 Consider a typical case where a transmitter designed for operating into a 50 S transmission line is The fraction of the incident power not reaching the made to feed an antenna with a nominal impedance of load because of mismatches and reflections is 50 Q. In the ideal world, apart from a little loss in the transmission line, all the energy produced by the trans- Plant mitter will be passed on to the antenna. In the practical (2.2.33) Pincident world, an exact antenna match to the transmission line is seldom achieved and most antenna manufacturers are honest enough to admit the discrepancy and they use Hence the mismatch loss (or reflection loss) is calcu- a term called the voltage standing wave ratio (VSWR) lated from to indicate the degree of mismatch. VSWR is useful because ML (dB) = -10 log (1 - r2 (2.2.34) it is relatively easy to measure - it is based on mod- ulus values rather than phasor quantities which en- ables simple diode detectors to be used for 2.2.11.5 Transmission coefficient measurement purposes; it indicates the degree of mismatch in a termination; The transmission coefficient (tv) is defined as the ratio of it is related to the modulus of the reflection coeffi- the load voltage (VL) to the incident voltage (Vinc) but cient (shown later). VL = Vinc + Vref. Hence VSWR is defined as (2.2.35) VSWR = (2.2.37) If we now use Equation 2.2.28 to substitute for , we obtain A VSWR of 1 represents the best possible match. 16 (2.2.36) Any VSWR greater than |1| indicates a mismatch and a large VSWR indicates a greater mismatch than a smaller Sometimes Equation 2.2.36 is normalised to Z and when VSWR. Typical figures of VSWRs for good practical ZL/Zo is defined as z, we obtain terminations range from 1.02 to 1.1. 15 This term is based on the Standing Wave Pattern principle which was introduced in Section 2.2.2.2 where you walked along a tunnel which produced echoes while your friend whistled at a constant amplitude and pitch. In the tunnel case, the loudest (maximum intensity) sound occurred where the incide