e arrays, slots Element and Array Mobile Communications (base Dipole arrays Element and Array stations) Mobile Communications (mobile) Monopoles, microstrip Element HF Communications Dipoles, monopoles Element The following sections describe first generic antenna characteristics, then the specific antenna types, and then, briefly the practical implementation of antennas in communica- tions systems. Basic Properties The principle of reciprocity is one of the most important properties of an antenna. It means that the properties of an antenna when acting as a transmitter are identical to the properties of the same antenna when acting as a receiver. For this to apply, the medium between the two antennas must be linear, passive, and isotropic, which is always the case for communi- cations systems. The directional selectivity of an antenna is represented by the radiation pattern. It is a plot of the relative strength of the radiated field as a function of the angle. A pattern taken along the principal direction of the electric field is called an E-plane cut; the orthogonal plane is called an H-plane cut. The most common plot is the rectangular decibel plot (Figure 9.3), which can have scales of relative power and angle chosen to suit the antenna being charac- terized. Other types of plots, such as polar (used for small and two-dimensional antennas), contour (or three-dimensional), and isometric are also used. A radiation pattern is character- ized by the main beam and sidelobes. The quality is specified by the beamwidth between the -3-dB points on the main beam and the sidelobe level. Communication antennas radiate in either linear polarization or circular polarization. In modern communications, cross-polarization is important. This is the difference between the two principal plane patterns and is specified relative to a reference polarization, called the copolar pattern. There are three definitions of cross-polarization. The one in normal use with reflector antennas and feed systems is Ludwig's third definition (Ludwi