Communications, Radar & Electronic Warfare (201.. - Index of

Management of Interference at a Radio Site267WhereHI = Horizontal isolation in dB .VI = Vertical isolation in dB .SI = Slant isolation in dB.2(}SI(dB) = (VI-HI)· - +HIx = horizontal distance in metres.y = Vertical distance in metres.}, = wavelength in metres.e = tan- l ey/x).These values are suitable for conditions where x is greater than 10.?c and y is greaterthan },.An example is shown for a frequency of 450 MHz for two vertically spaced dipoleantennas.TIus can be illustrated by an example. Consider two vertically spaced antennas operatingwith an EIRP of 25 W, wIDch is 14 dBW (44 dBm). If the median faded signal power to bereceived is -110 dBm and the assumption is that the transnlit signal is not faded, then inorder to prevent de-sensitisation of the receiver, then the loss required from the transnlitterantenna to the receiver is 154 dB. Tills equates to more than 950 metres - unreasonablefor masts. The situation is different for antennas tuned to different frequencies. If theoffset channel rejection were, say, 45 dB, then the minimum equivalent isolation wouldbe 109 dB . The dipoles would only have to be spaced by at least 70 metres. The greater theisolation, the more closely the antennas can be spaced. TIus is illustrated in Figure 16.1.n140Dipole antenna vertical separation loss: 450 MHz120-- -100iii' 80:s.(fj(fj0 60....JI402: I0 50 100 150 200 250 300Vertical distance (m)Figure 16.1Vertical isolation for two typical dipole antennas.