mselves. If power is fed to an isotropic antenna (i.e., an antenna that radiates equally in all directions in azimuth and elevation), the wavefront will radiate outward from the antenna in an ever expanding sphere at 3 X 108 meters (186,282 miles) per second. The strength of the signal obviously decreases with distance as a given amount of power is spread over a greater area, and the incident power density at a remote point can be calculated as in equation 8.2, where Pr is received power density in watts per meter2, Pt is transmitted power in watts, and r is distance in meters. (8.2) Because of the very large differences in power density over long propagation paths, particularly in the microwave bands, it is usual to measure power density in decibels (dB) relative to 1 watt (i.e., dBW), or 1 milliwatt (i.e., dBm). At frequencies below the microwave range, field strength in volts per meter is a more usual measurement than power density, partly because volts per meter have historically been used for the measurement of signal strength and partly because the thermal heating effect of power absorption is easier to use for microwave measurements. The conversion can be made for power density to field strength using the simple derivation of Ohm's law (Braun, 1986), as in equation 8.3, where V is voltage, R is resistance in ohms, and P is power in watts. V2=RP (8.3) In this case, R is substituted for Z, the characteristic impedance of free space, which is a constant of 87 ohms. Field strength can therefore be converted to power density by equation 8.4 (Maslin, 1987), where E is field strength in volts per meter, Z is the impedance of free space (87 ohms), and Pd is power density in watts per meter2. E2=ZoPa (8.4) Equation 8.5 can be used to convert directly from dBW to dBuV, where E is in dB. = =PidB(1W/m2)+145.6 (8.5) On the left side of this equation, dB represents a voltage and not a power ratio. The voltage at the center of an unterminated half-wave dipole in the path of a radiowave and aligned along the axis of the e