A corrugated horn antenna, adapted to be coupled to a waveguide at its apex for X-band excitation is further adapted to be connected to waveguides through a circumferential slot for S-band excitation at four distinct phases selected for the desired S-band polarization. The circumferential slot is positioned along the axial length of the horn for good impedance matching and is provided with an X-band choke in the form of two concentric choke slots. For further improvement in impedance matching, the second (outer) choke slot is divided by plugs into four segments that coincide with waveguide ports for the four distinct phases of the S-band

Cohn, S. B.

Williams, W. F.

English

NASA Technical Reports Server

United States Patent 1191 
Williams et al. 
[54] DUAL BAND COMBINER FOR HORN 
ANTENNA 
[76] Inventors: Robert A. Frosch, Administrator of 
the National Aeronautics and Space 
Administration, with respect to an 
invention of William F. Williams, La 
Canada; Seymour B. Cohn, Encino, 
both of Calif. 
[21] Appl. No.: 8,207 
[22] Filed: Jan. 31,1979 
[51] Int. (;a2 ............................................. H0lQ 13/00 
1521 U.S. Cl. ..................................... 343/786; 343/895 
E581 Field of Search ............... 343/729, 755, 776, 786, 
343/854, 895 
~561 References Cited 
U.S. PATENT DOCUMENTS 
2,398,096 4/1946 Katzin .................................. 343/786 
3,325,817 6/1967 Ajioka et al. ........................ 343/786 
3,389,394 6/1968 Lewis ................................... 343/786 
3,423,756 1/1969 Foldes .................................. 343/786 
3,568,204 3/1971 Blaisdell ............................... 343/786 
[111 4,199,764 
[45] Apr. 22, 1980 
3,631,502 12/1971 Peters, Jr. et al. ................... 343/786 
3,732,571 5/1973 Neale ................................... 343/786 
4,077,039 2/1978 Ren et al. ............................. 343/786 
Primary Examiner-David K. Moore 
Attorney, Agent, or Firm-Monte F. Mott; John R. 
Manning; Paul F. McCaul 
PI ABSTRACT 
A corrugated horn antenna adapted to be coupled to a 
waveguide at the apex thereof for X-band excitation is 
further adapted to be connected to waveguides through 
a circumferential slot for S-band excitation at four dis- 
tinct phases Si through S4 selected for the desired S- 
band polarization. The circumferential slot is positioned 
along the axial length of the horn for good impedance 
matching and is provided with an X-band choke in the 
form of two concentric choke slots. For further im- 
provement in impedance matching, the second (outer) 
choke slot is divided by plugs into four segments that 
coincide with waveguide ports for the four distinct 
phases of the S-band. 
9 Claims, 5 Drawing Figures 
https://ntrs.nasa.gov/search.jsp?R=19800015033 2020-03-21T03:07:42+00:00Z
U.S. Patent Apr. 22, 1980 4,199,764 
4.199.764 , ,  
1 
DUAL BAND COMBINER FOR HORN ANTEMVA 
ORIGIN OF THE INVENTION 
The invention described herein was made in the per- 
formance of work under a NASA contract and is sub- 
ject to the provisions of Section 305 of the National 
Aeronautics and Space Act of 1958, Public Law 85-568 
(72 Stat. 435; 42 U.S. 2457). 
BACKGROUND OF THE INVENTION 
This invention relates to a dual band horn antenna 
and more particularly to a dual-band horn antenna oper- 
ating in the linearly polarized mode and/or in the circu- 
larly polarized mode. 15 
Horn antennas are widely used as elements to feed 
paraboloid reflectors in Cassegrain and other reflector 
antenna. In some cases, in order to operate such a reflec- 
tor antenna in dual band, such as in X-band and S-band, 
a dichroic subreflector transparent to the S-band and 2o 
reflective to the X-band is used. The X-band fed 
through the horn antenna is reflected by the hyperbolic 
dichroic subreflector. The S-band is fed by another 
element at the paraboloid focus through the dichroic 
subreflector. This feed permits full performance for 25 
telementry within S-band, while at the same time allow- 
ing for operation within X-band. 
In another case, e.g., the reflex feed used within the 
Deep Space Network Stations of JPL/NASA, both 
frequencies are operated as Cassegrain by using a flat 30 
dichroic plate in the region of an S-band and X-band 
horn. The S-band horn energy is reflected by an ellipti- 
cal subreflector onto the dichroic plate and thence to 
the main hyperboloid. The X-band horn energy trans- 
mits through the dichroic plate and to the hyperboloid. 35 
The final phase center for the S-band system is the same 
as the X-band as determined by the shape (elliptic) and 
location of the S-band reflectors. 
The corrugated horns used in such a reflex feed yields 
very good performance, but the use of the dichroic 40 
subreflector and the large asymmetric feed structure 
results in degradation (about 0.5 dB) of X-band perfor- 
mance. This is because of some small loss in the dichroic 
subreflector and some back scatter at X-band resulting 
in an increase (2' or 3' K.) in the X-band antenna noise 45 
10 
