Dichroic plate has orthogonally-disposed, loaded dipole apertures with their orientations arranged so as to cancel cross-coupling effects which would otherwise result in power loss to circularly polarized signal

Ludwig, A. C.

Woo, R. T.

English

NASA Technical Reports Server

Main
Polarization
Vector Diagram
ross-
olarlzation 
January 1975	 B74-10283 
NASA TECH BRIEF 
NASA Pasadena Office 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Low-Loss, Circularly-Polarized Dlchroic Plate 
.
The problem: 
In order for a specialized antenna system to operate 
simultaneously on two frequencies, provision must be 
made for passing X-band signals to the X-band feed 
cone while reflecting S-band signals to the S-band 
feed cone. 
The solution: 
A dichroic plate has been designed which has 
orthogonally-disposed, loaded dipole apertures with 
their orientations arranged so as to cancel the cross-
coupling effects which would otherwise result in a 
power loss to a circularly polarized signal.
How it's done: 
The original circularly-polarized dichroic plate 
(Figure 1A) showed low loss (<0.6 dB for angles of 
incidence up to 600) for both parallel and perpendic-
ular polarizations. However, an ellipticity loss 
exceeding 2 dB was also measured. Further study 
revealed the presence of a cross-polarized component 
with a magnitude small enough (approximately 17 dB 
down) to cause no significant transmission loss but 
large enough to produce substantial ellipticity loss. 
The generating mechanism for cross-polarization in 
the circularly polarized plate of Figure 1A is entirely 
different from that in a comparable linearly polarized 
Segment of Circularly-Polarized Dichroic 
Plate Showing Dipole Array 
JfJ 
_r^ J^ A^ A 
flJL 
_n. 
Main 
Potarizationj. Cross- 
Polarization	 Cross- 
Polarization IN 
Main _-
Polarization	 B. Typical Dipole Cross-Polarization 
Figure 1. Circularly-Polarized Dlchroic Plate Details
(continued overleaf) 
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https://ntrs.nasa.gov/search.jsp?R=19740000283 2020-03-17T14:19:27+00:00Z
plate. Cross-polarization is produced through the 
mutual coupling of adjacent orthogonal dipoles as 
shown in Figure lB. The main polarization voltages 
are represented by solid arrows in one direction and 
by dashed arros in the direction orthogonal to it. 
The coupling to the adjacent dipole and the resulting 
cross-polarization are also shown. The vector diagram 
shows that the circular polarization resulting from the 
cross-polarization is opposite in sense to the main 
circular polarization and therefore represents a power 
loss in transmission through the plate. To reduce the 
loss, two new dipole configurations for circularly 
polarized plates were devised. 
The configuration in Figure 2 has the dipole 
elements so oriented that the cross-polarization 
generated by one dipole is cancelled by that generated 
by the adjacent orthogonal dipole. In the second new 
configuration (not shown), the cross-polarization 
generated by one dipole element cancels that 
generated in the other. The ellipticity loss for both 
modified plates is less than 0.2 dB, while the degrada-
tion in antenna gain is less than 0.4 dB. A previously-
devised, circularly-polarized dichroic plate is unsuit-
able in this application as it requires a dielectric 
backing to prevent the center pieces of the elements 
from falling out, and the lossy dielectric adds substan-
tially to the antenna noise temperature. 
iUi 
Main I _____1•1______ I 	 Polarization 
I
)sslarization 
ii	 -r 
Main 
Figure 2. Typical Dipole Elements of Modified 
Circularly-Polarized Dichroic Plate
Figure 3 illustrates one intended use of the dichroic 
plate. The reflex antenna feed system is a two-
frequency transmitting/receiving antenna feed system 
utilizing the JPL tricone assembly, an ellipsoidal 
reflector, and a 0.9-m by 0.9-rn (3-ft by 3-ft) dichroic 
plate. Such an antenna is capable of simultaneous 
operation at two different frequencies. 
I	 I	 I 
—+-
Offset 
Hyperboloidal 
I \	 I Subref lector 
/ \ I	 \	 I I	 \ I 
/ Ellipsoidal \Reflector
\t 
/	 Dichroic 
' Plate 
vl L"	 SBand XBand 
Note: 
Requests for further information may be directed 
to:
Technology Utilization Officer 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Reference: TSP74-10283 
Patent status: 
This invention has been patented by NASA (U.S. 
Patent No. 3,769,623). Inquiries concerning nonex-
clusive or exclusive license for its commercial develop-
ment should be addressed to: 
Patent Counsel 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Source: Richard T. Woo and

Arthur Ludwig

NASA Pasadena Office

(NPO-13171) 
Figure 3. Use of Dichrolc Plate in Reflex Antenna 
Dual-Frequency Feed System
	
is 
Category 01 (Electronics - Components and 
Circuitry)
	 0 
*(J.S.Government Printing Office: 1975 - 636-684 
B74-10283


Low-loss, circularly-polarized dichroic plate

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