Antenna feed system for receiving circular polarization and transmitting linear polarization

An object is to provide for receiving a circularly polarized signal from an antenna feed (10) connected to orthogonally spaced antenna elements (11, 12, 13, 14) and for transmitting a linearly polarized signal through the same feed without switches, and without suffering a 3 dB polarization mismatch loss, using an arrangement of hybrid junctions. The arrangement is comprised of two dividing hybrid junctions (15, 16), each connected to a different pair of antenna elements and a summing hybrid junction (17). In one embodiment, a receiver (18) is connected to the summing hybrid junction directly. A diplexer (19a or 19b) is used to connect a transmitter (20a or 20b) to only one pair of antenna elements. In an alternative embodiment, designated left and right circularly polarized (LCP and RCP) transmitters (21, 22) are connected to the summing hybrid junction by separate diplexers (23, 24), and separate LCP and RCP sensitive receivers (25, 26) are connected to the diplexers in order to transmit linearly polarized signals using all four antenna elements while receiving circularly polarized signals as before. An orthomode junction (30) and horn antenna (32) may replace the two dividing hybrid junctions (15, 16) and antenna feed (10).

TECHNICAL FIELD 
This invention relates to an antenna feed system which accepts circularly 
polarized signals while receiving and transmits linearly polarized 
signals. 
BACKGROUND ART 
Most of the deep space telecommunications links use circular polarization. 
One reason for this is because of the operational simplicity of tracking a 
circularly polarized signal as opposed to a linearly polarized signal. 
However, linear polarization provides an opportunity to perform scientific 
investigations and engineering tasks that cannot be done with circular 
polarization. This being the case, for some space missions it would be 
desirable to have a downlink from the spacecraft that is linearly 
polarized. Since the Deep Space Network (DSN) normally radiates a 
circularly polarized uplink, it has been recognized that a penalty for 
providing a linear downlink would be 3 dB loss in the uplink due to 
polarization mismatch. An implementation that provides a linear downlink 
capability without suffering the 3 dB uplink loss is desirable. 
The linearly polarized downlink on the spacecraft would provide an 
opportunity to conduct investigations, such as of the solar corona with 
Faraday rotation techniques. In addition, a linearly polarized downlink 
would provide an opportunity for calibrating the Earth's ionosphere. The 
calibration of the ionosphere is needed by some deep space missisons to 
provide precision navigation. Present ionospheric calibration methods are 
problematic (not line of sight) and use linearly polarized VHF satellites 
which are becoming less available. Use of a linearly polarized signal in 
the line-of-sight would be an attractive alternative, that is, use the 
same downlink for telemetry, ionospheric calibrations, and scientific 
investigations. 
STATEMENT OF INVENTION 
An object of this invention is to provide a system for transmitting a 
linearly polarized signal and for receiving a circularly polarized signal, 
without suffering a 3 dB polarization mismatch loss. 
In its broadest aspects, an antenna capable of receiving a circularly 
polarized signal is connected to means for separating the signal into 
orthogonal linear components which are then recombined at the input to the 
receiver by a summing hybrid junction to present to the receiver the total 
power carried in the circularly polarized signal received at the antenna. 
A diplexer is used in at least one linearly polarized component path to 
permit transmission of a linearly polarized signal through the antenna. In 
one embodiment, each of two pairs of orthogonally disposed antenna 
elements are connected to separate ports of separate 180.degree. hybrid 
junctions, each having a third port connected to the summing hybrid 
junction for receiving circularly polarized signals at the receiver. Due 
to the diplexer in the linearly polarized component path to which an 
active transmitter is connected, only one 180.degree. hybrid is fed while 
transmitting. Both 180.degree. junctions transfer linearly polarized 
signals that are orthogonal while receiving a circularly polarized signal. 
These linearly polarized signals are thus transferred to the summing 
hybrid junction which extracts all of the power from the circularly 
polarized signal and couples it to the receiver. Thus, while receiving, 
the entire energy in the uplink circularly polarized wave at the antenna 
is coupled to the receiver. 
