 adding a coil to provide the additional inductance, or more usually by making it physically slightly longer. Typically reflectors are made about 5 per cent Reflector Driven element (about 5% longer Directors than driven element) (about 5% shorter than driven element) maximum radiation feeder polar diagram Figure 4.23 A Yagi antenna Antenna systems 103 longer than the driven elements. Conversely directors are made by tuning the element above resonance. This can be done by adding some capacitance into the element, or making it slightly shorter than the driven element, again by about 5 per cent. Only one reflector is used, as the addition of further reflectors behind the main one adds very little to the performance. Further directors do give increased directivity, and it is not uncommon to see ten or more directors on an antenna. Normally the number is limited by size, cost and the required amount of gain. Typically a two element design consisting of a driven element and reflector or director will give around 5 dB gain over a dipole and a three element antenna with a director as well gives around 7 dB. Additional directors give less gain, starting at around 2 dB for the first one or two; however, as a rough guide each additional director gives around 1 dB. For example, a typical 11 element array is likely to have a gain of around 13 dB and a 12 element version, around 14 dB. It is also found that the element spacing has a small effect on the gain. Usually the spacing is between 0.1 and 0.3 wavelengths. The polar diagram of a Yagi is generally like that shown in Figure 4.23. As more elements are added the beamwidth becomes much narrower. This makes positioning the antenna more critical, a point to be remembered when choosing the required gain for an antenna. The feed impedance is altered by the parasitic elements. In the same way that the presence of the earth lowers the feed impedance of the Figure 4.24 A Yagi used for television reception 104 Newnes Guide to Radio and Communications Technology dipole, placing para