Resonant slots

This paper describes how the coupling of a resonant half-wave slot to a rectangular wave guide in the wall of which the slot is cut, came to be studied in order to solve the problem of linear microwave radiators fed from wave guides. The methods of experimental investigation are described and the results are presented in terms of a method of representing the loading of the dominant wave in the guide. The important conception is the transformation of the circle-diagram variable (w) representing the dominant wave-system in the guide. It is shown that wave guides may be coupled by resonant slots. If such a slot is cut in the wall of a wave guide and lies opposite a registering slot in a second guide in contact with the first, the wave guides are coupled if the slot can be excited by the dominant wave in both guides. The type of coupling depends on the aspect of the slot in each guide. The laws of guide coupling are explained in terms of the manner in which impedance is transferred from the position of the slot centre guide 2 into guide 1 at the same position. The coupling of variable reactances to the guide by resonant slots to produce a T-section load is described, with experimental confirmation of the transformation of impedance and phase by the load. The method of radiation coefficients is applied to deduce the law of guide-coupling in the general case; it may be applied to treat loading and coupling of two waves in the same guide. Finally, directive aerial coupling by a pair of slots is discussed. Finally, the elements of the design problem for a linear microwave array and the theory of the wave-guide feed are discussed. Both transverse and longitudinal polarization are considered, together with the effects of mutual interaction between the inclined slots cut in the narrow face of the guide in the longitudinally-polarized array. The bandwidth of arrays is treated and a broad-band array of inclined-displaced slots in the broad face is described with measurements of - its performance. The principle of the microwave Yagi aerial is briefly presented.