Horn radiator assembly with stepped septum polarizer

A circularly polarized wave entering the horn is converted to a linearly polarized wave which appears in one of the ports depending on the sense of the polarization. The stepped configuration of the impedance matching section minimizes mutual coupling among horns of the respective assemblies of the array antenna.

BACKGROUND OF THE INVENTION 
This invention relates to a microwave horn radiator assembly for radiating 
circularly polarized electromagnetic waves from a radiator at a front 
portion of the assembly, the assembly including an orthomode transducer 
providing a conversion between linearly and circularly polarized 
radiation. More particularly, the invention employs a septum increasing 
stepwise monotonically in height from a bottom wall to a top wall of a 
square waveguide to provide two rectangular waveguide ports at a back end 
of the assembly, opposite the horn radiator, to provide the conversion 
between linearly and circularly polarized radiation. The assembly also 
includes plural stepped waveguide sections forming an impedance matching 
section behind the horn radiator, and a set of capacitive posts between 
the matching section and the septum. 
An antenna comprising an array of radiators providing circularly polarized 
radiation may be employed in numerous situations, including a mounting of 
the antenna on board a spacecraft to provide for communication between the 
spacecraft and a station on the earth. In the construction of the antenna, 
each of the radiators is formed as a part of a radiator assembly which 
includes microwave structures for converting a linearly polarized 
electromagnetic wave to a circularly polarized electromagnetic wave for 
transmission of a microwave signal, and for converting from circularly 
polarized radiation to linearly polarized radiation upon reception of a 
microwave signal. It has been the practice to employ an orthomode 
transducer to provide for the conversion between the linearly and 
circularly polarized radiation. The microwave structure for polarization 
conversion is substantially larger than the radiator itself. In a typical 
construction of orthomode transducer, perpendicularly oriented rectangular 
waveguides have been employed to provide for both right-hand and left-hand 
circularly polarized waves. 
A problem arises in that the foregoing construction is inconvenient because 
of the excessively large size required of the microwave structure, 
including the orthomode transducer, which feeds electromagnetic power to 
the radiator, and which receives incoming signals from the radiator for 
each of the radiators of the array antenna. A further disadvantage in the 
foregoing construction is excessive complexity in the manufacturing 
process required to produce the microwave structure. Also, it is noted 
that a large bandwidth is advantageous in the use of communication 
equipment, and the foregoing construction has been disadvantageous in 
respect to a limitation of the maximum bandwidth available for 
communication. The physical size has been enlarged also because of a need 
for numerous tuning screws, the need for such tuning also complicating the 
manufacture and set-up procedure. Also, the radiator should be operated in 
such a fashion as to minimize mutual coupling between signals of the 
various radiators of the antenna array. 
SUMMARY OF THE INVENTION 
The aforementioned problems are overcome and other advantages are provided 
by a multiple-band horn radiator assembly and an antenna comprising an 
array of such horn radiator assemblies which incorporate the invention to 
provide sufficient bandwidth to combine functions of transmit, receive, 
and tracking frequencies in each horn radiator assembly. The construction 
of the invention minimizes hardware, reduces weight, and saves 
manufacturing time. 
Each horn radiator assembly comprises a waveguide section of square 
cross-section arranged coaxially with a circular cylindrical horn. The 
horn provides a radiating aperture for radiation of circularly polarized 
electromagnetic waves. The waveguide section is coupled via an impedance 
matching section to the horn radiator. Within the waveguide section, and 
extending in the direction of a longitudinal axis of the waveguide 
section, there is provided a septum which increases gradually in height 
from the bottom wall to the top wall via a series of steps with 
progression in a direction away from the horn radiator. At the maximum 
height of the septum, the septum extends from the bottom wall to a top 
wall of the waveguide section and bisects a rear portion of the waveguide 
section into two rectangular waveguides which serve as input ports of an 
orthomode transducer for injecting linearly polarized radiation to be 
converted to either right or left-handed circularly polarized waves. The 
septum introduces a phase-shift characteristic of decreasing phase shift 
with increasing frequency. This phase-shift characteristic is 
counterbalanced by a line of capacitive teeth disposed on the bottom wall 
in front of the septum to provide a phase-shift characteristic wherein 
phase shift increases with increasing frequency. The diameter of the horn 
radiator is larger than the height of a sidewall of the waveguide section, 
and the impedance matching section comprises two waveguide sections of 
decreasing cross-sectional sides wherein a forward section connecting with 
the horn is circular and a back section connecting with the aforementioned 
waveguide section is square in cross section.

