Abstract :
Equipment is described that provides a continuous display on a cathode-ray tube of the reflection coefficient in a rectangular wave-guide over a frequency band of 50 Mc/s at around 35 Gc/s. A reflectometer method is employed, by which the reflected signal is measured directly, using 12 kc/s square-wave modulation of a microwave signal generated by a reflex klystron oscillator. The accuracy of this method is limited by the directivity of the directional coupler; in the arrangement described a microwave bridge enables reflection coefficients down to 2% to be measured fairly accurately, when using directional couplers with only 25 dB directivity. The microwave bridge is formed by using two similar directional couplers in conjunction with two hybrids in a symmetrical arrangement. One hybrid is connected as a shunt-T, and the other as a series-T, in such a way that the spurious signal due to the finite directivity of the directional couplers is cancelled at the detector. Optimum balance is obtained by adjustment of a variable attenuator and a variable phase-shifter in one arm of the bridge. The effective directivity of the microwave bridge exceeds 52 dB over at least 50 Mc/s, the band being limited only by the electronic tuning range of the klystron, as the method itself is inherently broad-band. Automatic gain control is provided to reduce to a negligible amount the power variations occurring throughout the klystron scan.
