating power in such a phase that the directional properties of the antenna are altered. This is achieved having the phase of the current in the parasitic element or elements in such a phase that it reinforces the signal in a particular direction, or cancels it out. There are two main types of parasitic element: reflectors that reflect power back towards the driven element, and directors that increase the power levels in the direction of the directors. The properties of a parasitic element are determined by their spacing and their electrical length. When a parasitic element is made inductive the induced currents are in such a phase that they re-radiate power back towards the driven element, i.e. reflecting the power. To make an element into a reflector it is tuned slightly below resonance. This can be done by physically