The design and testing of the prototype horn for the proposed 1-megawatt radar are described. The unique features of this square horn include a multiflare design in which flare angle changes rather than corrugations are used to generate the required higher-order modes. A five-port combining section is used at the input. The design of this section and the multiflare section are described. Measured radiation patterns are in good agreement with theoretical patterns

Hoppe, D. J.

English

NASA Technical Reports Server

N89..27884
TDA Progress Report 42-97 January-March 1989
A Multiflare Horn With 1-Megawatt Power Handling Capability
D. J. Hoppe
Radio Frequency and Microwave Subsystems Section
This article describes the design and testing of the prototype horn fi_r the proposed
1-megawatt radar. The unique features of this square horn include a multiflare design in
which flare angle changes rather than corrugations are used to generate the required
higher-order modes. A five-port combining section is used at the input. The design of this
section and the multiflare section are described in this article. Measured radiation patterns
are in good agreement with theoretical patterns.
I. Introduction
Deep Space Network (DSN) plans call for increasing the
transmitting power of the Goldstone Solar System Radar from
the present 365 kW to 1 MW [11 . In the proposed I-MW con-
figuration, the output from four 250-kW klystron amplifiers
must be combined to form the 1-MW signal. This article focuses
on the proposed horn for the 1-MW radar, including the input
section where the four 250-kW signals will be combined. The
horn design closely resembles the design of the horn used at
the Haystack Observatory in Westford, Massachusetts [2],
with some important simplifications.
The first section of this article describes the input section
of the horn and bow it interfaces with the four high-power
amplifiers and waveguide system. Computer simulation of the
combiner section and multiflare sections of the horn are
described, and radiation patterns for the final design are pre-
sented. The following section compares these theoretical pat-
terns with measured patterns for the prototype horn. Finally,
conclusions are drawn and remaining areas of concern are
described.
II. Horn Input Section
A. Combiner Section
A simplified block diagram for the proposed 1-MW radar
transmitter system is shown in Fig. 1. Four 250-kW klystron
amplifiers are fed by a common drive signal and produce the
required 1-MW signal. These four signals are carried through a
WR125 waveguide to the horn input, where they are further
split into eight 125-kW signals. Four of these signals (one pair
of klystrons) feed the in-line ports of four orthomode junc-
tions, while the other klystron pair feeds the ortho ports. By
correctly adjusting the relative phase of the signals of these
klystron pairs, either circular or linear polarization can be
generated in the output port of the four orthomodes. For this
design, the orthomode output is through 0.95-in. square wave-
guides rather than the circular waveguides used in the DSN.
These four square waveguides then enter the combiner section
of the horn, which is described next.
In order to design the combiner section, the possible propa-
gating waveguide modes for a particular dimension of the
149
https://ntrs.nasa.gov/search.jsp?R=19890018513 2020-03-20T01:53:00+00:00Z
square waveguide must be known. The critical dimension area x
where a TEmn or TMmn begins to propagate is given by:
l/
ml2x
_ko(m2 +//2)1/2
For the frequency of interest (8.51 GHz), the critical dimen-
sion for several of the most important modes is given in Table 1.
It is seen that in the feeding waveguides where a = 0.95 in.,
only the two orthogonally polarized TEIo modes propagate.
When the four guides are combined as shown in Fig. 2, the
output waveguide has an inner dimension of 2.060 in. From
Table 1 it is seen that this is just below the critical dimen-
sion for the TE3o and TEo3 modes. For symmetric (in-phase)
excitation of the four feeding waveguides, this is the first
higher-order mode that is excited in the large waveguide.
Therefore, for the chosen dimensions, only the TEIo and
TEoi modes exist in the large output waveguide section of the
combiner. Since the TE3o and TEo3 modes are strongly
excited at the junction but decay away from the edge, a
straight section follows the combiner in order to allow these
modes to decay at least 30 dB before the horn begins to flare
out. A wall thickness of 0.160 in. between the four input
waveguides was chosen to allow sufficient room for cooling
channels in the web region. Maximum dimensions were chosen
to obtain the highest possible power handling capability.
B. Multiflare Section
