sitic elements close to the driven element lowers its impedance. The element spacing has a larger effect on the feed impedance than it does on the gain. In most designs a folded dipole is used and the spacing adjusted to give the optimum match. As there is only one driven element and the remainder are parasitic elements it is not possible to achieve complete control of the phasing and magnitude of the currents in all the elements. As such it is not possible to have complete control over the radiation pattern. In view of the large number of variables involved in the design, and the compromises that need to be made, computer programs are widely used in the design of these antennas. This enables their performance to be accurately assessed before any antennas are made. As a result there have been considerable performance improvements in Yagi antennas in recent years. Discone The discone is often used where a wideband receiving antenna is needed. For example, it is very popular with scanner enthusiasts, and it is also used in many commercial applications. It is almost omnidirectional and it can operate over a frequency range of up to 10:1 in certain instances. In addition to this it offers a low angle of radiation over most of its operating range. It also presents a relatively good match to the feeder over its operating range, although there is obviously some variation, particularly at the extremes of the range. The discone derives its name from its distinctive shape. Looking at Figure 4.25 it can be seen that the antenna basically consists of a disc section and a cone section that are simulated by a number of rods. The disc section is insulated from the cone by a block of material that also acts as a spacer keeping the two sections a fixed distance apart. In fact this distance is one of the factors that determines the overall frequency range of the antenna. When designing a discone the length of the cone elements should be a quarter wavelength at the minimum operating frequency. The disc elements are generally made t