SWR Meter - SWR Bridge

SWR Bridge

SWR can also be measured using an impedance bridge circuit. The bridge is balanced (0 volts across the detector) only when the test impedance exactly matches the reference impedance. When a transmission line is mismatched (SWR > 1:1), its input impedance deviates from its characteristic impedance; thus, a bridge can be used to determine the presence or absence of a low SWR.

To test for a match, the reference impedance of the bridge is set to the expected load impedance (for example, 50 ohms), and the transmission line connected as the unknown impedance. RF power is applied to the circuit. The voltage at the line input represents the vector sum of the forward wave, and the wave reflected from the load. If the characteristic impedance of the line is known to be 50 ohms, we know the magnitude and phase of the forward wave; it is the same wave present on the other side of the detector. Subtracting this known wave from the wave present at the line input yields the reflected wave. Properly designed, a bridge circuit can be used not only to indicate a match, but the degree of mismatch - thus making it possible to calculate the SWR. This usually involves alternately connecting the reference wave and the reflected wave to a power meter, and comparing the magnitudes of the resulting deflections.