A diplexer for electromagnetic signals of higher and lower frequency is formed of a common waveguide channel for both signals, the common channel branching into a through waveguide channel and a side waveguide channel. The through channel includes a filter having a pass band for propagation of the lower frequency signal and inhibiting propagation of the higher frequency signal. The side channel is formed as a waveguide below cut-off frequency with respect to the lower frequency signal for inhibiting propagation of the lower frequency signal while permitting propagation of the higher frequency signal. A coupling aperture formed as a slot resonant at the higher frequency is located in a waveguide wall at an integral number of quarter guide wavelengths in front of the filter for coupling the higher frequency signal between the common and the side channels.

BACKGROUND OF THE INVENTION 
This invention relates to a microwave diplexer providing separate 
propagation paths to electromagnetic radiations of two different 
frequencies and, more particularly, to a diplexer having one branch 
constructed as a waveguide below cutoff to radiation at a lower of the two 
frequencies while passing radiation at a higher of the two frequencies, 
and having a second branch with a band pass filter for allowing 
propagation of the lower frequency radiation while inhibiting propagation 
of the higher frequency radiation. 
Microwave diplexers are employed in circuits handling signals at more than 
one frequency band. A typical circuit is found in a communication system 
such as a broadcast system employing a satellite for retransmission of 
radio and television signals. The satellite carries an antenna with a feed 
structure that illuminates the antenna, or receives from the antenna, 
signals at different frequencies. A diplexer couples the feed structure to 
transceivers operating at the different frequencies. 
Generally speaking, the diplexer may be described as having three branches, 
or channels, which are constructed of waveguide. These branches are a 
common branch operative at both the higher and the lower frequencies to 
couple electromagnetic power between the antenna and the two transceivers. 
The common channel branches into a through channel and a side channel 
which are operative to separate microwave signals at the two frequencies. 
The through channel connects with circuitry such as a receiver or 
transceiver operative at one frequency, while the side channel connects 
with circuitry such as a receiver or transceiver operative at the other 
frequency. 
The diplexer is usually constructed with tuned elements in both the through 
and the side channels to form filters in these channels so that each of 
these two channels propagates radiation at only one of the two frequency 
bands while inhibiting propagation at the other of the frequencies. 
Thereby, the diplexer can separate incoming signals at the two frequency 
bands, and can couple outgoing signals at the two frequency bands to a 
common feed of the antenna. 
A problem arises in that the construction of a diplexer with filters of 
discrete microwave elements in two of the branches adds undue complexity 
to the manufacturing process, and may also prevent a minimizing of the 
physical size of the diplexer. 
SUMMARY OF THE INVENTION 
The aforementioned problems are overcome and other advantages are provided 
by a microwave diplexer constructed in accordance with the invention, the 
diplexer comprising a first section of waveguide and a second section of 
waveguide joining the first section. The first section serves as a common 
channel of the diplexer and terminates in a common port for propagation of 
radiation at the two frequency bands. The second section serves as a 
through channel of and terminates in a through port for propagation of 
radiation at the lower frequency. 
A third section of waveguide joins the first section of waveguide, the 
third section serving as a side channel of the diplexer and terminates in 
a side port for propagation of radiation at the higher frequency. A filter 
is provided in the through channel for inhibiting a propagation of the 
higher frequency radiation while permitting propagation of the lower 
frequency radiation. The filter is constructed of a series of two or more 
inductive irises spaced apart by one or more resonant cavities. 
The side channel is provided, in at least a portion thereof, with 
cross-sectional dimensions large enough to sustain a propagating mode of 
the higher frequency radiation and small enough to provide an evanescent 
mode for the lower frequency radiation to inhibit propagation of the lower 
frequency radiation. There is a coupling aperture in a broad wall of the 
first waveguide section for coupling radiation from the common channel to 
the side channel. The aperture is configured as a slot resonant at the 
higher frequency. A first of the filter irises closest the coupling 
aperture is located an odd number of one-quarter guide wavelengths at the 
higher-frequency radiation from the coupling aperture to reflect the 
higher frequency radiation back to the coupling aperture with a maximum 
value of electric field at the coupling aperture to maximize propagation 
of the higher frequency radiation between the common channel and the side 
channel.

