260 BELL S YSTEM TECH NIC A L JOURNA L 



8.5 Parabolic Cylinders beticceii Parallel Plates 



In «S.O we saw thai parabolic cylinders may be illuminated by line sources 

 or that they may be confined between parallel plates and illuminated by 

 point sources to produce line sources. In either of these two cases the char- 

 acteristics which the feed should have are specilled accurately by the con- 

 ditions stated at the beginning of 8.4 for paraboloidal feeds with the excep- 

 tions that condition c must be reworded so that it applies to cylindrical 

 rather than to spherical optics. 



We will first consider parabolic cylinders bounded by parallel plates 

 because in doing so we describe in passing one form of feed for unbounded 

 parabolic cylinders. Two forms of transmission between parallel plates 

 are used in practice. 





r!" 



Fig. 23 — Dual Aperture Rear Feed Horn. 



a. The transverse electromagnetic (TEM) mode in which the electric 

 vector is perpendicular to the plates. This is simply a slice of the 

 familiar free space wave and can be propagated regardless of the spacing 

 between the plates. It is the only mode that can travel between the 

 plates if they are separated less than half a wavelength. Its velocity 

 of propagation is independent of plate spacing. 



b. The TEoi mode in which the electric vector is parallel to the plates. 

 This mode is similar to the dominant mode in a rectangular waveguide 

 and differs from it only in that it is not bounded by planes perpen- 

 dicular to the electric vector. It can be transmitted only if the plate 

 spacing is greater than half a wavelength, is the only parallel mode 

 that can exist if the spacing is under a wavelength and is the only sym- 

 metrical parallel mode that can exist if the plate spacing is under three 



