U.S. patent number 3,573,838 [Application Number 04/771,178] was granted by the patent office on 1971-04-06 for broadband multimode horn antenna.
This patent grant is currently assigned to Hughes Aircraft Company. Invention is credited to James S. Ajioka.
United States Patent |
3,573,838 |
Ajioka |
April 6, 1971 |
**Please see images for:
( Certificate of Correction ) ** |
BROADBAND MULTIMODE HORN ANTENNA
Abstract
The apparatus of the present invention provides a horn antenna
capable of generating a pattern with rotational symmetry and
polarization purity over a broadband of frequencies with
comparatively low side lobes. In achieving this operation, a first
mode which propagates as a "slow wave" is launched through a first
portion of the horn and is converted to a second mode which
propagates as a "fast wave" through the remaining portion. At the
aperture of the horn, the resulting wave combines with a "medium
wave" which normally propagates along the entire length of the
horn, to achieve the desired aperture distribution. Since the slow
wave and the fast wave compensate each other for frequency changes,
the overall desired aperture distribution can be sustained over a
broad range of frequencies.
Inventors: |
Ajioka; James S. (Fullerton,
CA) |
Assignee: |
Hughes Aircraft Company (Culver
City, CA)
|
Family
ID: |
25090961 |
Appl.
No.: |
04/771,178 |
Filed: |
October 28, 1968 |
Current U.S.
Class: |
343/783;
343/786 |
Current CPC
Class: |
H01Q
13/025 (20130101) |
Current International
Class: |
H01Q
13/00 (20060101); H01Q 13/02 (20060101); H01q
013/00 () |
Field of
Search: |
;343/772,776,777,778,779,786,783 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lieberman; Eli
Claims
I claim:
1. A broadband multimode horn antenna comprising a conductive horn
having a conical configuration, means for launching and propagating
a first electromagnetic wave in the TE.sub.11 mode of predetermined
velocity through first and second successive portions of the length
of said conductive horn, means for launching and propagating a
second electromagnetic wave in the TEM mode having a velocity
slower than said predetermined velocity through said first portion
of the length of said conductive horn, and means for converting
said second wave into a third electromagnetic wave in the TM.sub.11
mode having a velocity faster than said predetermined velocity
through said second portion of the length of said conductive horn
whereby said first and third waves combine to provide a
predetermined field intensity pattern at the aperture of said horn
over a broad range of frequencies.
2. A broadband multimode horn antenna comprising a conductive horn
having an aperture at one extremity thereof; a plurality of
longitudinal conductive spaced rods disposed along a first portion
of the length of said horn spaced from said aperture; means coupled
to said horn for launching a wave of predetermined velocity
therethrough; and means coupled to said spaced rods for launching a
TEM wave therealong of a velocity less than said predetermined
velocity whereby said TEM wave converts to a wave of a velocity
faster than said predetermined velocity at the termination of said
first portion and propagates through a second portion of the length
of said horn extending from said first portion to said aperture to
combine with said wave of predetermined velocity to provide a
predetermined field intensity pattern.
3. The broadband multimode horn as defined in claim 2 wherein said
conductive horn has a conical configuration with a constant
diameter throat section, and said plurality of longitudinal
conductive spaced rods constitutes first and second rods disposed
opposite each other along said first portion of the length of said
horn.
4. The broadband multimode horn antenna as defined in claim 2
wherein the ratio of the cross-sectional dimensions of the diameter
of said rods and the relative spacing thereof to the dimensions of
said horn at said cross section are constant thereby to provide a
constant impedance to said TEM wave.
5. The broadband multimode horn as defined in claim 2 wherein said
conductive horn has a conical configuration with a constant
diameter throat section, and said plurality of longitudinal
conductive spaced rods constitutes first, second, third and fourth
rods disposed at quadrature points along said first portion of the
length of said horn.
6. The broadband multimode horn as defined in claim 2 wherein the
respective end portions of said plurality of spaced longitudinal
conductive rods nearest said aperture each include no less than one
step discontinuity therealong thereby to provide impedance matching
between said rods and said second portion of the length of said
horn.
