A multi-port, multi-frequency combiner comprising a main waveguide having a cross-section in the shape of a right-angle parallelogram and dimensioned to simultaneously propagate co-polarized signals in different frequency bands and at least one signal that is orthogonally polarized with respect to the co-polarized signals, at least a portion of the waveguide being overmoded; a plurality of junctions spaced along the length of the main waveguide for coupling selected signals in the different frequency bands in and out of the waveguide, at least one of the junctions being located in an overmoded portion of the waveguide, each of the junctions having an unbalanced or pseudo-balanced feed with only a single side-arm waveguide for transmitting and receiving the signals; and filtering means disposed within the main waveguide and operatively associated with each junction therein for signals in the highest frequency band, the filtering means having (1) a stopband characteristic for coupling signals in the highest frequency band between the main waveguide and the junction and the side-arm waveguide connected thereto, and (2) a passband characteristic for passing signals in lower frequency bands past the junction. In the preferred embodiment of the invention, the waveguide has an overmoded section with a square cross-section and a single-moded section with a rectangular cross-section, with the overmoded and singlemoded sections being joined by a transition section having at least one side wall which is tapered to effect the transition from the square cross-section to the rectangular cross-section.

TECHNICAL FIELD 
The present invention relates generally to microwave systems, and, more 
particularly, to microwave combining networks commonly referred to as 
"combiners". Combiners are devices that are capable of simultaneously 
transmitting and/or receiving two or more different microwave signals. The 
present invention is particularly concerned with combiners which can 
handle co-polarized signals in two or more frequency bands and, if 
desired, in combination with one or more orthogonally polarized signals; 
the orthogonally polarized signals can also be handled in two or more 
frequency bands. 
BACKGROUND ART 
In the propagation of microwave signals, it is generally desired to confine 
the signals to one propagation mode in order to avoid the distortions that 
are inherent in multimode propagation. The desired propagation mode is 
usually the dominant mode, such as the TE.sub.10 mode in a square 
waveguide. The higher order modes can be suppressed by careful 
dimensioning of the waveguide such that the higher order modes are cut 
off. In certain instances, however, it is necessary for portions of the 
waveguide to be large enough to support more than one frequency band, and 
a discontinuity in such a waveguide can give rise to undesired higher 
order modes. For this reason, such waveguide sections are often referred 
to as "multi-mode" or "overmoded" waveguide. 
One example of a waveguide system that requires an overmoded waveguide 
section is a system that includes a multi-port, multi-frequency combiner. 
For example, four-port combiners are typically used to permit a single 
antenna to launch and/or receive microwave signals in two different 
frequency bands in each of two orthogonal polarizations. Each of these 
frequency bands is usually at least 500 MHz wide. For instance, present 
telecommunication microwave systems generally transmit signals in 
frequency bands which are referred to as the "4 GHz", "6 GHz" and "11 GHz" 
bands, but the actual frequency bands are 3.7 to 4.2 GHz, 5.925 to 6.425 
GHz, and 10.7 to 11.7 GHz, respectively. Signals of a given polarization 
in any of these bands must be propagated through the combiner without 
perturbing signals in any other band, without perturbing orthogonally 
polarized signals in the same band, and without generating unacceptable 
levels of unwanted higher order modes of any of the signals. 
Elaborate and/or costly precautions have previously been taken to avoid the 
discontinuities that could give rise to undesired higher order modes in 
multi-frequency combiners of the type described above. For example, U.S. 
Pat. No. 4,077,039 discloses such a combiner that uses a pseudo-balanced 
feed in the tapered portion of a flared horn, in combination with 
evanescent mode waveguide filters in the side arms of the high frequency 
port of the combiner. The basic dilemma posed by the multi-port, 
multi-frequency combiners is that undesired mode-generating 
discontinuities must be avoided in the overmoded waveguide sections, and 
yet some means must be provided for coupling selected signals with one or 
more ports located in the overmoded section of waveguide. Previous 
solutions of this dilemma have involved various complex, costly and/or 
physically cumbersome designs. 
In co-pending U.S. patent application Ser. No. 384,997, filed June 4, 1982, 
for "Multi-Port Combiner for Multi-Frequency Microwave Signals", assigned 
to the assignee of the present invention, there is described an improved 
multi-port combiner that can be economically manufactured and yet provides 
excellent performance characteristics when used with co-polarized signals 
in two or more frequency bands. 
