AN EXPERIMENTAL DUAL POLARIZATION ANTENNA FEED 393 



in all three bands. To keep the coupler length within a reasonable size, 

 the individual hole dimensions must be on the order of Xo/4 (at 4 kmc), 

 which will permit coupling hole resonance within the 3-to-l frequency 

 l)and. A further consideration is the selection of the hole shape to avoid 

 perturbing the TEii° wave that is orthogonal to the strongly coupled 

 TEii^ wave. 



Spacing of the coupling holes nuist not be \g/2 to avoid: (1) large re- 

 flections in the driven waveguide, and (2) large backward-travelling 

 waves in the adjacent coupled waveguide. This requirement is easily 

 met in the 11-kmc coupler where only one band is present; however, the 

 presence of signals in two or three bands makes the non Xg/2 spacing 

 more difficult in the 6- and 4-kmc couplers. 



Another phenomena of importance exists in coupled waveguides operat- 

 ing over an extended frequency range. A coupling aperture in the side 

 wall (see Fig. 1) may interact with a high-order mode at the latter's 

 cutoff frequency, resulting in a significant perturbation of the desired 

 coupling. For example, at the frequency where TEoi^ passes through cut 

 off, the coupling between TEu^ and TEio° will be perturbed if the coup- 

 ling hole is of sufficient size. Small coupling holes do not allow this pertur- 

 bation to manifest itself. Coupling holes in a realistic design do become 

 large enough to allow this effect to appear. Since dominant mode guides 

 in the 4- and 6-kmc bands can support other modes in the higher fre- 

 quency bands, considerable caution must be exercised in selecting the 

 round guide sizes on this account alone. (The size of the round guide is 

 determined also by the phase velocity in the rectangular guide). 



SELECTION OF COUPLING APERTURES 



A series of holes in either the narrow or broad side of the rectangular 

 guide can, in principle, be used to achieve complete power transfer from 

 TEio° waves to TEn° waves. The specific consequences of coupling 

 through holes located along the center line of the broad side will be 

 considered first. (Off center holes are not of interest because they couple 

 TEio'"' waves to both polarizations of TEii° waves in a frequency-sensitive 

 way.) The transverse magnetic field H^ and the electric field Ep of the 

 TEii° waves can couple to TEio° waves. When two fields couple, the back- 

 ward wave in the undriven guide can be greater than the forward wave 

 in the same guide. To avoid this possibility, transverse slots can be used 

 to prevent electric field coupling. The coupling of a transverse slot in- 

 creases as the frequency is increased which suggests that 11 kmc signals 

 be introduced at the position nearest to the antenna because the largest 

 tolerable apertures for an 11-kmc coupler would not perturb 6- or 4-kmc 