7. A broadband multimode horn antenna comprising a conical horn
having a cylindrical throat section at the narrow extremity thereof
and an aperture at the remaining extremity; first and second
longitudinal rods disposed along a portion of the length of said
conical horn adjacent said throat section on opposite sides of the
centerline thereof, said longitudinal rods being supported by
right-angle extensions thereof at the extremities nearest said
throat section to the inner surface of said conical horn; and means
for launching a TE.sub.11 wave through said throat section thereby
to generate a multimode field intensity pattern at said
aperture.
8. A broadband multimode horn antenna system comprising a conical
horn having a cylindrical throat section at the narrow extremity
thereof and an aperture at the remaining extremity; first and
second longitudinal rods disposed through said throat section and
along a portion of the length of said conical horn adjacent thereto
on opposite sides of the centerline thereof; means responsive to a
signal to be radiated from said horn for dividing the power and
controlling the relative phase thereof along first and second
paths, said first path being coupled to said throat section for
exciting a TE.sub.11 wave therethrough; and means including a
hybrid junction having an input connected to second path and
outputs connected to said first and second rods for launching said
signal as a TEM wave therealong thereby to generate a controllable
field intensity pattern representative of said signal at said
aperture.
9. A broadband multimode horn antenna system comprising a conical
horn having a cylindrical throat section at the narrow extremity
thereof and an aperture at the remaining extremity; a plurality, n,
of longitudinal conductive rods disposed through said throat
section and along a portion of the length of said conical horn
adjacent thereto at equal intervals about the centerline thereof;
and a hybrid feed network responsive to a signal to be radiated
from said aperture and connected to said n longitudinal conductive
rods for exciting said rods with said signal with a phase
difference between successive rods equal to cos(2.pi.m/n) where m
is an integer.
10. A broadband multimode horn antenna comprising a rectangular
horn having a rectangular throat section at the narrow extremity
thereof and a rectangular aperture at the remaining extremity; a
plurality of conductive rods disposed longitudinally through said
throat section and along a portion of the length of said
rectangular horn adjacent thereto; and means connected to said rods
for generating a predetermined multimode field intensity pattern at
said rectangular aperture.
11. The broadband multimode horn antenna as defined in claim 10
wherein said rectangular throat section has a width a and a height
b and wherein x is a variable denoting the respective positions of
a portion of said rods along said width from a first reference side
of said throat section and y is a variable denoting the respective
positions of the remainder of said rods along said height from a
second reference side of said throat section orthogonal to said
first reference side and wherein said last-named means constitutes
discrete sampling of cos(n.pi.m/a) along said width for said
portion of said rods and constitutes discrete sampling of
cos(m.pi.y/b) along said heights for said remainder of said rods
wherein m and n are integers that determine the modes of
propagation.
Description
BACKGROUND OF THE INVENTION
Contemporary single conical or pyramidal horns which combine higher
order modes with a dominant mode can give a variety of radiation
patterns. Good control of the radiation pattern can be achieved by
controlling the relative amplitudes and phases of the various modes
at the horn aperture. High efficiency feed horns for cassegrain
antennas utilize this technique. The principal disadvantage to
contemporary multimode horns is that the bandwidth is extremely
narrow. The reason for this narrow bandwidth is that the higher
order modes are generated prior to or in the region of the throat
of the horn and since various modes have different velocities of
propagation through this region, the relative phases at the horn
aperture will vary rapidly with frequency. The longer the horn, the
greater is this frequency sensitivity.
One present method of generating higher order modes is the use of a
step discontinuity followed by a phasing section to adjust the
relative phases of the modes. The relative amplitudes are
determined by the height of the step discontinuity and the length
of the phasing section is adjusted to give the proper relative
phase at the horn aperture. Since the higher order modes have phase
velocities that vary much more rapidly with frequency change than
the dominant mode, particularly in the phasing section and in the
throat region of the horn, the radiation pattern will also change
rapidly with changes in frequency. That is, such horns are narrow
band devices. Other methods of generating higher order modes are
series stub mode generators, and pins or corrugations for mode
generation. These methods are narrow band for the same reason as
above.