In another embodiment, two transmitters are used, one transmitting a signal 
with right circular polarization, and the other transmitting a signal with 
left circular polarization. When both are transmitting simultaneously, the 
output results in transmission of a linearly polarized signal. If only one 
transmitter is excited, only right or left circular polarization is 
transmitted. Two receivers can be connected through diplexers to receive 
left and/or right circularly polarized signals. 
The novel features that are considered characteristic of this invention are 
set forth with particularity in the appended claims. The invention will 
best be understood from the following description when read in connection 
with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
Before referring to the schematic diagrams of the invention, several useful 
forms of hybrid junctions will be described, all of which are described in 
Radar Handbook edited by Merrill I. Skolnik and are widely used in 
microwave systems as power dividing and summing devices. One is the 3 dB 
directional coupler sometimes called a quadrature hybrid junction or 
coupler because it divides the power inserted in one port equally between 
two other ports, with phase quadrature between the two output signals, and 
provides no power to the fourth port. Another is the magic-T hybrid 
junction which divides power inserted in one port equally between two 
other ports, with 0.degree. or 180.degree. phase difference between the 
two output signals, depending on which input port is used, and provides no 
power to the fourth port. A third is the hybrid ring which has four ports, 
1, 2, 3 and 4, spaced electrically 90.degree. apart, with an electrical 
spacing of 270.degree. from port 4 to port 1 to complete a ring. A signal 
into port 1 divides equally with 180.degree. phase difference between port 
2 and port 4, and a signal into port 3 divides equally between ports 2 and 
4 with zero phase difference. 
In the schematic diagrams to be described, the hybrid junctions are 
represented by rectangles, each having four ports, with an indication of 
whether it is of the 90.degree. phase difference type, namely a quadrature 
coupler, or of the 180.degree. phase difference type, namely a magic-T or 
hybrid ring. In each case, the phase relationships of the ports are 
indicated, and an unused port is terminated by a suitable impedance to 
absorb any incidental energy out of that port. However, it should be 
understood that the invention is not limited to these particular hybrid 
junctions. The concept of the invention is given to any suitable 
implementation with due consideration to bandwidth and power handling 
capabilities. 
Referring now to FIG. 1, an antenna feed 10 having orthogonally disposed 
antenna elements 11, 12, 13 and 14 are grouped in pairs, namely 11, 13 and 
12, 14, for coupling through 180.degree. hybrid junctions 15 and 16. The 
hybrid junctions 15 and 16 are, in turn, coupled by a 90.degree. hybrid 
junction 17 to a receiver 18 for uplink circularly polarized reception. A 
diplexer 19a connects the hybrid junction 15 to a transmitter 20a for 
downlink linearly polarized signals by excitation of only elements 11 and 
13. A diplexer 19b connects the hybrid junction 16 to a transmitter 20b 
for downlink transmission of linearly polarized signals by excitation of 
only elements 12 and 14. Note that the two linearly polarized signals 
transmitted are orthogonal, so if both are excited simultaneously in phase 
quadrature, circular polarization could be achieved. However, in practice 
only one transmitter would be used for transmitting a linearly polarized 
signal. The other transmitter would serve as a back-up (with its 
transmission polarized in space orthogonal to the first transmitter). 
This arrangement of hybrid junctions provides for a linearly polarized 
downlink transmission from a spacecraft while allowing the spacecraft to 
receive a circularly polarized uplink transmission. No switches are 
required to achieve linear transmit and circular receive operation either 
individually or simultaneously, and no spacecraft commands are required; 
the circular receive and transmit capabilities are inherently present for 
use at all times. All that is necessary besides the hybrid couplers is a 
diplexer for connecting the transmitter to the hybrid junction 15 (16), 
and for isolating the hybrid junction 17 connected to the receiver while 
transmitting. 
While receiving, 180.degree. hybrid junctions 15 and 16 feed the summing 
hybrid junction 17, thereby coupling the circularly polarized uplink 
transmission to the receiver 18. The diplexer 19a automatically couples 
the hybrid junction 15 to the hybrid junction 17, with essentially no loss 
to the signal received and essentially no coupling to the transmitter 20a. 
