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VSWR is a measure of impedance mismatches in a circuit. A large VSWR can cause many problems in RF circuits. Worst-case effects include permanent damage to RF/microwave high-power amplifiers (HPA), commonly termed as VSWR failure. It is essential to protect the HPA from such catastrophes. This article demonstrates a scheme to detect a VSWR condition and protect the HPA from such failures using directional couplers and high-performance RF logarithmic amplifiers. A prototype of the VSWR detection and protection scheme was designed and tested. A specific HPA design that gets damaged for VSWR > 4:1 was functional even after subjected to a VSWR > 15:1 when equipped with the proposed protection scheme.

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The voltage and the current along a transmission line are related by a specific ratio known as the characteristic impedance (Z_O). When conducted RF energy traveling along the transmission line encounters a load equal to the characteristic impedance, all of the available power is delivered to the load. Any discontinuity (mismatch) along the transmission line that changes the load impedance causes reflected currents and voltages along the line, creating a standing wave. Incident and reflected waves interfere constructively and destructively resulting in maximum (V_max) and minimum (V_min) as shown in Figure 1. Voltage standing wave ratio (VSWR), a measure of this mismatch, is defined as a ratio of V_max/V_min.

Perfectly matched impedances (VSWR=1:1) result in ideal power transfer, while heavily mismatched impedances (high VSWR) result in reduced power transfer to the load. A high VSWR can cause problems anywhere in the system, but it is the PA preceding the antenna that is the most sensitive to these events. An excessive VSWR can reduce the operating range of the radio, cause the transmitted signal to saturate the receive section or cause the radio to heat up. More severe effects would damage the transmitter and break down the transmission line dielectric through some catastrophic failure mechanism, such as combustion. High VSWR can also cause shadowing in a TV broadcast system as the signal reflected off the antenna reflects again off the power amplifier and is then rebroadcast resulting in multipath-like phenomenon.