SUMMARY OF THE INVENTION
In accordance with the present invention, a plurality of spaced
longitudinal conductive rods are disposed in the throat section of
a conical horn which as a cross-sectional shape that can be
circular, rectangular, triangular, etc. These conductive rods may
be parasitically excited or driven externally from the horn to
allow remote control of the relative phase and amplitude of the
modes to achieve a desired field distribution pattern across the
aperture of the horn. In instances where the longitudinal
conductive rods are driven externally, hybrid junctions may be used
to control the amplitude and phase. Also, for efficient launching
of the higher order modes from the TEM lines, the impedance match
in the transition from TEM to horn modes must be good. The
impedance match can be enhanced by adjustment of the parameters of
conductor position in the horn, conductor size and shape, any or
all of which can vary along the lines.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a conical multimode horn of circular cross section
having parasitically energized longitudinal rods;
FIGS. 2A and 2B illustrate field distribution in the horn of FIG.
1;
FIG. 3 shows a circular multimode horn with externally controlled
mode amplitudes and phases;
FIG. 4 shows a circular multimode horn illustrating at TM.sub.11
mode field configuration launched from TEM line;
FIGS. 5A and 5B show a circular multimode horn adapted for
multimode generation with multiconductor TEM lines excited by a
hybrid network;
FIG. 6 shows a conical multimode horn having longitudinal rods with
impedance matching;
FIGS. 7A and 7B show a rectangular horn with four longitudinal rods
for mode launching;
FIG. 8 shows a cross section of the throat portion of a rectangular
horn having a plurality of longitudinal rods for mode
launching;
FIG. 9 shows an E-plane aperture distribution from a TEM launched
multimode conical horn;
FIG. 10 shows an H-plane aperture distribution from a TEM launched
multimode conical horn; and
FIG. 11 shows a diagonal-plane aperture distribution from a TEM
launched multimode conical horn.
DESCRIPTION
Referring now to FIG. 1 of the drawings, there is shown an
embodiment of the present invention including a conical horn 10
having a throat section 12 which is terminated with a flange 14.
Longitudinal conductive rods 15, 16 are attached diametrically
opposite each other to the inner surface and at the right extremity
of the throat section 12, as viewed in the drawing, by means of
right angle extensions of length h . Thus, the rods 15, 16 commence
substantially at the termination of the throat section 12 a
distance, h, from the surface and extend for a distance, L.sub.1,
towards the aperture of the conical horn 10. Although not critical,
the rods may extend along lines emanating from the vertex of
conical horn 10 and intersecting points a distance, h, from
opposite sides of the throat section 12 at the junction with the
conical horn 10.
In the operation of TE.sub.multimode circular conical horn of FIG.
1, a dominant TE.sub.11 mode is initially launched through the
throat section 12. The incident TE.sub.11 mode excites the TEM mode
on the two conductor lines formed by longitudinal rods 15, 16 at
the commencement thereof. The magnitude of the TEM mode is
controlled by the "antenna height," h, i.e., the support portions
of the rods 15, 16. Since only a portion of the TE.sub.11 mode is
converted to the TEM mode, both the TE.sub.11 and TEM modes exist
in the region L.sub.1 of the conical horn 10. The configuration of
the TEM mode in the region L.sub.1 in relation to the rods 15, 16
is illustrated in FIG. 2A. At the right extremity of the rods 15,
16, as viewed in the drawing, a TM.sub. 11 mode is launched by the
TEM mode. Thus, the TE.sub.11 and TM.sub.11 modes exist in the
region L.sub.2 of the conical horn 10. Since the longitudinal rods
15, 16 terminated at the end of region L.sub.1, the TEM mode cannot
exist in the region L.sub.2. The TE.sub.11 and TM.sub.11 modes
combine at the aperture of the horn 10 as illustrated in FIG. 2B to
generate a field intensity pattern having very low side lobes. E, H
and diagonal plane patterns generated by a TEM launched TM.sub.11
mode are illustrated in FIGS. 9, 10 and 11. The side lobes in all
these patterns are of the order of 30 db. down from the peak, the
patterns are essentially identical in shape showing rotational
symmetry and finally, the cross polarization as measured in the
diagonal plane where it is greatest is seen to be entirely
negligible which illustrates the desireable property of
polarization purity.
The relative phase of the TE.sub.11 and TM.sub.11 modes at the
aperture of the horn 10 are retained over a broad frequency range.