temperature. What is required is a feeding technique 
that will more nearly optimize X-band performance, 
with only slight degradation of S-band performance. To 
accomplish that, it is evident that the dichroic subreflec- 
tor must be removed, but then the two bands must be 
fed from the same or coaxial apertures. Some obvious 
approaches to accomplish this are: an X-band horn 
within (coaxial with) the S-band horn, an X-band end 
fire element (disc-on-rod or helix) within the S-band 
horn, and an array of four or more S-band horns sur- 
rounding the X-band radiator, much like a monopulse 
system. All of these approaches would result in a con- 
siderable S-band performance compromise (- 2 dB), 
and the use of anything but a good horn for X-band 
might well have as much loss as the dichroic subreflec- 
tor. The only obvious approach available is to actually 
use the same horn with both hands and develop a tech- 
nique that will result in acceptable illumination func- 
tions in both of these widely separated frequencies. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, the same 
horn operable at two widely spaced frequency bands 
50 
55 
60 
65 
2 
with nearly identical radiation patterns is achieved by 
using a corrugated horn and a combiner to excite the 
horn in its two frequency bands, and operating in a 
beamwidth saturation mode. The combiner, which must 
have a low loss at the X-band (excited through the apex 
of the horn) of less than 0.02 dB for useful application, 
is comprised of a circumferential slot for S-band injec- 
tion and designed with a choke for the X-band rejec- 
tion. The circumferential slot position in the horn is 
chosen to obtain good S-band impedance matching. 
Excitation of the slot is through four equally spaced 
waveguide ports. Two opposite ports are fed with 180" 
phase difference to yield a horizontal polarization, and 
the other two ports are fed with 180" phase difference to 
yield a vertical polarization. Feeding the ports around 
the circumferential slot with a 90" phase difference from 
port to port clockwise or counter-clockwise yields a 
circular polarization of one sense or the other. 
The novel features of the invention are set forth with 
particularity in the appended claims. The invention will 
be best understood from the following description when 
read in conjunction with the accompanying drawings. 
BRIEF DESCRIPTION OF THE DRAWINGS 
FIG. 1 illustrates schematically a cross section of a 
novel combiner or diplexer for a dual-band horn an- 
tenna. 
FIG. 2 illustrates schematically an end view of a horn 
antenna according to the invention shown schemati- 
cally in FIG. 1. 
FIG. 3 illustrates the design of the choke slots for 
X-band in an S-band circumferential slot. 
FIGS. 4u and 4b illustrate the two parts of a horn 
made in accordance with the teachings of FIGS. 1, 2 
and 3 and separated at the S-band circumferential slot. 
DESCRIPTION OF PREFERRED 
EMBODIMENTS 
Referring now to FIG. 1, there is shown schemati- 
cally a cross section of a feed horn capable of operation 
at two widely spaced frequency bands, X-band and 
S-band, and having nearly identical radiation patterns in 
the two bands made possible by a new and improved 
dual band combiner used to excite this horn in its two 
frequency bands. The X-band (8.4 to 8.55 GHz) excita- 
tion of the horn antenna is coupled through the apex 11 
by a waveguide 12. The horn is excited in the S-band 
(2.1 to 2.3 GHz) through a circumferential slot 13 with- 
out increasing the noise temperature at X-band, and 
virtually without additional loss at X-band over and 
above the loss at X-band that would occur if the horn 
were used alone at X-band without using a combiner. 
Circumferential choke slots 14 are designed to reject the 
X-band from the combiner. In that manner, a corru- 
gated horn operable at two widely spaced frequency 
bands with nearly identical radiation patterns is 
achieved. The horn is excited in its two frequency bands 
and operated in a beam width saturation mode, which is 
with the horn of a length increased to the point where 
additional length does not make the pattern narrower 
nor does it make the horn develop higher gain. A depth 
of corrugation is selected to satisfy the requirements of 
corrugated feed horn operation within both frequency 
bands. 
The combiner has a low loss at X-band of less than 
0.02 dB. That is important because beyond this value, 
the additional noise in the system from this loss would 
4,199,764 
3 4 
render the horn useless for some applications, such as circular polarization at X-band with losses at S-band 
deep space telemetry reception and navigation. The less than 0.2 dB and losses at X-band of less than 0.02 dB 
technique which makes this possible is to feed the S- relative to the horn antenna without the combiner. The 
band signal into the horn through the circumferential technique for this combiner is to feed the S-band into 
feed slot 13 that has designed into it the choke 14 that 5 the horn through a circumferential slot that is designed 
