Directional SWR Meter
A directional SWR meter measures the magnitude of the forward & reflected waves by sensing each one individually, with directional couplers. A calculation can then be performed to arrive at the SWR.
Referring to the above diagram, the transmitter (TX) and antenna (ANT) terminals are connected via an internal transmission line. This main line is electromagnetically coupled to two smaller sense lines (directional couplers) which are terminated with resistors at one end, and diode rectifiers at the other. Sometimes a printed circuit board using three parallel traces is used to make the transmission line and the two sensing lines. The resistors are chosen to match the characteristic impedance of the sense lines. The diodes convert the magnitudes of the forward & reverse waves to FWD and REV DC voltages, respectively, which are then smoothed by the capacitors.
To calculate the VSWR, first calculate the reflection coefficient:
Then calculate the VSWR:
In a passive meter, this is usually indicated on a non-linear scale.
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