The reason for this is because the phase of the TM.sub.11 mode at
the horn aperture depends on the "relatively slow" phase velocity
of the TEM mode over the distance L.sub.1 and the "relatively fast"
phase velocity of the TM.sub.11 mode over the distance L.sub.2 and
the phase of the TE.sub.11 mode at the horn aperture is a function
of the "medium" velocity of the TE.sub.11 mode only over the
distance L.sub.1 + L.sub.2; that is, the composite average phase
velocity of a slow wave and a fast wave can be made essentially
equal to the average phase velocity of a medium velocity wave over
a broad frequency range thus assuring the relative phase between
the TE.sub.11 and TM.sub.11 waves at the horn aperture to be
essentially constant over a broad frequency range (eg., over the
entire frequency band of normal operation of a waveguide). Stated
mathematically: ##SPC1##
In the equation (1) above, the terms to the left of the equal sign
represent the phase of the TM.sub.11 wave at the aperture of the
horn 10 while the term to the right of the equal sign represents
the phase of the TE.sub.11 wave at the aperture.
Changes in the frequency of the TEM and TM.sub.11 modes are
self-compensating, i.e., as one increases the other decreases
relative to the TE.sub.11 mode, whereby equation (1) can remain
valid for a broad range of frequencies. In general, the slow TEM
wave over the distance L, plus the fast TM.sub.11 wave over the
distance L.sub.2 can equal the medium speed TE.sub.11 wave over the
distance L.sub.1 +84 L.sub.2 and this equality is essentially
maintained over a broad frequency range.
Another embodiment of the multimode horn of the present invention
adapted to allow external control of the amplitude and phase of the
modes is shown in FIG. 3. Referring to FIG. 3, there is shown, in
partial section, a conical horn 22 including a throat section 23
with an arm 24. Longitudinal conductive rods 25, 26 extend from the
center conductors 27, 28, respectively, of coaxial lines 29, 30
through throat section 23 after which they fan out along the inner
surface of conical horn 22. The end of throat section 23 of horn 22
is terminated by a conductive disc 30 which serves as a termination
for the outer conductors of the coaxial lines 29, 30. Coaxial lines
29, 30 are connected to the opposite polarity outputs of a hybrid
junction 32 which, in turn, is connected through a variable phase
shifter 33 to one output of a variable power divider 34. A
remaining output from the variable power divider 34 is coupled to
the arm 24 of horn 22 in manner to launch a TE.sub.11 wave therein.
An input 36 to the variable power divider 34 serves as an input to
the multimode horn 22. Lastly, a remaining output 37 from the
hybrid junction 32 is terminated by means of an impedance 38, or
may be used as an odd mode generator for an error (difference)
channel for monopulse operation.
In operation, a microwave signal is applied to the input line 36
which divides it between the arm 24 and the hybrid junction 32. The
hybrid junction 32 applies opposite polarity signals through the
coaxial lines 29, 30 to the longitudinal rods 25, 26 to launch a
TEM wave therealong. As in the case of the device of FIG. 1, the
TEM wave launches at TM.sub.11 wave through the conical horn 22
commencing with the terminal of rods 25, 26. Concurrently, a
TE.sub.11 wave is launched in arm 24 in which is directed through
the horn 22 in the same manner as the distance L.sub.1 + L.sub.2 in
the horn 10 of FIG. 1. The relative amplitude and phase of the
TE.sub.11 and TM.sub.11 waves at the aperture may be controlled to
achieve a desired pattern by the variable power divider 34 and the
variable phase shifter 33.
Referring to FIG. 4, there is shown a multimode horn in accordance
with the invention illustrating the electric field configuration
for a TM.sub.11 mode. In particular, the apparatus of FIG. 4
illustrates a conical horn 40 in partial section with a throat
section 41 terminated by a conductive disc 42. Conductive disc 42
supports coaxial inputs 44, 46, the center conductors of which
connect to longitudinal rods 47, 48. Longitudinal rods extend
longitudinally through the throat section 41 and then fan out
slightly in the conical section of the horn 10. In addition, a
cylindrical waveguide arm 50 terminated by a disc 51 is connected
to the throat section 4. In operation, the coaxial inputs 44, 46
are fed with signals of opposite phase and of a frequency
sufficiently high to enable the conical section of the horn 10 to
support the TM.sub.11 mode of propagation. Under these
circumstances, a TEM mode propagates along the longitudinal rods
47, 48 and launches at TM.sub.11 mode at the terminations thereof.