acts like an X-band stop filter. This is illustrated sche- to stop the X-band with a choke, or band stop filter. 
matically in FIG. 1. This is best illustrated schematically in FIG. 1. The 
, The circumferential feed slot is excited through four radial line injection region is shown within the horn 
waveguide ports 15-18 shown in FIG. 2 designed into proper at a position along the horn proper selected to 
the horn antenna structure in order to be able to feed 10 obtain good impedance matching. The dimension b is 
signals Si and S 2  from opposite sides with 180" phase chosen at less than one-half wavelength at the highest 
difference to yield horizontal polarization, and to feed X-band frequency. This limits any attempts at X-band 
signals S3 and S4 through opposite sides with 180" phase propagation within the line to TMmJ radial modes, as 
difference to yield vertical polarization. Feeding the noted hereinbefore. 
ports around the circumferential dot with a 90" phase 15 To test operation of the combiner made in accor- 
difference from port to port clockwise or counter- dance with the foregoing description, opposite pairs of 
clockwise yields circular polarization Of one sense or S-band inputs, SI and S2, must be measured and devel- 
the other. oped together since they are used together to create a 
The circumferential feed slot 13 is located within the linear polarization (a HE1 circular waveguide hybrid 
corrugated horn Proper at a Position empericalb' se- 2 0  mode) and there is significant cross-coupling between 
lected to obtain good impedance matching, and the them. The other inputs, S3 and S4, are decoupled from 
dimension b of the slot is chosen at less than one-half the first pair and used to create the orthogonal linear 
wavelength at the highest X-band frequency, such as mode. The two pairs, taken together, will generate 
0.350 inches. This limits any attempts at X-band propa- circular polarization. These opposite pairs must be ex- 
gation within the line to TMmo radial modes, where m is 25 cited in phase opposition, i.e., 180" out of phase with 
the number of A/2 variations around the circumference, each other, in order to properly generate the TEll mode 
and there are no A/2 variations in the b direction. The instead of the next higher mode, the TMo,. 
TE20' (m=2) radial mode is excited by X-band HE1 1 The S-band test generator is therefore fed into an E-H 
wave (Or by TM I 1 wave if Present). Therefore, the plane tee (1 80" hybrid). This will immediately develop 
radial h e  band stop filter is designed to Stop X-band in 30  the 180" phase difference when using the &plane input 
the TE20 radial line mode, and also to Present xin=o arm. The arm lengths to the combiner inputs must then 
looking into the annular opening at X-band. This will be equal in order to maintain this 180" phase differential. 
result in negligible effect on the X-band HEii wave i.e., A slotted line is used to perform the measurement on an 
negligible leakage reflection, or mode conversion. input arm and therefore a straight waveguide section of 
The design of the X-band choke slots is illustrated in 35 the Same phase length is used in the other input arm to 
FIG. 3. Dimension b of FIG. 1 is chosen at 0.35", about maintain this exact phase relationship. In this manner 
a guide wavelength in X-band. Using the Radiation the mutual coupling between opposite arms is ''tuned 
Laboratory Waveguide Handbook, vel. 10, PP. out" as though part of a mismatch reflection. 
337-350, the remaining dimensions for the choke are It was determined by these measurements that an 
obtained and are indicated in FIG. 3. A second choke 40 inductive iris was needed at the waveguide inputs to the 
Slot was added according to the same dimension. Be- combiner. This S-band waveguide is only 0.89 cm (0.35 
yond these X-band chokes, the radial line continues for in.) high and the standard 10.92 cm (4.3 in.) wide. This 
a short distance, and is then terminated in four places matched input in narrow waveguide was then trans- 
with step Junction tramformers (not shown). These formed up to the standard WR430 size using a 4-step, 
transformers have four steps and terminate in standard 45 3+,ection, waveguide transformer. 
WR430 waveguide. The construction of this combiner Below are tabulated the final voltage standing wave 
is shown in FIGS. 4a and 4b. 
The structure has been separated into two sections 
for viewing. The section in FIG. 4b is the input end, 
showing the taper to a small X-band input end. The 50 
section in FIG. 4u is the output end with the pair of 
X-band choke slots 14. The addition of four plugs 21 to 
ratios (VSWR) for this combiner at S-band. 
Frequency (GHz) VSWR 
2. 100 9.50 
2.150 4.50 
2.200 2.68 
2.225 2.01 
2.250 1.50 
2.275 1.23 
2.300 I .04 
2.325 1.19 
2.350 1.54 
2.375 2.11 
2.400 2.94 
2.450 6.20 
2.500 16.50 
24 in the second choke slot improves matching. These 
plugs are situated next to respective comer sections "21 
26, forms the waveguide ports 15 to 18 indicated in 
