New Equipment for Impedance Matching and Measurement at Very High Frequencies

The slotted transmission line has long been used as a standard device for determining impedance. Over the v.h.f. range, however, the constructional difficulties due to the physical length required for the slotted line make it primarily a laboratory instrument. The need for an alternative device of comparable or improved accuracy, capable of giving much more rapid measurements, and sufficiently robust to be used on field tests of aerial arrays, has led to a modified approach. A test section of low-loss coaxial line, which may be as short as one-third of the equivalent slotted line, is fitted with fixed probes to measure the relative voltage amplitudes at three known points. From these ratios the unknown impedance can be found, either analytically or graphically, each ratio defining a circular locus on the Smith chart. The frequency range covered by a line of this type is approximately 3:1; near mid-frequency the present equipment can measure standing-wave ratios up to 5:1, corresponding to resistive loads ranging from 15 to 375 ohms for a line of 75 ohms. An important advantage of the new equipment is that the adjustment of a load to a prescribed value is shown by the simultaneous zero reading of two meters.