re an- of length = This serves to illustrate many of the features of tennas. The parabolic reflector is widely used in communications 17/6 Antennas J1(X)/X SIN(X)/X Figure 17.5 Dipole and radiation pattern and is often fed by a conical horn. Both the reflector and the horn ance matching between the antenna and the transmitting or receiv- are circular apertures. For an aperture distribution which is inde- ing circuit becomes a major design constraint. pendent of azimuthal angle the simplest case is uniform illumina- tion which gives a radiated field as in Equation 17.8, where J (x) a Bessel function of zero order. Table 17.2 Radiation characteristics of circular apertures E p = 2 ( ru ) (17.8) Electric field aperture Level of distribution beamwidth first sidelobe sin (x) Uniform -17.6dB This can be compared to and is also plotted in Figure 17.5. The first sidelobe level is at -17.6dB. Table 17.2 lists a number of Taper to zero at edge -24.6dB circular aperture distributions and corresponding radiation pattern properties. The pedestal distribution is representative of many re- flector antennas which have an edge tapers of about - -10dB corre- sponding to E (a) = 0.316. The Gaussian distribution is also important because high performance feed horns ideally have Gaus- sian aperture distributions. The Fourier transform of a Gaussian taper which decreases to zero at the edge of the aperture gives a Taper on a pedestal Gaussian radiation pattern which has no sidelobes. -26.5dB 17.3.2 Radiation from small antennas 0.5 + 1 - (2n)2 Small antennas are needed for mobile communications operating at frequencies from HF to the low microwave region. Most of these Gaussian -40dB are derivatives of the simple dipole, Figure 17.6, which is an electric (p=3) current element which radiates from the currents flowing along a small metal rod. The radiation pattern is always very broad with exp -n(20)2 energy radiating in all directions. An important design parameter is the impedance of the dipole which can vary considerably depending on