 Equation 17.16 applies. (17.16) Figure 17.8 Geometry of reflector antenna The depth of the paraboloid is usually specified by its F/D ratio. Common sizes are between F/D = 0.25 (  = 90 degrees) to F/D = Y = d+kd cos 8 (17.13) The pattern for small spacings will be almost omnidirectional and 17.4.1.2 Aperture fields and radiation patterns as the spacing is increased the pattern develops a maxima perpen- dicular to the axis of the array. At a spacing of half a wavelength, a null appears along the array axis, Figure 17.7(h). This is called a Ray optics indicates that the path length from F to P to A is equal to broadside array. If a phase difference of 180 degrees exists between twice the focal length. Hence the phase across the aperture is constant. The amplitude across the aperture plane will peak at the the two elements then the pattern shown in Figure 17.7(c) results. centre and taper towards the edge for two reasons. Firstly because Now the main beam is along the direction of the array and the array the feed will have a tapered radiation pattern and secondly because is called the end-fire array. This illustrates one of the prime advant- the action of a parabola in transforming a spherical wave from the ages of the array, namely by changing the electrical phase it is possible to make the peak beam direction occur in any angular feed into a plane wave across the aperture introduces a path loss which is a function of angle 0. The aperture electric field is then direction. Increasing the spacing above half a wavelength results in given by Equation 17.17, where F( 0,0 ) is the pattern of the feed. the appearance of additional radiation lobes which are generally undesirable. Consequently the ideal arrays spacing is half wave- (17.17) length, though if waveguides or horns are used this is not usually possible because the basic element is greater than half a wavelength in size. Changing the relative amplitudes, phases and spacings can Feeds suitable for reflector antennas are discussed in a later produce a wide variet