Properties of the Brewster angle and internal reflection in a dielectric prism are proposed as the basis of a new type of element for use in oversize waveguide in quasi-optical microwave components. Waveguide loss is reduced and precision broadband attenuators, phase shifters, and directional couplers can be constructed on the basis of the properties

Carson, J. W.

English

NASA Technical Reports Server

October 1967	 Brief 67-10416 
.
	
NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
•.	 ••. 
Dielectric Prisms Would Improve Performance of 
Quasi-Optical Microwave Components 
The problem: 
Use of conventional, single-mode, rectangular 
waveguide in the millimeter and submillimeter range 
is accompanied by large waveguide losses, which 
increase with frequency. One way of minimizing these 
losses is to use oversize waveguide for various func-
tional elements such as attenuators, couplers, and 
phase shifters. In these cases the microwave signals 
behave in a quasi-optical manner, permitting the use 
of many standard optical components and techniques. 
In one technique, a double prism has been used as the 
basic element in conjunction with oversize waveguide. 
In this technique the input signal is split at the air gap 
between the two prisms, part of the signal being 
reflected as one output and part transmitted as a 
second output. The energy division depends on the 
gap distance and the wavelength. Reflection losses 
occur at the input and output surfaces, making it 
necessary to use impedance matching methods to 
reduce the VSWR. One way of accomplishing the 
reduction is to use a fine comb structure cut into the 
input and output faces of the prisms. This double-
prism approach has a number of disadvantages, which 
include the following: (I) The comb structure is nar-
rowband, and becomes increasingly difficult to fabri-
cate as the frequency increases. Other impedance-
matching methods are either narrowband, or lossy, 
or both. (2) The parallelism of the inner surfaces of 
the prisms must be accurately maintained as the gap is 
varied in order to have accurate and repeatable per-
formance. This requirement presents a mechanical 
alignment problem. (3) The energy division is fre-
quency dependent. 
The solution: 
The properties of the Brewster angle (the one angle 
of incidence of a polarized light wave in the plane of
incidence at a dielectric interface for which the re-
flected signal is zero) and internal reflection in a di-
electric prism are proposed as the basis of a new type 
of element for use in oversize waveguide. Precision 
broadband attenuators, phase shifters, and direc-
tional couplers can. be constructed on the basis of 
these properties. In principle, the performance of 
these devices depends only on the geometrical con-
figuration and the dielectric constant of the constit-
uent material. Because this constant is nearly inde-
pendent of wavelength over a waveguide bandwidth, 
the devices should be inherently broadband. With the 
input and output signals at the Brewster angle, 
reflection losses would be at a minimum, and no 
impedance matching networks should be required. 
The overall reflectance and admittance may be cal-
culated directly by means of the Fresnel equations. 
Low-loss dielectric materials (plastics, ceramics, 
quartz) commonly used in microwave devices are 
suitable for the dielectric prisms. For an amplitude 
modulator, the dielectric prism would consist of a' 
ferrite. 
Note: 
Inquiries concerning these devices may be di-
rected to:
Technology Utilization Officer 
Electronics Research Center 
575 Technology Square 
Cambridge, Massachusetts 02139 
Reference: B67-10416 
Patent status: 
Inquiries about obtaining rights for the commercial 
use of this invention may be made to NASA, Code 
GP, Washington, D.C. 20546.
Source: John W. Carson 
(ERC- 100 Ii) 
Category 01 
• - /	 This document was prepared under the sponsorship of the National 	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States 	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States 	 will be free from privately owned rights.
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Dielectric prisms would improve performance of quasi-optical microwave components

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