n good antenna systems will enable a receiver to pick up stations at good strength which are totally inaudible on a receiver with a poor antenna. 78 Newnes Guide to Radio and Communications Technology Polarization It has been mentioned (in Chapter 2) that electromagnetic waves have a certain polarization. This is important because antennas are sensitive to polarization. Most antennas are linearly polarized and it is found that antennas in which the element or elements are vertical are sensitive to vertically polarized signals, and antennas with horizontal elements are sensitive to horizontally polarized signals. For optimum reception the antenna should be polarized in the same plane as the signal to be received. If it is polarized in a different plane then the received signal level is reduced. If it is at right angles (i.e. 90 degrees) to the received signal then in theory no signal is received. Similarly when transmitting the antenna will radiate a signal with a polarization in the same plane as the elements. Obviously antennas can be developed for elliptical or circular polarization. It is found that some multi-element antennas such as the Yagi which is described later have complicated radiation patterns. They may have side lobes that have signals with a different polarization to that of the main beam. These lobes may be linearly polarized, or even elliptically polarized, and as a result signals with a different polarization may be received at a greater strength when the antenna is aligned away from the source. Impedance Any antenna will present a certain impedance at the point at which it is fed. This is often called the feed impedance. The impedance of an antenna results from many factors including the size and shape of the antenna, the operating frequency and its environment. The impedance is normally complex, consisting of inductive and capacitive elements as well as a resistive one. The resistance emanates from two sources. One is the resistance of the conducting element of the antenna. This is known as th