o have an overall length of 0.7 of a quarter wavelength. The diameter of the top of the cone is mainly dependent upon the diameter of the coaxial cable being used. The spacing between the cone and the disc should be about a quarter of the inner diameter of the cone. Making the minimum diameter of the cone small will increase the upper frequency limit of the antenna. Operation of the antenna is fairly complicated but it can be visualized in a simplified manner. The elements that form the two sections of the antenna electrically simulate a complete disc and cone from which the energy is radiated. Naturally the more elements that are used the better the simulation of the cone, but typically a good balance between cost and Antenna systems 105 Figure 4.25 A discone antenna electrical performance is achieved when six or eight elements are used. Addition of further elements would also add to the wind resistance of the antenna. In operation the energy from the feeder spreads out over the surface of the cone towards the base of the antenna until a point on the cone is reached where the vertical distance between the point on the cone and the disc is a quarter wavelength. At this point 'resonance' is seen and the energy is radiated. The radiated signal is vertically polarized and the radiation pattern is very similar to that of a vertical dipole. Although some variation is seen over the operating band particularly at the top, it maintains a very good low angle of radiation over most of the range. It is found that the current maximum is at the top of the antenna and that below the minimum frequency the antenna presents a very bad mismatch to the feeder. However, once the frequency rises to within the operating band a good match to 50 ohm coax is maintained over virtually the whole of the operating range. Log periodic The log periodic antenna is a wideband antenna that is directional and capable of providing significant levels of gain and operating over a 106 Newnes Guide to Radio and Communications Technology frequency range