just the segments into which the waveguide slots open, 
to 24" on a flange 25 which, when fastened to a flange 55 
FIG. 2. These plugs thus limit the second choke slot to 
while the first choke slot is a complete circumferential 
choke slot in the circumferential feed slot 13. Noise 60 
measurements with this horn-combiner combination 
indicate essentially no addition of noise due to the com- 
biner. When compared to an X-band corrugated horn 
without the combiner, no additional noise could be 
noted with a resolution of tenths of a Kelvin. 
In summary an X-S combiner is provided in a corru- 
gated horn antenna with different polarizations, such as 
right circular polarization at S-band and right and left 
From this it is noted that the bandwidth is only 100 
MHz for a VSWR of less than 1.5. This is suitable for 
65 receiving operations only; another combiner could be 
used for S-band transmission. 
The most important characteristic of the combiner is 
that it have extremely low loss at X-band, Le., does not 
4,199,764 
5 6 
contribute any further noise to the system. This noise for excitation 180" out of phase, whereby, excitation of 
was measured by using the combiner in a full scale horn. said first and second pairs of waveguide ports may be 
This combination was used with an X-Band maser am- selected for a desired polarization. 
plifier setup for measuring total noise temperature by 4. The combination of claim 3 wherein said circum- 
comparing it with an identical system using an X-band 5 ferential choke is comprised of two annular slots of a 
horn without the combiner. The horn without the com- width about one fourth a guide wavelength in the one 
biner has a certain measurable noise level when looking frequency band. 
to the open sky (receiving from space.) 'I%e horn and 5. The combination of claim 4 wherein the outer one 
combiner were then substituted to determine a different of said annular slots is divided by plugs into four 
noise level as caused by this different configuration. A 10 sections, each section coinciding with a waveguide 
long sequence of these substitution measurements were port, to improve impedance matching. 
made. The result of all measurements indicated that 6. A dual band antenna operable at two widely 
essentially no difference existed between the two sys- spaced frequency bands with nearly identical radiation 
patterns comprising a corrugated horn and a combiner tems. 
trated and described, it is recognized that modifications ating in a beamwidth saturation mode, said combiner 
having a low loss at the band excited through the apex and variations may readily occur to those skilled in the 
of the horn of less than 0.02 dB and being comprised of art. Consequently, it is intended that the following claims be interpreted to include such modifications and 
2o a circumferential feed slot with circumferential choke variations. slots for the other band being diplexed, said circumfer- 
entia1 feed slot position being chosen to obtain good 
impedance matching. 
7. A dual band antenna as defined in claim 6 wherein 
a particular embodiment has been l5 to excite said horn in its two frequency bands, and oper- 
What we claim is: 
In a corrugated horn antenna adapted to be excited 
through the apex thereof at one frequency band, a com- 
biner for excitation of the horn at a lower frequency 
band comprised of a circumferential feed slot in a plane 25 said circumferential slot is coupled to four equally 
perpendicular to the horn axis, the position of said plane spaced waveguide Ports for excitation with energy of 
along the length of the horn axis being for the other band being diplexed at distinct phases. 
optimum impedance matching, said feed slot being of a 8. A dual band antenna as defined in claim 7 wherein 
width, b, less than half a guide wavelength of the high- two of said waveguide Ports opposite each other and on 
est of the one frequency band, and a circumferential 30 a hOrizontal axis are fed with 180' Phase difference to 
choke in said feed slot. yield a horizontal polarization, and two of said wave- 
2. The combination of claim 1 including waveguide guide ports opposite each other and on a vertical axis 
ports into said circumferential slot for coupling excita- are fed with 180" phase difference to yield a vertical 
tion of said lower frequency from said circumferential polarization. 
slot. 9. A dual band antenna as defined in claim 7 wherein 
3. The combination of claim 2 wherein said wave- said four equally spaced waveguide ports are excited 
guide ports consist of a first pair of waveguide ports with 90" phase difference from port to port in a selected 
opposite each other for excitation 180' out of phase and direction for circular polarization in the selected direc- 
a second pair of waveguide ports opposite each other tion. 
35 
and oriented 90' from said first pair of waveguide ports 40 * * * * *  
45 
50 
55 
60 
65 


Dual band combiner for horn antenna

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