Two quarterwave-plate polarizers inserted between rotary waveguide joints transform received signals from arbitrary linear to circular polarizations and then from circular to fixed linear polarizations. Fixed linear polarizations are applied to amplifiers and filters in usual fashion

Bathker, D. A.

Levy, G. S.

Mccrea, F. E.

English

NASA Technical Reports Server

March 1973	 B73-10134 
•	 NASA TECH BRIEF 
NASA Pasadena Office
	 4r -'-	 -I - 
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-Noise Microwave Polarimeter 
The problem: 
To equip a microwave antenna with a system 
which can track and measure the polarization of 
weak microwave signals. 
The solution: 
A combination of two quarterwave-plate polarizers 
inserted between rotary waveguide joints transforms 
received signals from arbitrary linear to circular 
polarizations and then from circular to fixed linear 
polarizations; the fixed linear polarizations are ap-
n" 
plied to amplifiers and filters in the usual fashion. 
How it's done: 
The polarizer is constructed integral with the front 
end of the receiver in the feed-cone system of a Cas-
segrainian antenna. The component parts of the 
polarizer system are indicated in the block diagram. 
The system can receive or transmit signals which have 
right circular polarization, left circular polarization, 
or rotatable linear polarization. The feed cone em-
ploys a dual mode horn followed by two quarterwave 
plates mounted between three rotatable joints (re-
motely controlled). The angular displacement of the 
quarterwave plates is sensed by synchros through 
gears mounted on the quarterwave plates and is in-
dictated on a monitoring panel in the control room. 
When the upper quarterwave plate is correctly 
aligned with the incident signal, it converts the lin-
early-polarized signal to one sense of circular polar-
ization. The lower quarterwave plate converts the 
circular polarization back to the linear mode and 
transmits the signal to one of the two orthogonal out-
put ports of the orthomode assembly elements. Mode 
dampers precede each transducer in order to suppress
the higher-order modes that are generated by the 
presence of the asymmetric rectangular-to-cylindrical 
waveguide junctions in each transducer. The cosine 
taper adaptor is necessary for matching the electrical 
Feed Horn 
L-J	 Rotary Joint 
Upper Quarterwave Plate 
F-ZJ Rotary Joint 
Lower Quarterwave Plate 
Rotary Joint 
Cosine Taper Adaptor 
F__1 Mode Damper 
Upper Orthomode Transducer 
[]]-.,Error Port - To Error Channel
Mode Damper 
Ej	 Lower Orthomode Transducer Reference Port 
- To Reference Channel 
properties of the larger quarterwave plate assembly

to the smaller diameter orthogonal-mode transducers.

When the upper quarterwave plate is incorrectly

aligned, both senses of circular polarization (left and 
(continued overleaf) 
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned rights. 
https://ntrs.nasa.gov/search.jsp?R=19730000134 2020-03-17T07:08:37+00:00Z
right) are generated, and some of the signal appears 
at the second transducer port for injection into an 
error channel receiver. The alignment of the lower 
quarterwave plate is at 450 with respect to the posi-
tion of the orthomode ports. The lower quarterwave 
plate is capable of a neutral position, as well as ±450 
positions with respect to the two orthogonal outputs. 
The lower quarterwave plate position is therefore a 
basic adjustment which lends versatility to the polar-
izer, i.e., the ability to adapt to different space mis-
sion polarization measurement requirements, such as 
switchable and/or circular polarizations. For circu-
larly-polarized reception or transmission, the lower 
quarterwave plate is set to the neutral position and 
only the upper unit is used. 
The polarizer can be adjusted manually for high-
accuracy polarimetric measurements; additionally, it 
has been used in an automatic polarization tracking 
system in which the lower quarterwave plate was 
used as the reference orientation and the upper 
quarterwave plate was used as the rebalance element 
of the servo loop. When the polarizer is operated in 
automatic servo mode, both the reference channel 
and error channel ports transmit signals through a 
rectangular waveguide to maser amplifiers which are 
operated in their own closed-cycle cryostats at 4.2° K. 
In this mode, the reference channel maser is con-
nected to the space communications receiver in its 
normal configuration, and the error-channel maser is 
connected to a receiver which develops an error volt-
age proportional to the difference in phase between 
a sample of the reference-channel signal and the error-
channel signal. The error voltage is used to adjust 
the position of the upper quarterwave plate, and the 
plate displacement is rapid enough to track the an-
gular change in polarization of an incident signal 
from space under the influence of Faraday rotation, 
or spacecraft tumbling at a low rate. 
The rotary joints are low-loss, low-noise cylindrical 
waveguide sections designed for the S-band applica-
tion. The use of large cylindrical waveguide allows 
the rotary joint to be wideband. 
The quarterwave plate is a waveguide device which 
retards the phase of half the incoming signal by 90 
electrical degrees with respect to the other half, such 
that the incoming plane polarized signal is split into 
two spatially orthogonal components which travel at
different velocities. Upon emergence, the two com-
ponents will have the proper time and space phases 
to form a circularly polarized signal. If the retarda-
tion in phase corresponds to 180 electrical degrees (as 
by adding a second quarterwave plate), the circularly-
polarized signal will again be linearly polarized. Any 
error in the alignment angle between the incoming 
signal and the upper quarterwave plate gives rise to 
orthogonal modes in the rotary joint which are not 
compensated by the reference quarterwave plate. 
These orthogonal modes develop an error signal at 
the error output port of the orthogonal mode trans-
ducer. The quarterwave plate is then rotated to com-
pensate for the change of polarization of the incoming 
signal to null the error signal. 
Notes: 
1. The same combination of elements is also appli-
cable to future experimental systems in the X-band 
region. In a more generalized version, it would be 
possible to use the apparatus to optimize reception 
or transmission of arbitrarily polarized elliptical 
waves simply by not restricting the second quarter-
wave plate to exactly ± 45° positions. 
2. The polarimeter has been mounted on the 210-foot 
Cassegrainian spacecraft tracking antenna at Gold-
stone, California and was used to measure the 
Faraday rotation suffered by the 2292-MHz, 5-
band telemetry carrier of Pioneers when the sig-
nals interacted with the plasma and the magnetic 
field in the solar corona. 
3. Requests for further information may be directed 
to:
Technology Utilization Officer 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Reference: TSP 73-10134 
Patent status: 
NASA has decided not to apply for a patent. 
Source: Gerald S. Levy, Dan A. Bathker, 
and Frank E. McCrea of 
Caltech/JPL 
under contract to 
NASA Pasadena Office 
(NPO-11512)
I 
B73-10134	 Category 02


Low-noise microwave polarimeter

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