The electric field configurations of the TM.sub.11 mode is
illustrated by the lines 55.
Referring to FIGS. 5A and 5B there is shown a multimode horn in
accordance with the invention with more than two TEM mode launching
rods. In particular, FIGS. 5A and 5B show a conical horn 60 with
longitudinal TEM mode launching rods 61, 62, 63, 64. The rods
61--64 are tapered and spacing flared such that the characteristic
impedance for the TEM mode is constant (i.e. the ratio of rod
diameter and spacing to horn diameter are constant to give
geometrical similarity at any cross section). In particular, FIGS.
5A and 5B show a conical horn 60 with longitudinal TEM mode
launching rods 61, 62, 63, 64 shown in cross section in FIG. 5B.
The longitudinal rods 61, 62, 63, 64 are driven by means of a
hybrid feed network 65 through coaxial lines 66, 67, 68, 69. In
operation, the hybrid feed network 65 excites the multiconductor
TEM lines 61--64 with signals of the proper amplitudes and phases
which, in turn, launch the higher order modes in the horn 60. The
relative phase of the signals applied to the multiconductor TEM
lines 61--64 proceeding in a clockwise direction as viewed in FIG.
5B is 2.pi.m/n where m32 .+-.0, 1, 2, 3... and n is the number of
lines. Thus, for m=1 in the present case, the relative phase
between adjacent lines is 90.degree. . This phase difference is
employed for exciting a TEM mode which, in turn, launches dual
orthogonal (right and left circularly polarized) modes in the horn
60.
For efficient launching of the higher order modes from the TEM
lines or longitudinal rods, the impedance match in the transition
from TEM to horn modes must be good. Referring to FIG. 6, there is
shown a conical horn 70 including longitudinal rods 71, 72.
Conductive ferrules 73, 74 are disposed near the end of the rods
71, 72, respectively, and ferrules 75, 76 at the extremities
thereof. The ferrules 73, 74 are of the order of twice the length
of the ferrules 75, 76 and are spaced of the order of one length
away therefrom. In addition, the diameter of ferrules 73, 74, 75,
76 is of the order of two to three times the diameter of the rods
71, 72. The same configuration could, of course, be achieved by
undercutting a heavier rod. The principal consideration to achieve
impedance matching is to generate reflections to cancel out
reflections developed at the transition between the rods 71, 72 and
free space within the horn 70.
Longitudinal rods can, of course, be used with horns of other
shapes such as horns with rectangular or triangular cross section.
Referring to FIGS. 7A and 7B there is shown a rectangular horn 80
with longitudinal rods 81, 82, 83, 84. Connections to the rods
81--84 are made through coaxial connectors 85. A more complex
pattern of longitudinal rods in a rectangular horn 86 is shown in
cross section in FIG. 8, wherein there are two horizontal rows 87,
88 and two vertical rows 89, 90, as viewed in the drawing, of six
rods each. The height of the horn 86 is b, the width a, the
vertical distance from the lower side, as viewed in the drawing, y,
and the horizontal distance from the left side, as viewed in the
drawings, x. In general, the rows 87--90 of rods can be driven in a
manner to achieve any desired pattern as might be determined by a
finite Fourier series. Higher are modes and launched, however, by
driving the discrete rods in accordance with corresponding points
of the desired field intensity pattern within the rectangular horn
86. In this latter case, excitation of rods in rows 87, 88 is in
accordance with discrete sampling of cos(n.pi.x/a) where x denotes
position of rod in x direction. Similarly, excitation of rods in
rows 89, 90 is in accordance with discrete sampling of
cos(m.pi.y/b) where y denotes position or rod in y direction. In
the foregoing situation, m and n are integers which denote the
particular mode to be launched. The rods 81--84 in the horn 80 of
FIGS. 7A and 7B are excited in accordance with the manner described
in connection with FIG. 8.
* * * * *