DETAILED DESCRIPTION 
With reference to FIG. 1, there is shown a diplexer 10, constructed in 
accordance with the invention and being suitable for use with microwave 
circuitry in the processing of electromagnetic signals. By way of example 
in the use of the diplexer 10 in a satellite communication system, the 
diplexer 10 is employed with an antenna 12 having a reflector 14 and a 
feed 16. The diplexer 10, the feed 16 and the reflector 14 are supported 
by a support 18, indicated in phantom, which support may be a satellite 
circumnavigating the earth for use in a communication system. The diplexer 
10 is constructed of a waveguide 20 with a section 22 of waveguide 
extending from the side of the waveguide 20 to form three channels, 
namely, a common channel 24, a through channel 26 and a side channel 28. 
The three channels terminate respectively in three ports, namely, a common 
port 30, a through port 32 and a side port 34. The through port 32 and the 
side port 34 are connected, by way of example, to transceivers 36 and 38. 
The common port 30 is connected to the feed 16 for transmission of signals 
from the transceivers to the reflector 14 to form a beam 40 of radiation. 
With reference also to FIGS. 2, 3 and 4, the diplexer 10 further comprises 
a filter 42 which is formed of three inductive irises 44, 46 and 48 which 
are spaced apart along an axis 50 of the waveguide 20 to define a series 
of two cavities 52 and 54 of the filter 42. Capacitive tuning screws 56 
are provided for tuning the filter 42. Also included in the diplexer 10 is 
a coupling aperture in the form of a resonant slot 58 for coupling 
electromagnetic energy between the side channel 28 and a first section of 
the waveguide 20. The first section of the waveguide 20 extends from the 
common port 30 to a first one of the irises, namely the iris 44, the first 
section being co-terminous with the common channel 24. A second section of 
the waveguide 20 extends from the first iris 44 to the through port 32 and 
houses the filter 42, the second section being co-terminous with the 
through channel 26. Two shims 60 are located in the waveguide section 22 
at opposite ends of the slot 58 and are disposed parallel to an axis 62 of 
the waveguide section 22. 
The waveguide 20 comprises a broad top wall 64 and an opposed broad bottom 
wall 66 which are joined by narrow sidewalls 68 to provide a rectangular 
cross section to the waveguide 20. In a preferred embodiment of the 
invention, the ratio of the widths of the broad top wall 64 to a sidewall 
68 is 2:1. The waveguide section 22 comprises broad walls 70 which are 
joined by narrow sidewalls 72 to provide a rectangular cross section to 
the waveguide section 22. In a preferred embodiment of the invention, the 
ratio of the widths of a broad wall 70 to a sidewall 72 is 2:1. 
The slot 58 passes through the top wall 64, is located symmetrically about 
the axis 62, and extends in its longitudinal dimension parallel to the 
broad wall 70 of the waveguide section 22 and perpendicular to the 
sidewalls 68 of the waveguide 20. The perimeter of the slot 58 is equal to 
one free-space wavelength at the center of the band of radiation to be 
coupled by the slot 58 between the channels 24 and 28. The center of the 
slot 58 is located between the first iris 44 and the common port 30 at 
distance equal to an odd number of one-quarter guide wavelengths, 
preferably one-quarter guide wavelength, from the iris 44. 
In the filter 42, the irises 44, 46 and 48 extend from the top wall 64 to 
the bottom wall 66, and abut the sidewalls 68. The outermost irises 44 and 
48 define apertures 74 and 76, respectively, which are of equal width and 
are wider than the aperture 78 defined by the central iris 46. If the 
filter 42 were constructed with only one cavity, then there would be only 
two irises defining equal apertures. If the Filter 42 were constructed 
with three or more cavities, then there would be additional irises with 
varying aperture sizes symmetrically positioned about a center of the 
filter, the aperture sizes narrowing toward the center of the filter. The 
filter 42 is constructed in accordance with well known technology to 
provide a pass band at the microwave frequencies which are to propagate 
via the through channel 26, and to provide a stop band at the microwave 
frequencies which are to propagate via the side channel 28. The tuning 
screws 56 are located along the center of the bottom wall 66 and penetrate 
into the waveguide 20 a relatively small distance, typically less than ten 
percent of the distance between the broad walls 64 and 66. 
