to RF systems. This permits better identification of multiple targets and, also, gives improved directional accuracy. Also, the antennas are small so they can be assembled into compact arrays and, in addition, they can achieve well defined beam patterns which can provide the narrow beams with high gain favoured by radars and Data links. Other advantages of the newly available microwave band were the absence of fading (often a problem in the Shortwave radio (SW) band) and great increase in signal spectrum, compared to the congested RF bands already in use. In addition to being able to accommodate many more signals, the ability to use Spread spectrum and frequency hopping techniques now became possible. Once microwave techniques had become established, there was rapid expansion into the band by both military and commercial users. Antennas for DF Antennas for DF have to meet different requirements from those for a radar or communication link, where an antenna with a narrow beam and high gain is usually an advantage. However, when carrying out direction finding, the bearing of the source may be unknown, so antennas with wide beamwidths are usually chosen, even though they have lower antenna boresight gain. In addition, the antennas are required to cover a wide band of frequencies. The figure shows the normalized polar plot of a typical antenna gain characteristic, in the horizontal plane. The half-power beamwidth of the main beam is 2 × Ψ0. Preferably, when using amplitude comparison methods for direction finding, the main lobe should approximate to a Gaussian characteristic. Although the figure also shows the presence of sidelobes, these are not a major concern when antennas are used in a DF array. Typically, the boresight gain of an antenna is related to the beam width. For a rectangular horn, Gain ≈ 30000/BWh.BWv, where BWh and BWv are the horizontal and vertical antenna beamwidths, respectively, in degrees. For a circular aperture, with beamwidth BWc, it is Gain ≈ 30000/BWc2. Two antenna types, popular for DF, are cavity-backed spirals and horn antennas. Spiral antennas are capable of very wide bandwidths and have a nominal half-power beamwidth of about 70deg, making them very suitable for antenna arrays containing 4, 5 or 6 antennas. For larger arrays, needing narrower beamwidths, horns may be used. The bandwidths of horn antennas may be increased by using double-ridged waveguide feeds and by using horns with internal ridges. Microwave receivers Early receivers