While transmitting, the diplexer 19a couples the transmitter 20a to the 
hybrid junction 5 with essentially no loss of the signal transmitted and 
with essentially no coupling to the summing hybrid junction 17. The 
transmit power is simply divided once into signals 180.degree. out of 
phase for excitation of the antenna elements 11 and 13. Operation for the 
diplexer 19b and transmitter 20b is similar. 
The diplexer 19a is essentially a short circuit between hybrid junctions 15 
and 17 while receiving, and it is then convenient to view the reciprocal 
arrangement as transmitting, with the energy source assumed to be 
occupying the receiver position 18. The hybrid junction 17 divides the 
assumed energy source into two signals 90.degree. out of phase, and each 
signal is again divided into two signals 180.degree. out of phase to 
thereby produce four signals of equal amplitude at the elements of the 
antenna feed 10 at phase angles of 0.degree., 90.degree., 180.degree. and 
270.degree.. The signals from one 180.degree. hybrid junction are applied 
to one pair of diametrically opposite antenna elements, and the signals 
from the other 180.degree. hybrid junction are connected to the other pair 
of diametrically opposite antenna elements. The result is a circularly 
polarized signal. With the receiver back in place, the reciprocal action 
of receiving the circularly polarized uplink signal results in all of the 
circularly polarized energy being delivered to the receiver. For 
transmitting the downlink signal, energy is made available to only one 
pair of antenna elements for a linearly polarized signal transmission. 
This arrangement thus provides a solution to the problem of providing a 
linearly polarized downlink and a circularly polarized uplink, without 
suffering a 3 dB polarization mismatch loss, and without switches and 
commands. The arrangement of hybrid junctions and a diplexer enables the 
polarization change without switches. 
In the embodiment of FIG. 2, the hybrid junctions 15, 16 and 17 are 
connected to an antenna feed 10 for a circularly polarized uplink as in 
the embodiment of FIG. 1, and are therefore identified by the same 
reference numerals. What is different is that all four antenna elements 
are exicted for a linearly polarized downlink signal using two 
transmitters 21 and 22 coupled to the 90.degree. hybrid junction 17 by 
respective diplexers 23 and 24. A combination of a left circularly 
polarized (LCP) signal derived from connections associated with 
transmitter 21 and a right circularly polarized (RCP) signal derived from 
connections associated with transmitter 22, for example, produces a linear 
polarization of the downlink signal. If only one transmitter is energized, 
all antenna elements will still be excited, but then a circularly 
polarized signal is transmitted, for example, LCP from transmitter 21 or 
RCP from transmitter 22. While receiving, a receiver 25 coupled to the 
90.degree. hybrid junction 17 is sensitive to, for example, RCP energy, 
and a receiver 26 coupled to the hybrid junction 17 is sensitive to, for 
example, LCP energy. 
In a third embodiment shown in FIG. 3, an orthomode junction 30 and a horn 
antenna 32 effectively replace the 180.degree. hybrid junctions 15 and 16, 
and the antenna feed 10, respectively, in the arrangements described above 
with reference to FIG. 1. The output of the orthomode junction while 
receiving is two orthogonal (0.degree. and 90.degree.) and 90.degree. 
phase shifted linearly polarized signals which are the components of a 
circularly polarized uplink signal. The hybrid junction 17 combines the 
components and presents the total power to the receiver. The diplexer 19a 
is transparent to received signals, and couples the linear transmitter 20a 
to one port of the orthomode junction. The reciprocal action of the 
orthomode is then one of transmitting through the horn antenna a linearly 
polarized downlink signal. The diplexer 19b and transmitter 20b coupled to 
the other port of the orthomode junction transmit an orthogonal downlink 
to that of diplexer 19a and transmitter 20a. 
Although particular embodiments of the invention have been described and 
illustrated herein, it is recognized that modifications and variations may 
readily occur to those skilled in the art. Consequently, it is intended 
that the claims be interpreted to cover such modifications and equivalents 
.