DISCLOSURE OF THE INVENTION 
It is a primary object of the present invention to provide an improved 
multi-port, multi-frequency combiner having a different physical 
structure, new coupling mechanisms, and significantly improved operating 
characteristics. More particularly, an objective of this invention is to 
provide such a combiner which does not require the use of balanced feeds 
in many applications, thereby reducing the cost of the combiner; which 
permits relatively wide separation of frequency bands; which provides high 
power-handling capability; which has excellent isolation among junctions, 
frequency bands and polarization planes; which is relatively easy to tune, 
thereby further reducing manufacturing costs; and/or which permits 
relatively wide mechanical tolerances while still meeting competitive 
performance specifications. 
The present invention realizes the foregoing objectives by providing a 
multi-port, multi-frequency combiner comprising a main waveguide having a 
cross section in the shape of a right-angle parallelogram and dimensioned 
to simultaneously propagate co-polarized signals in different frequency 
bands and at least one signal that is orthogonally polarized with respect 
to the co-polarized signals, at least a portion of the waveguide being 
overmoded; a plurality of junctions spaced along the length of the main 
waveguide for coupling selected signals in the different frequency bands 
in and out of the waveguide, at least one of the junctions being located 
in an overmoded portion of the waveguide, each of the junctions having an 
unbalanced or pseudo-balanced feed with only a single side-arm waveguide 
for transmitting and receiving the signals; and filtering means disposed 
within the main waveguide and operatively associated with each junction 
therein for signals in the highest frequency band, the filtering means 
having (1) a stopband characteristic for coupling signals in said highest 
frequency band between the main waveguide and the junction and the 
side-arm waveguide connected thereto, and (2) a passband characteristic 
for passing signals in lower frequency bands past the junction. 
In the preferred embodiment of the invention, the waveguide has an 
overmoded section with a square cross-section and a single-moded section 
with a rectangular cross-section, with the overmoded and single-moded 
sections being joined by a transition section having at least one side 
wall which is tapered to effect the transition from the square 
cross-section to the rectangular cross-section. 
It is to be understood that the term "rectangular" is used herein in a 
limited sense, meaning a right-angle parallelogram with unequal sides. The 
generic term "right-angle parallelogram" is used to encompass both squares 
(equal sides) and rectangles (unequal sides).

BEST MODE FOR CARRYING OUT THE INVENTION 
While the invention has been shown and will be described in some detail 
with reference to specific exemplary embodiments, there is no intention 
that the invention be limited to these particular embodiments. On the 
contrary, it is intended to cover all modifications, alternatives and 
equivalents which may fall within the spirit and scope of the invention as 
defined by the appended claims. 
Turning now to the drawings and referring first to FIGS. 1 through 7, there 
is shown a four-port combiner whose forward portion includes a square 
waveguide 10 with an open end or mouth 11 through which signals are 
propagated to and from four junctions A, B, C and D. The other end 12 of 
the combiner is also open, serving as the junction D. The four junctions 
A, B, C and D are spaced along the length of the combiner for transmitting 
and receiving two pairs of co-polarized signals in two different frequency 
bands. More specifically, junctions A and B are longitudinally aligned 
with each other for supporting one pair of co-polar signals, and junctions 
C and D are similarly aligned for supporting the other pair of co-polar 
signals. One of the junctions in each aligned pair, namely junction A in 
one pair and junction C in the other pair, is dimensioned to transmit and 
receive signals in the higher frequency band, while the other two 
junctions B and D are dimensioned to transmit and receive signals in the 
lower frequency band. For example, in a typical application junctions A 
and C handle orthogonally polarized signals in the 6-GHz frequency band 
(5.925 to 6.425 GHz), and junctions B and D handle orthogonally polarized 
signals in the 4-GHz frequency band (3.7 to 4.2 GHz). The microwave 
signals can be transmitted in one of these frequency bands and received in 
the other frequency band, or the signals can be simultaneously transmitted 
and received in both frequency bands using the different polarizations. 
The square waveguide 10 is wide enough, along both transverse axes, to 
permit the propagation therethrough of the two orthogonally polarized, 
low-frequency signals, as well as the orthogonally polarized 
high-frequency signals. Thus, the square waveguide 10 is necessarily 
overmoded. The rear portion of the combiner, on the other hand, handles 
only one pair of co-polarized signals, and thus is formed from a 
single-moded rectangular waveguide section 13. Between the rectangular 
rear section 13 and the square front section 10 is a transition section 14 
which tapers from a rectangular cross-section at one end to a square 
cross-section at the other end. 
