Coaxial Swept-Frequency VSWR Measurements Using Slotted Lines

A system is described using a sweep generator, a slotted line with a broad-band probe, a sliding load which can be coupled mechanically to the probe carriage, a logarithmic amplifier, and a read-out device which can measure the VSWR of coaxial components continuously versus frequency. The technique is especially effective for the measurement of the VSWR of low-loss two-port networks and of residual VSWR of a slotted line where the elimination of termination reflections is accomplished automatically without computation. The technique of making swept-frequency VSWR measurements using slotted lines is described in detail. The use of a logarithmic RF ratio meter and dual-channel arrangement enables the operator to read the VSWR continuously versus frequency directly from the X-Y plot or scope trace independent of frequency sensitivity of the probe or of the sweep generator output or amplitude instability of the sweep generator. It is shown that the mechanical coupling of a termination sliding in a reference line with the probe carriage provides the equivalent of a reflection-free termination and allows the measurement of the magnitude of the reflection scattering coefficients independent of the loss or phase shift within the microwave component. The heart of the measurement system is a slotted line having a dielectric support at the measurement end with a reflection coefficient below 0.005 up to 18 GHz. By connecting the reference airline with the sliding termination directly to the slotted line output the overall residual VSWR of the measurement system is determined directly and continuously versus frequency.