ired to be smaller. There are a number of ways of reducing the physical size of an antenna. One method is to make the antenna electrically the right length even 110 Newnes Guide to Radio and Communications Technology Non-conducting former Antenna (a) Centre (b) Base (c) Helical loaded loaded Figure 4.29 Adding inductive loading to an antenna though it is physically small. It is found that when an antenna is too short for its resonant frequency it becomes capacitive. The capacitive compo- nent can be removed by adding inductance to the antenna to bring it to resonance. This can be achieved by adding a coil to a section of the antenna as shown in Figure 4.29, or by making what is called a helical antenna and distributing the inductance over the whole length. When a single coil is used to give the inductance, this is either placed in the centre as shown or at the base. Another alternative is to make the antenna electrically short and feed it accordingly. If this method is adopted arrangements need to be made to match the impedance of the antenna to the feeder, as the two will not be matched. Parabolic reflector One type of antenna which has been seen far more in recent years is the parabolic reflector or 'dish'. It is widely used for frequencies above 1 GHz where very high levels of gain can be achieved. Initially these antennas Antenna systems 111 Reflector follows a parabolic curve Radiating or receiving element at the focus of the curve Figure 4.30 A parabolic reflector were only used for professional applications, especially radio astronomy or satellite communications. However, with the advent of satellite television these antennas are often seen on the sides of houses for reception of these broadcasts. The antenna consists of a radiating element that may be a simple dipole or a waveguide horn antenna. This is placed at the focal point of the parabolic reflecting surface. The energy from the radiating element is arranged SO that it illuminates the reflecting surface. Once the energy is reflected it leaves the an