As can be seen most clearly in FIGS. 4 and 5, the slots which are formed in 
the walls of the waveguide sections 10, 14 and 13 to define the locations 
of the three junctions A through C have rectangular configurations, and 
each of these slots is connected to a corresponding side-arm waveguide of 
rectangular cross-section. Each of the two high-frequency junctions A and 
C includes a pair of diametrically opposed slots to form a pseudo-balanced 
coupling between the main waveguide 10 or 13 and the side-arm waveguides 
at these junctions. The rectangular slots at all three junctions A, B and 
C are of the H-plane type and have their long dimensions extending in the 
longitudinal direction, i.e., parallel to the main axis of the combiner. 
It has been found that with the main waveguide used in the combiner of this 
invention, the slots leading to the side-arm waveguides can be made of 
different sizes. For example, the slots may have a length of about 40 to 
100% of the broad dimension of the side-arm waveguide and a width of about 
40 to 100% of the narrow dimension of the side-arm waveguide. Although the 
illustrative combiner utilizes such a wide slot only at junction C, the 
slots at junctions A and B could be widened to increase the power-handling 
capability of the combiner, as well as to widen the bandwidth. 
Examining the first high-frequency junction A in more detail, the slots at 
this junction are in the form of two diametrically opposed irises 20 and 
21 coupled to a rectangular side-arm waveguide 22 and a stub waveguide 23, 
respectively. A shorting plate 24 closes the outer end of the stub 
waveguide 23. The purpose of the stub waveguide 23 and its iris 21 is to 
produce the desired impedance matching at the high-frequency junction A, 
providing shunt stub tuning that reduces the return loss while at the same 
time eliminating excitation of non-symmetrical higher order modes. As can 
be seen most clearly in FIGS. 1 and 3, a plurality of tuning screws 22a-d 
are provided in one wall of the side arm 22 to facilitate the tuning of 
junction A. 
The structure of the other high-frequency junction, junction C, is similar 
to that of junction A, except that everything is rotated 90.degree. around 
the main axis of the combiner, and there are no irises in the slots. Thus, 
junction C has two diametrically opposed slots 30 and 31 coupled to a 
rectangular side-arm waveguide 32 and a stub waveguide 33, with a shorting 
plate 34 closing the outer end of the stub waveguide 33. The stub 
waveguide 33 is provided with tuning screws 33a-d, and the side-arm 
waveguide 32 is provided with a single tuning screw 32a. 
Turning next to the low-frequency junction B, this junction has only a 
single rectangular slot 40 connected to a single rectangular side-arm 
waveguide 41. The center of this junction is preferably aligned with the 
center of the tapered side wall 42 of the transition section 14 so that 
the tapered wall 42 operates as a miter bend that, in conjunction with the 
pins and tuning screws described below, guides the low-frequency signals 
between the slot 40 and the combiner mouth 11 leading to the antenna. The 
tapered side wall 42 also operates as a transformer between both junctions 
C and D and the antenna. 
The second low-frequency junction D is formed by the open end 12 of the 
single-moded rectangular waveguide section 13. This junction handles the 
low-frequency signals which are polarized orthogonally with respect to the 
low-frequency signals handled at junction B. 
In order to couple the desired signals into the slots at the respective 
junctions A, B and C, and to pass the other signals past each slot, 
filtering means are provided at the two high-frequency junctions A and C. 
More particularly, the filtering means associated with each of the high 
frequency junctions A and C have stopband characteristics for coupling the 
high frequency signals between the main waveguide section 10 or 13 and the 
high-frequency slots and side arms, and a passband characteristic for 
passing low-frequency signals past the slots of the high-frequency 
junctions. In addition, the filtering means and the geometry of the 
high-frequency junctions suppress spurious excitation of signals in 
undesired propagation modes different from the mode in which the desired 
signals are being propagated. 
No filters are required in any of the side-arm waveguides, though side-arm 
filters may be added as optional features if desired. The high-frequency 
slots and side arms at junctions A and C are dimensioned to support only 
the high frequency signals; thus, these slots and side arms themselves 
serve to filter out any low frequency signals. At the low frequency 
junction B, both the low frequency and high frequency signals to be passed 
by this junction are orthogonally polarized relative to the slot 40, and 
thus no filters are required in the side arm 41. At the low frequency 
junction D, only the desired low-frequency signal is present, and thus 
there is no need for any filters whatever. 
