00kHz 300KHz 10MHz 30MHz MICROWAVE MILLIMETRIC v.l.f. u.h.f. 30MHz 100MHz 300MHz 10GHz 30GHz 100GHz 300GHz VISIBLE LIGHT 300GHz 1 THz 10 THz 100THz 1000THz Figure 27.1 Radio frequency bands and designations 27/4 Radiowave propagation hxf=3x108 (27.1) v - E2 (27.6) Free space propagation If the dipole is connected to a feeder of matching impedance, E is halved. If polarisation and feeder losses are taken into account, the input to a receiver can be calculated. This is seldom a practical situation, but it occurs when both trans- mitting and receiving antennas are situated away from the influence A useful formula for calculating free space attenuation, derived of the earth's surface or other reflecting and absorbing objects, from the power density Equation 27.1 and from 27.6, and published including the transmitter and receiver themselves. If power is fed to in CCIR Report 252-2 is given by Equation 27.7, where Lfr is the an isotropic antenna, i.e. an antenna that radiates equally in all free space loss in decibels, f is the frequency in megahertz and d is distance in kilometres. directions in azimuth and elevation, the wave front will radiate outwards from the antenna in an ever expanding sphere at 3x108 metres per second (186,282 miles per second). The strength of the = 32.44 + log d (27.7) signal obviously decreased with distance as a given amount of power is spread over a greater area, and the incident power density It will be noted that Equation 27.7 introduces a frequency compo- at a remote point can be calculated as in Equation 27.2 (CCIR, nent into the calculation. 1978), where Pr is received power density in watts per meter2, Pt is As an example, the power density from a 500 watt transmitter transmitted power in watts and r is distance in metres. working into a lossless isotropic antenna at a distance of 1 kilometre is 0.039W3/m², and at 16 kilometres is This is a (27.2) power ratio of 24dB, which is to be expected as there is a 6dB power loss every time the distance is doubled. The CCIR formula (Equa- tion