DETAILED DESCRIPTION 
FIGS. 1-8 show a horn radiator assembly 30 which, in accordance with the 
invention, comprises a circular cylindrical horn 32, a waveguide section 
34 of square cross-section, and an impedance matching section 36 which 
connects a front end of the waveguide section 34 with a back end of the 
horn 32. The waveguide section 34 includes a septum 38 which extends along 
a center line of the waveguide section 34, and bisects the back end of the 
waveguide section 34 to form two ports 40 and 42. A transmit/receive 
circuit 44, indicated diagrammatically, connects with the ports 40 and 42 
for applying linearly polarized radiation to one or both of the ports 
40-42 to be converted by the assembly 30 to circularly polarized radiation 
which radiates as a beam 46 from the horn 32. The assembly 30 operates 
also in reciprocal fashion such that a circularly polarized wave, incident 
upon the horn 32, is converted to a linearly polarized electromagnetic 
wave appearing in either the port 40 or 42, depending on the direction of 
circular polarization, to be received at the circuit 44. 
The impedance matching section 36 comprises a forward section of a 
waveguide 48 of circular cross-section which connects with a back end of 
the horn 32, and a rear section of waveguide 50 of square cross-section 
which interconnects the forward waveguide section 48 with a front end of 
the waveguide section 34. The diameter of the forward waveguide section 48 
is smaller than the diameter of the horn 32. The height of a wall, such as 
a sidewall 52 of the rear waveguide section 50 is smaller than a diameter 
of the forward waveguide section 58. The decreasing magnitude of the 
dimensions of the waveguide sections 48 and 50 relative to the dimension 
of the horn 32 provides for a stepped configuration to the impedance 
matching section 36. The square waveguide section 50 includes the 
aforementioned sidewall 52, and a sidewall 54, and top and bottom walls 56 
and 58 which are joined by the sidewalls 52 and 54. The forward waveguide 
section 48 comprises a cylindrical sidewall 60 and a back wall 62. The 
horn 32 comprises a cylindrical sidewall 64 and a shelf 66 at an interface 
between the horn 32 and the forward waveguide section 48. The horn 32 and 
the impedance matching section 36 are formed integrally as a unitary 
structure which is provided with a flange 68 at the back end of the 
impedance matching section 36 for mating with the waveguide section 34 via 
a flange 70 located at the front end of the waveguide section 34. A 
further flange 72 is provided at the back end of the waveguide section 34 
for connection with other microwave components, such as components of the 
circuit 44. 
In accordance with a feature of the invention, the waveguide section 34 
comprises a top wall 74 and a bottom wall 76 which are joined by sidewalls 
78 and 80. The septum 38 stands on the bottom wall 76, and extends from a 
middle portion of the waveguide section 34 towards the back end at the 
flange 72. At a front end 82 of the septum 38, the septum 38 has a 
relatively short height, the height extending stepwise, via a succession 
of steps 84, to a back portion 86 of the septum wherein the septum 38 
extends the full height of the waveguide section 34 from the bottom wall 
76 to the top wall 74. 