In the particular embodiment illustrated, the filtering network associated 
with the first 6-GHz junction (junction A) takes the form of two opposed 
rows of conductive posts, 50h-l and 51h-l extending into the square 
waveguide 10 along a plane located midway between and parallel to the two 
irises 20 and 21, plus a pair of offset posts 50m, 51m. These posts 50h-m 
and 51h-m form a filter which is virtually invisible to the orthogonally 
polarized signals of junctions C and D. This filter has a stopband 
characteristic which couples one of the two orthogonally polarized 6-GHz 
signals into the side arm 22 of junction A, and a passband characteristic 
which allows the co-polarized 4-GHz signal to pass junction A unimpeded. 
That is, the locations and lengths of posts 50h-m and 51h-m are selected 
to pass the 4-GHz signals for junction B and to reject the co-polarized 
6-GHz signals, thereby diverting the latter into the desired side arm 22. 
Both the 4-GHz and the 6-GHz signals that are orthogonally polarized 
relative to the 6-GHz signal coupled to junction A pass the junction-A 
filter unimpeded. 
Two additional sets of opposed conductive posts 50a-g and 51ag on the front 
side of junction A match both the 4-GHz and the 6-GHz signals for 
junctions A and B, thereby minimizing the VSWR for those signals. 
In addition to the posts 50a-m, 51a-m, two further rows of opposed posts 
50n-r, 51n-r extend into the square waveguide 10 along a plane that is 
perpendicular to the plane of the posts 50a-l 51a-l. That is, the plane of 
the posts 50n-r, 51n-r longitudinally bisects the irises 20, 21 of 
junction A. These posts cooperate with certain of the posts at junction B 
to match both the 4-GHz and 6-GHz signals for junctions C and D. 
The particular filter arrangement illustrated is only one example of a 
configuration that has been found to produce good results in a 
four-junction combiner for orthogonally polarized 4 and 6 GHz signals; it 
will be understood that other configurations will produce similar results 
for the same or different frequency bands and/or for different waveguide 
configurations. Similarly, the posts which are in the form of screws for 
easy adjustment of radial length in the illustrated embodiment, may be 
replaced by balanced vanes, fins, rods, pins or other tunable devices. 
The filtering network associated with the second 6-GHz junction (junction 
C) is formed by a set of conductive posts 60a-l extending into the 
rectangular waveguide 13. Posts 60a-h and 60m-p are centered on a plane 
located midway between the two irises 30 and 31, while posts 60i-l are 
located symmetrically on opposite sides of that plane. The filter formed 
by this set of posts 60a-l is similar to the filter formed by the two sets 
of posts 50h-m and 51h-m at junction A, in that both filters have similar 
stopband and passband characteristics, i.e., the filter formed at junction 
C by the posts 60a-l has a stopband characteristic which couples the 6-GHz 
signal into the side arm 32 of junction C, and a passband characteristic 
which allows the co-polarized 4-GHz signal to pass junction C unimpeded. 
Turning next to the 4-GHz junction B, two opposed sets of posts 70a-b and 
71a-b and a further single set of posts 70c-i associated with this 
junction, in cooperation with posts 50n-r, 51n-r at junction A, match the 
4 and 6-GHz signals of junctions C and D. Additional posts 70j-o and 71j-l 
match the 4-GHz signal for junction B, helping to guide this signal into 
the junction B side arm 41. This junction also includes a set of 
transverse pins 72 which cooperate with the tapered side wall 42 to direct 
the 4-GHz signal from the antenna to the junction B side arm 41. 
The electrical characteristics of the components of the present invention 
are well known. For example, in Starr, Radio and Radar Technique, Sir 
Isaac Pitman & Sons, London, 1953, pp. 126-133, the author describes the 
electric characteristics of discontinuities in waveguides, and 
specifically describes the impedance of posts having a certain diameter, 
depth of waveguide penetration, and length. Further, in Harvey, Microwave 
Engineering, Academic Press, New York, 1963, 214-219, the author discloses 
the use of posts in the construction of a pass-band filter. In addition, 
the effect of stub guides for impedance matching is also well known, and 
is generally described in Terman, Electronic And Radio Engineering, 
McGraw-Hill, New York, 4th Ed. 1965, pp. 149-150. 
