ional. A small loop is least responsive along its axis, and most responsive in the plane perpendicular to the axis of the antenna. Loop antennas generally have several turns, and often a variable capacitor is added to resonate the antenna and improve its performance. This is normally done if the antenna forms part of a complete receiver, otherwise they are often operated in a non-resonant mode where they provide a wideband response that is fairly flat. 114 Newnes Guide to Radio and Communications Technology To radio Figure 4.33 A small loop antenna with capacitor resonance Large loops are classified as those with a circumference of greater than 0.1 wavelengths. Typically they have a circumference of half or a full wavelength and this makes them self-resonant without the need for external tuning. A half wave loop presents a high impedance to the feeder, but the full wave loop has a much lower impedance, of the order of 75 ohms. The maximum radiation occurs at right angles to the axis of the antenna. Active antenna In a number of receiving applications active antennas are used. As the name implies the antennas employ an active or amplifying element. They consist of an electrically short antenna element. An active amplifier at the base of the antenna element is used to amplify the signal and act as a matching circuit to ensure a good match to the feeder. This is normally achieved easily because a short antenna will present a high impedance and this can usually be accommodated very easily with a semiconductor amplifier. Power for the amplifier is normally supplied along the coax from a power supply at the receiver end of the feeder. The centre conductor of the coax allowing the supply to be passed to the amplifier, while also carrying the signal to the receiver. In this way it is possible to have the power source near the receiver without the need for additional wires to be routed to the antenna. Antenna systems 115 The design of active antennas is not easy. If they are to perform well they must be capable of good st