The steps 84 are of differing heights and widths. For example, the widths 
of the steps 84 vary from approximately 0.1 to 0.25 wavelength at a 
nominal frequency of the electromagnetic radiation propagating through the 
waveguide section 34. By way of example, in the construction of a 
preferred embodiment of the invention, the radiator assembly 30 operates 
over a frequency range of 11.0 to 13.4 GHz (gigahertz) and over a range of 
13.7 to 18.0 GHz. In use of the radiator assembly 30, by way of example, a 
frequency band of 11.7 to 12.2 GHz or a band of 12.2 to 12.7 GHz could be 
used for transmission; a band of 14.0 to 14.5 GHz or a band of 17.3 to 
17.8 GHz could be used for reception, and a band of 15.5 to 16.5 GHz could 
be used for satellite tracking. For purposes of constructing the preferred 
embodiment of the radiator assembly 30, a nominal value of frequency of 
12.45 GHz is selected, this corresponding to a free-space wavelength of 
0.948 inches. The largest dimensions of the steps are found in the middle 
of the series of steps 84. Smaller dimensions of the steps 84 are found 
near both ends of the series of the steps 84. Incremental heights of the 
steps 84 vary from approximately 0.035 to 0.200 wavelengths at the nominal 
value of the radiation frequency, and the incremental widths of the steps 
84 vary from approximately 0.1 to 0.25 wavelengths. The actual heights of 
the steps 84, as represented by the legends A2-A8 in FIG. 7, are provided 
relative to a reference plane at the outside edge of the bottom wall 76. 
The actual locations of the riser portions of each of the steps 84, as 
represented by the legends B1-B8 in FIG. 7, are provided relative to a 
reference plane at the front surface of the front flange 70 of the 
waveguide section 34. The following dimensions are employed in the 
preferred embodiment of the invention. The dimensions A2, A3, and A4 
measure, respectively, 0.080, 0.126, and 0.174 inches. The dimensions A5, 
A6, A7, and A8 measure, respectively, 0.254, 0.313, 0.572, and 0.614 
inches. The dimensions B1, B2, B3, and B4 measure, respectively, 1.017, 
1.100, 1.215, and 1.478 inches. The dimensions B5, B6, B7, and B8 measure, 
respectively, 1.713, 1.958, 2.055, and 2.423 inches. Thus, the smaller 
step widths are approximately one-tenth wavelength, and the larger step 
widths are approximately one-quarter wavelength. The septum 38 has a 
thickness of 0.030 inches. 
A characteristic of the septum 38 is that it introduces a phase shift 
versus frequency to radiation propagating past the septum 38 wherein the 
phase shift decreases with increasing frequency. Such a phase shift 
characteristic is similar to that disclosed for a capacitive ridge in U.S. 
Pat. No. 4,654,611 of Wong et al. In order to provide a broad-band 
transmission characteristic to the waveguide section 34, as well as to the 
entire horn radiator assembly 30, a set of teeth 88 are provided 
upstanding from the bottom wall 76 and are arranged in a line colinear 
with the septum 38 to introduce capacitance to the waveguide section 34. 
The capacitance introduced by the teeth 88 has a phase-shift 
characteristic to radiation propagating in the waveguide section 34 
wherein the amount of phase shift increases with increasing frequency of 
the radiation. Four of the teeth 88 are provided in the preferred 
embodiment of the invention, the teeth 88 being spaced apart from each 
other and from the front end 82 of the septum 38 by spaces 90. With 
increasing frequency of the radiation, the increment in phase shift 
introduced by the row of teeth 88 tends to cancel the decrement in phase 
shift introduced by the septum 38 so as to obtain the desired wide 
bandwidth characteristic of the radiator assembly. In the preferred 
embodiment of the invention, the teeth 88 have the same height and the 
same width, and the spaces 90 are all equal. The height of the teeth 88, 
relative to the reference plane, as designated by the legend A1 in FIG. 7 
is 0.047 inches. The thickness of the bottom wall 76, as represented by 
the legend A0, is 0.040 inches. The teeth 88 are positioned periodically 
with a period of 0.242 inches as measured between centers of the teeth. 
