 the exact size and shape of the rod. This means that the imped- Generic antenna types 17/7 can become zero in which case the dipole is easily matched to a coaxial cable of impedance 75ohms. The half wave dipole has a gain of 2.15dB. A monopole is a dipole divided in half at its centre feed point and fed against a ground plane, Figure 17.6. The ground plane acts as a mirror and consequently the image of the monopole appears below the ground. Since the fields extend over a hemisphere the power Image radiated and the radiation resistance is half that of the equivalent dipole with the same current. The gain of a monopole is twice that of a dipole. The radiation pattern above the ground plane is the same as that of the dipole. 17.3.3 Radiation from arrays Array antennas consist of a number of discrete elements which are usually small in size. Typical elements are horns, dipoles, and microstrip patches. The discrete sources radiate individually but the pattern of the array is largely determined by the relative amplitude and phase of the excitation currents on each element and the geome- tric spacing apart of the elements. The total radiation pattern is the multiplication of the pattern of an individual element and the pattern of the array assuming point sources, called the array factor. Array theory is largely concerned with synthesising an array factor to form Figure 17.6 Dipole and radiation pattern: (a) dipole; (b) a specified pattern. In communications most arrays are planar arrays monopole; (c) polar pattern of dipole with the elements being spaced over a plane, but the principles can be understood by considering an array of two elements with equal The radiation fields from a dipole are obtained by integrating the amplitudes, Figure 17.7(a). This has an array factor given by Equa- radiation from an infinitesimally small current element over the tion 17.12, where is given by Equation 17.13. length of the dipole. This depends on knowing the current distribu- tion which is a function not only of the length but also 