the noise level at the receive because of the large number of cancellations required, rather than site. The frequency of optimum transmission (FOT) is the frequency 27/10 Radiowave propagation with maximum availability and minimum path loss, and is generally the transmitting antenna, and r is the distance in metres between taken as 90% of the MUF. antennas. H.F. groundwave and skywave communications are still widely used for long distance low capacity services such as aeronautical ground to air transmissions, despite the increasing availability of P, = P,G,G,A (27.12) satellite services. Defence forces are a major user, but the effects of the electromagnetic pulse (EMP) released by a nuclear explosion on the ionosphere and on equipment require special consideration in For systems calculations, the formula can be expanded to include military systems. Much of the information on this topic is classified, transmit and receive antenna VSWR and polarisation mismatch as but some papers are available. in Equation 27.13, where P, is the magnitude of the voltage reflec- tion coefficient at the receive antenna, P, is the magnitude of the voltage reflection coefficient at the transmit antenna, and p is the 27.7 Terrestrial line of site propagation polarisation mismatch. In the v.h.f., u.h.f. and microwave bands, the ionosphere has a (27.13) minimal effect on propagation although anomalous conditions such as sporadic E propagation do affect the lower frequencies in this range. The frequencies are generally well above the critical fre- quency so that transmissions pass through the ionised layers and out Voltage reflection coefficient can be calculated from the VSWR into space. This offers the considerable benefit that the same fre- as in Equation 27.14. quencies can be used and re-used many times without causing mutual interference, provided sensible frequency planning is car- ried out and adequate physical separation is provided. p = VSWR - VSWR (27.14) Communications link calculations can be carried out using the Friis power