In the operation of the preferred embodiment of the invention, the diplexer 
10 operates at signal frequencies 12 and 14 GHz (gigahertz). Both signals 
propagate through the common channel 24. The lower frequency 12 GHz signal 
propagates in the through channel 26 centered in a pass band having a 
width of approximately 1.0 GHz provided by the filter 42. The higher 
frequency 14 GHz signal propagates in the side channel 28 centered in a 
pass band having a width of approximately 1.0 GHz provided by the slot 58. 
Operation of the diplexer 10 is reciprocal such that microwave signals can 
propagate in either direction between the ports 30 and 32, and between the 
ports 30 and 34. The waveguide 20 is fabricated of WR-75 waveguide having 
interior dimensions of 0.75 inch by 0.375 inch. The waveguide section 22 
is fabricated of WR-62 waveguide having interior dimensions of 0.622 inch 
by 0.311 inch. The shims 60 are positioned contiguous the sidewalls 72, 
extend the full distance between the broad walls 70, and abut the top wall 
64 of the waveguide 20. Each of the shims 60 has a length of 1.0 inch 
which is greater then the guide wavelength of the waveguide section 22 at 
14 GHz. 
The shims 60 reduce the distance between the sidewalls 72 to 0.460 inch 
resulting in a cutoff frequency of approximately 12.8 GHz in the region of 
the waveguide section 22 between the shims 60. In the filter 42, each of 
the cavities 52 and 54 extends along the waveguide axis 50 a distance of 
approximately 0.5 inch, this being slightly less than one-half the guide 
wavelength at 12 GHz, and functions as a resonator tuned to resonate at 12 
GHz. The aperture of the iris 44 and of the iris 48 is 0.45 inch as 
measured in a direction parallel to the top wall 64. The aperture of the 
iris 46 is 0.25 inch. The higher frequency 14 GHz signal is attenuated 
sufficiently by the filter 42 so that, as a practical matter, the higher 
frequency signal may be regarded as not propagating through the filter 42. 
The cross-sectional dimensions of the waveguide section 22 within the 
region of the shims 60 are large enough so as to allow propagation of the 
higher frequency signal. The cross-sectional dimensions of the shim region 
are too small to sustain a propagating mode at the lower 12 GHz frequency, 
and provide for an evanescent mode which severely attenuates the lower 
frequency signal so that, as a practical matter, the lower frequency 
signal may be regarded as not propagating in the waveguide section 22. 
The first iris 44 reflects the higher frequency signal back towards the 
common port 30 to produce a standing wave having a maximum value of 
electric field one-quarter guide wavelength in front of the first iris 44. 
The placement of the slot 58 one-quarter guide wavelength at the higher 
frequency in front of the first iris 44 maximizes coupling of the higher 
frequency signal via the slot 58 between the common channel 24 and the 
side channel 28. The aforementioned bandwidth at the higher frequency 
signal is dependent of the dimensions of the slot 58, a narrower slot 
providing a narrower bandwidth. The length of the slot 58 is 0.42 inch, 
this being approximately one-half the free-space wavelength at the higher 
frequency. The width of the slot 58 is 0.040 inch. If desired, the slot 
width may be enlarged to 0.060 inch or decreased to 0.030 inch to increase 
or decrease the bandwidth of the signals coupled between the common and 
the side channels. 
The microwave signals at both frequency bands are transverse electric 
signals TE10 with the electric vector being perpendicular to the broad 
walls 64 and 66 in the waveguide 20, and perpendicular to the broad walls 
70 in the waveguide section 22. In the slot 58 the electric field extends 
across the slot perpendicular to the long sides of the slot. The overall 
length and width of the diplexer 10 measure 3.5 inch by 1.7 inch. Thus, 
the diplexer of the invention has a compact structure which is simpler and 
more readily manufactured than other diplexers heretofore. 
It is to be understood that the above described embodiment of the invention 
is illustrative only, and that modifications thereof may occur to those 
skilled in the art. Accordingly, this invention is not to be regarded as 
limited to the embodiment disclosed herein, but is to be limited only as 
defined by the appended claims.