An evanescent-mode waveguide frequency-scanning feed

Abstract

INTRODUIXION-beam-scanning sensitivity to frequency changes. This is usually accomplished by means of a folded waveguide (serpentine) series feed with many electrical degrees of pathlength between adjacent output ports. While this scheme is relatively simple, reliable and economical. the hardware implementation is often bulky and heavy. This paper discusses the feasibility of utilizing an alternative and weight.type of device, the evanescent-mode waveguide (EMW) feed, to achieve a large reduction in size A frequency-rcanning antenna requires a highly dispersive delay line in order to obtain high EMW differs from conventional waveguide in that the width is reduced sa that the unloaded guide will not propagate the dominant TElo mode. Ihe cutoff guide is then made propagating and highly dispersive by periodically loading it with capacitive elements to form a series of directly coupled waveguide cavities. A rigorous theoretical analysis was previously made that relates propagation coeficient to the physical parameters of the EMW feed; however, in order to determine the practicality of various EMW cross sections. further analytical expressions are derived for R F loses and power capacity. THEORETICAL DEVELOPMENTA practical configuration for a high-power EMW feed is shown in Figure 1. Dielectric slabs are chosen for periodic loading elements rather than capacitive posts. as high fields would exist near the post edges that severely limit peak power capacity. The guide interior consisLs of are normally fded with air, but could instead contain a lowdielectric material or dielectric gas. dielectric-loaded regions, designated "I", and cavity regions, designated "2". The cavity regionsThe dispersion relationship for the EMW is:where 0 = propagation constant in the EMW. Q = the period of the dielectric slabs, el = relative dielectric mnstant of slab, region I , and e2 = relative dielectric cnnstant of cavi ty, region 2.