The spacing between the teeth 88, as represented by the spaces 90, is 
0.095 inches. 
In the construction of the horn 32 and the impedance matching section 36, 
the horn 32 has an axial length, as measured from the shelf 66 to the 
front of the horn 32, of 0.675 inches. The thickness of the sidewall 64 is 
0.007 inches. In the forward waveguide section 48, the axial length, as 
measured from the back wall 62 to the shelf 66 is 0.310 inches. The axial 
length of the rear waveguide section 50 is 0.593 inches. The inside 
diameter of the horn 32 is 1.039 inches. The wall thickness of the forward 
waveguide section 48 is 0.018 inches. The inside diameter of the forward 
waveguide section 48 is 0.850 inches. The cross-sectional dimensions of 
the rear waveguide section 50 are the same as those of the waveguide 
section 34 wherein the interior wall heights are 0.583 inches. The 
stepwise construction of the impedance matching section 36 minimizes 
mutual coupling among horns 32 in an array of radiator assemblies 30 such 
as that to be described in FIG. 19. 
With reference to FIGS. 9A-13, a right-handed circularly polarized wave is 
presumed to be incident upon the horn 32, the wave having horizontally 
polarized components of electric field depicted as arrows in FIG. 9A, and 
vertically polarized components of electric field as depicted by arrows in 
FIG. 9B. The electric field components represented by FIGS. 9A and 9B 
occur in the front portion of the waveguide section 34. Upon reaching the 
steps 84 of the septum 38, changes occur in the electric field components 
as indicated by FIGS. 10A-10B and FIGS. 11A-11B. FIGS. 10A-10B represent a 
region of the septum 38 towards the front end of the septum, while FIGS. 
11A-11B represent a region of the septum 38 towards the back end of the 
septum. In FIG. 10A, the horizontally polarized electric field components 
are reconfigured, the reconfiguration continuing into FIG. 11A wherein the 
energy of the electric field has now been converted into opposed electric 
field vectors located on opposite sides of the septum 38 and extending in 
opposite direction. 
With respect to the vertically polarized electric field components, a part 
of the electric field appears on each side of the septum 38, as shown in 
FIGS. 10B and 11B, however, the direction of the electric field vectors 
remains the same on both sides of the septum 38. At the back portion 86 of 
the septum 38, the operation of the orthomode transducer (OMT) takes place 
to combine the electric fields resulting from the horizontally polarized 
fields of FIG. 9A and the vertically polarized fields of FIG. 9B. FIG. 12A 
shows the opposed vertical fields on both sides of the septum 38 resulting 
from the horizontally polarized field of FIG. 9A, and FIG. 12B shows 
electric fields pointing in the same direction on opposite sides of the 
septum 38 resulting from the vertically polarized field of FIG. 9B. The 
amplitudes of the fields of FIG. 9B and FIG. 9A are in phase quadrature to 
produce the right-handed circular polarization. However, in the 
transformation of the electric fields represented by FIGS. 10A, 11A, and 
12A, there has been a phase shift of 90 degrees which brings the 
amplitudes of the electric fields of FIGS. 12A in phase with the 
amplitudes of the electric fields in FIG. 12B. This results in a summation 
of the commonly directed fields of FIG. 12B, and a cancellation of the 
opposed fields of FIG. 12A to produce, in FIG. 13, a vertically polarized 
electric field in the port 42 with essentially no radiation being present 
in the port 40. 
