A phased array uses two or more driven elements (radiators connected directly to the feed line) to produce gain in some directions at the expense of other directions.
At A, a unidirectional phased system. At B, a bidirectional phased system.
A typical end-fire array consists of two parallel λ/2 dipoles fed 90° out of phase and spaced λ/4 apart (above figure A). This produces a unidirectional radiation pattern. Or, the two elements can be driven in phase and spaced at a separation of λ (above figure B). This results in a bidirectional radiation pattern. In the phasing system, the branches of the transmission line must be cut to precisely the correct lengths, and the velocity factor of the line must be known and be taken into account.
A wire antenna measuring λ or more, and fed at a high-current point or at one end, is a longwire antenna. A longwire antenna offers gain over a half-wave dipole. As the wire is made longer, the main lobes get more nearly in line with the antenna, and their amplitudes increase. The gain is a function of the length of the antenna; the longer the wire, the greater the gain. A longwire antenna must be as straight as possible for proper operation.
A broadside array.The elements are all fed in phase.
Above figure shows the geometric arrangement of a broadside array. The driven elements can each consist of a single radiator, as shown in the figure, or they can consist of more complex antennas with directive properties. In any case, all the elements are identical. If a reflecting screen is placed behind the array of dipoles in above figure, the system is known as a billboard antenna. The directional properties depend on the number of elements, whether or not the elements have gain themselves, and on the spacing among the elements. In general, the larger the number of elements, the greater the forward gain, and the greater the f/b and f/s ratios.