An investigation of the characteristics of cylindrical surface waves

Abstract

Some quantitative measurements on cylindrical surface waves have been made at a frequency of approximately 10 000 Mc/s. For this purpose an axial standing-wave indicator and an instrument for determining the radial field distribution have been designed and tested by application to a wave of known configuration. The axial standing-wave indicator was used for simple impedance measurements and is now established as a practical device. A series of quantitative observations on cylindrical surface waves have been made with the instrument for examining the radial field distribution, which at a long distance (say 30 ft) from a launching horn, was found to agree with that expected from theory. In the case of thin bare copper wires and enamelled wires of small size the radial decay coefficient deduced from the field-distribution graph approximated closely to the value obtained from the Sommerfeld-Goubau theory. On the other hand, the field distribution in the vicinity of the horn was always considerably distorted by radiation from the horn.For accurate work in the limited space available, it was found necessary to dispense with the horn and set up a surface-wave resonator. This arrangement is capable of yielding the same information on field distribution, since the standing-wave and travelling-wave patterns are identical. The wavelength of the surface wave supported by the resonator at its resonant frequency was measured by a special method described in the paper, and from a knowledge of the resonant frequency the decay coefficient was calculated.Measurements on No. 24 S.W.G. copper wire thickly coated with polythene showed that the corresponding values of the decay coefficient derived from the Goubau approximate theory were greatly in error. This applied in particular to conductors having a coating of polythene whose thickness exceeded 0.010 in. A curve relating the coating thickness with the measured decay coefficient was obtained and compared with the Goubau predictions. Later the assumptions made in the original theory were revised, and accurate curves displaying the dependence of the decay coefficient on the thickness of dielectric coating, wire diameter and frequency were computed. These curves were found to be in agreement with the experimental results.The surface-wave resonator can also be used for the measurement of surface reactance of corrugated surfaces.