The FIGS. 14A-18 provide a description of the operation of the waveguide 
section 34 for the case of the left-handed circularly polarized (LHCP) 
electromagnetic wave. The vectorial representations of the electric fields 
of FIGS. 14B, 15B, 16B, and 17B are the same as the electric fields 
portrayed in the FIGS. 9B, 10B, 11B, and 12B, respectively. In FIG. 14A, 
the electric field vectors are oriented in the opposite sense to the 
electric field vectors of FIG. 9A. Similarly, in FIGS. 15A, 16A, and 17A, 
the electric fields have the same patterns as do the fields of FIGS. 10A, 
11A, and 12A, respectively, but are oriented in the opposite sense. As a 
result, there is a summation of vectors to produce the electric fields in 
port 40, while there is a cancellation of electric fields to produce 
essentially no electric fields in port 42 of FIG. 18. Thus, upon comparing 
FIGS. 13 and 18, it is noted that an electric field is received in port 42 
in the case of right-handed circularly polarized waves, while for a 
received left-handed circularly polarized wave, the electric fields are 
presented in port 40. Since the operation of the radiator assembly 30 is 
reciprocal, the assembly 30 is operative to transmit a right-handed 
circularly polarized wave by applying the electric field to port 42 and, 
for transmission of a left-handed circularly polarized wave, the electric 
field is to be applied to port 40. In this manner, the transmit receive 
circuit 44 of FIG. 1 can provide for either a right-handed or left-handed 
circularly polarized wave by applying the electric field respectively to 
either port 42 or Port 40. Coupling of the circuit 44 to the port 40 
(FIGS. 1-2) is provided via line 92, and the coupling of the circuit 44 to 
the port 42 is provided by line 94. 
FIG. 19 shows an antenna 96 comprising an array of radiator assemblies 30 
with their horns 32 arranged side-by-side to produce a beam of radiation 
for transmission and reception of radiant signals. The transmit/receive 
circuit 44 is coupled via lines 98 and 100 to a beamformer 102. The 
beamformer 102 connects with the ports 40 and 42 of each of the radiator 
assemblies 30 via diplexers 104 and 106 to allow operation of transmit and 
receive functions in different portions of the frequency bands over which 
the radiator assemblies 30 are operative. In accordance with well-known 
circuitry, the beamformer 102 is operative to provide phase shift and/or 
delays of signals applied to one of the radiator assemblies 30 relative to 
other ones of the radiator assemblies 30 so as to form and to direct a 
beam of radiation produced by the horns 32. The beamformer 102 is 
operative to divide power equally among the radiator assemblies 30 for the 
transmission of radiation, and to combine the power of radiant signals 
received from the radiator assemblies 30 during reception of an incoming 
electromagnetic signal. By selection of either the port 40 or the port 42, 
respectively, in each of the radiator assemblies 30, a left-handed or 
right-handed circularly polarized wave can be transmitted or received. 
FIG. 20 shows a graph representing the frequency response of a horn 
radiator assembly 30. The graph includes two traces, the upper trace 
representing amplitude of a transmitted or received signal as a function 
of frequency, and the lower trace representing phase shift of the 
transmitted or received signal as a function of frequency. The amplitude 
variations are indicated in decibels, the phase shift is indicated in 
degrees, and the frequency is presented in units of gigahertz. A region of 
attenuation and rapid phase shift occurs in a relatively narrow frequency 
band centered at a frequency of approximately 13.5 GHz. This divides the 
useful spectrum of the radiator assembly 30 into a lower frequency band 
and a higher frequency band. 
As disclosed above, the invention provides for a radiator assembly 30 
having a smaller overall configuration than has been possible heretofore. 
A significant savings in space, over that of previous microwave 
structures, is afforded by the lack of tuning screws, by the parallel 
arrangement of the two waveguide ports 40 and 42, and by the reduction in 
overall length of the septum 38 through use of the numerous steps 84. The 
use of the numerous steps 84 also provides for a significant reduction in 
reflected waves, and the use of the capacitive teeth 88 serves to provide 
the desired broad bandwidth. The reduction in size facilitates 
construction of the array antenna 96, and the stepped configuration of the 
impedance matching structure 36 reduces mutual coupling between the 
radiating horns 32 of the antenna 96. 
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.