the increas- ing availability of satellite services. Defense 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 military systems. Much of the information on this topic is classified, but some papers are available. Terrestrial Line-of-Site Propagation In the VHF, UHF, and microwave bands, the ionosphere has a minimal effect on propaga- tion, although anomalous conditions, such as sporadic E propagation, do affect the lower frequencies in this range. The frequencies are generally well above the critical frequency SO that transmissions pass through the ionized layers and out into space. This offers the con- siderable benefit that the same frequencies can be used and reused many times without causing mutual interference, provided sensible frequency planning is carried out and ade- quate physical separation is provided. Communications link calculations can be carried out using the Friis power transmission formula as in equation 8.12, where Pr is received power in watts, Pt is transmitted power in watts, Gt is the gain of the transmitting antenna in the direction of the receiving antenna, Gr is the gain of the receiving antenna in the direction of the transmitting antenna, and r is the distance in meters between antennas. (8.12) For systems calculations, the formula can be expanded to include transmit and receive antenna VSWR and polarization mismatch, as in equation 8.13, where Pr is the magnitude of the voltage reflection coefficient at the receive antenna, Pt is the magnitude of the voltage reflection coefficient at the transmit antenna, and p is the polarization mismatch. (8.13) The voltage reflection coefficient can be calculated from the VSWR as in equation 8.14. p=VSWR-vswr +1 (8.14) The mismatch between two elliptically polarized waves is given by equation 8.15, where Rt is given by equation 8.16 and Rr by equation 8.17. Rt is the transmit antenna axial ratio, and Rr is the receive antenn