an also travel over similar distances. As distant galaxies and quasars emit radio signals these can be detected by radio telescopes which can pick up the minute signals and then analyse them to give us further clues about what exists in the outer extremities of the universe. 26 Newnes Guide to Radio and Communications Technology The loss of a signal travelling in free space can easily be determined as the only two variables are the distance and the frequency in use. The distance is the straight line distance between the transmitter and receiver. The loss resulting from the frequency in use arises from the fact that at higher frequencies the antennas are smaller and hence the received signal is smaller. Loss (dB) = 32.45 + 20 log10 (frequency in MHz) + 20 log10 (distance in km) Using the figure for a loss in a system it is quite easy to calculate the receiver levels for a given transmitter power. Transmitter and receiver power levels should both be expressed in dBW (dB relative to one watt) or dBm (dB relative to a milliwatt). The antenna gain naturally has an effect and gain levels are expressed relative to an isotropic radiator, i.e. one that radiates equally in all directions. Further details of antenna gains and isotropic radiators are given in Chapter 4. Feeder losses should also be taken into account as these have an effect on the overall signal levels and may be significant in some instances. Pr(dBm) = Pt(dBm) + Gta(dB) - Ltf(dB) - Lpath + where P = received power level = transmitter power level = gain of the transmitter antenna = loss of the transmitter feeder Lpath path loss = receiver antenna gain = loss of the receiver feeder Refraction, reflection and diffraction In the same way that light waves can be reflected by a mirror, SO radio waves can also be reflected. When this occurs, the angle of incidence is equal to the angle of reflection for a conducting surface as would be expected for light. When a signal is reflected there is normally some loss of the signal, either through absorption, or as a resul