One specific example of the combiner shown in FIGS. 1-7 was made of brass 
with a waveguide section 10 of square cross-section, 1.812".times.1.812", 
joined to an intermediate waveguide section 14 of similar square 
cross-section at one end and tapered down to a rectangular cross-section, 
1.812".times.0.872", at the other end. The third waveguide section 13 had 
a rectangular cross-section along its full length, tapering from 
1.812".times.0.872" to 2.290".times.1.145". The 6-GHz junction A had 
0.94".times.0.30" rectangular iris, while the 6-GHz junction C had a WR137 
rectangular waveguide side arm, stub and slots. The stub at junction A was 
0.813" in length, and the junction-C stub was 2.34" long. The 4-GHz 
junction in the intermediate section 14 had a 1.7".times.0.3" rectangular 
iris, and the 4-GHz side arm was WR181 rectangular waveguide. The 
locations and lengths of the posts and pins associated with the various 
junctions were as shown in FIGS. 1-5 described above. 
In tests using orthogonally polarized signals (each signal being linearly 
polarized) in each of two frequency bands extending from 3.690 to 4.210 
GHz and from 5.915 to 6.435 GHz, this combiner produced the following 
results: 
______________________________________ 
Return Loss, Junctions D, C: 
30 dB 
Return Loss, Junctions B, A: 
33 dB 
Polarization Isolation, 4 GHz: 
39 dB 
Polarization Isolation, 6 GHz: 
43 dB 
Isolation Between Ports A & B: 
60 dB 
Isolation Between Ports A & D: 
92 dB 
Isolation Between Ports C & B: 
46 dB 
Isolation Between Ports C & D: 
60 dB 
______________________________________ 
In the tests described above, the TM.sub.11 and TE.sub.11 higher order 
modes were excited and observed as mode pips in the discrimination curve 
in the 6-GHz band. To eliminate or at least reduce the generation of such 
higher order modes, the transition between the square and rectangular 
sections of the main waveguide can be effected by symmetrically tapering a 
pair of opposed side walls, rather than tapering only a single side wall. 
One example of such a transition is illustrated in FIG. 8. In this 
embodiment, the transition waveguide section 114 has a pair of opposed 
side walls 114a and 114b which are tapered symmetrically relative to the 
central axis of the combiner. The tapered side walls 114a, 114b do not 
serve as a miter bend for the coupling of signals to and from the side arm 
41, and thus additional pins 172 and posts 170 are added to perform this 
function. It will be noted that the tapered side walls 114a, 114b are not 
only symmetrical, but also are tapered in a non-linear configuration to 
reduce VSWR and avoid excitation of the TM.sub.11 and TE.sub.11 modes; 
this non-linear taper is useful with either the dual tapered side walls of 
FIG. 8 or the single tapered side wall of FIGS. 1-7. Another example of a 
suitable non-linear configuration is a stepped side wall. 
While the invention has been described above with particular reference to 
an exemplary four-port combiner, it will be appreciated that the invention 
is applicable to a large number of different combiner configurations 
having two or more longitudinally spaced junctions for handling signals in 
two or more different frequency bands. The signals in one or all of the 
different frequency bands may be orthogonally polarized, and the 
cross-section of the main waveguide can be square along the entire length 
thereof if desired. 
As can be seen from the foregoing detailed description, this invention 
provides an improved multi-port, multi-frequency combiner which does not 
require the use of balanced feeds in many applications, thereby reducing 
the cost of the combiner. The main waveguide of the combiner has a 
right-angle parallelogram cross-section along its entire length, and thus 
the longitudinal slots at the junctions do not generate any higher order 
modes in many dual-band applications (e.g., 4 and 6 GHz). The square 
and/or rectangular cross-sections of the main waveguide also provide 
extremely good polarization-holding properties as well as good isolation 
among the various junctions in different frequency bands. The improved 
combiner is also relatively easy to tune, particularly in the absence of 
any balanced feeds, thereby reducing the manufacturing cost. The power 
handling capability of the combiner can also be significantly increased by 
increasing the width of the irises opening into the various side arms. The 
particular embodiment illustrated in FIG. 8, with the symmetrically 
tapered side walls in the transition between the square and rectangular 
waveguide sections, is particularly advantageous in avoiding the 
excitation of the TE.sub.11 and TM.sub.11 modes and in improving VSWR.