Next, the horn must be flared out in order to properly
illuminate the 70-m dual-reflector antenna. Since the antenna
is a dual-shaped system, optimum efficiency is obtained when
the feed radiation pattern matches that of the design feed (in
this case the DSN standard 22.37-dB corrugated horn) over the
16 degrees of angle subtended by the subreflector. The TEl0
mode itself has unequal E- and H-plane patterns and is, there-
fore, not suitable for illuminating the 70-m antenna alone. In
order to obtain equal E- and H-plane patterns for a square
horn, higher-order modes must be added to the TEIo mode. A
method for generating the required modes by using changes in
horn flare angle rather than steps or corrugations has been
described by Cohn [3]. The multiflare horn is ideally suited
for high-power applications, as has been shown by the success
of the Haystack system.
The required modes were determined by calculating the
radiation from square apertures of different sizes with varying
mode mixtures. An extensive computer study determined that
the mixture given in Table 2 most closely reproduced the
22.37-dB corrugated horn pattern over the 16-degree range. It
is evident from the table that three modes must be added to
the TElo mode. These three modes will contain about 17 per-
cent of the propagating power in the ideal design.
The next task was to determine the flare angle changes
required to generate such a mode mixture. Initial guesses were
made from inIbrmation in Cohn's original paper, and a more
detailed analysis was carried out to fine-tune the horn dimen-
sions. For this part of the analysis, the horn was step-approxi-
mated by 200 segments, and the aperture modes were calcu-
lated for a TEl0 mode incident at the input, using a mode-
matching method [4J. Two flare angle changes were found to
be necessary to generate the required modes. The dimensions
of the final horn design are shown in Fig. 3.
Many iterations were required to determine these final
dimensions. The mode generation along the length of the horn
may be traced by examining the mode content after each of
the segments during the analysis. This is only strictly true if
reflections from the remainder of the horn are zero, but in this
case reflections in the horn are small enough for this to be an
excellent approximation. The modes present at various posi.
tions along the horn are plotted in Fig. 4. At the input (z = 0),
only the TElo mode carries power, and this remains the case
through the first 6-in. section of straight guide. At the abrupt
flare angle change (z = 6 in.), three additional modes are ex-
cited, and coupling between these four modes takes place
continuously during the first flared section. This coupling
occurs because the waveguide modes are only independent
(uncoupled) in a perfectly straight waveguide, not in a tapered
horn. Once the guide straightens out again (z = 14.75 in.), the
modes propagate independently through the phasing section.
The next flare angle change near z = 32 in. generates the mode
content required in the aperture. Note that the mode ampli-
tudes are plotted on a linear scale, and the power carried by a
particular mode is given by the amplitude squared. At each
point in the horn, the sum of the squares of the four mode
amplitudes is unity, indicating that power is conserved.
C. Radiation Patterns
The theoretical radiation patterns for the horn in the princi.
pal planes are shown in Fig. 5. Important characteristics are
the nearly perfectly identical E- and H-plane patterns over the
16-degree angle subtended by the subreflector, and the rela-
tively low sidelobe level. When comparing these patterns with
the 22.37-dB circular corrugated horn patterns, one notes that
they are virtually identical over the required 16-degree angle.
The H-plane pattern for this horn has a higher first sidelobe
than the 22.37-dB pattern, while the first sidelobe in the E-
plane is lower. The 22.37-dB pattern actually has a filled-in
first sidelobe that peaks at about -25 dB. Calculated patterns
for the multiflare horn are nearly sidelobe free in the 45-degree
plane. These effects help to compensate for the higher side-
150
lobe in the H-plane. Calculated 70-m antenna gain l\)r this
multiflare horn and the 22.37-dB horn are within 0.03 dB of
agreelnent.
horns analyzed using the mode-matching method, the accuracy
is adequate for design purposes.
lU. Measured Results
A photograph of the prototype horn fl)r which radiation
patterns were measured is shown in Fig. 6. The E-plane and
[l-plane patterns of this horn were measured and are compared
to the theoretical patterns in Fig. 7. Good agreement can be
seen throughout the main beam region, with discrepancies of a
few dB in the sidelobe levels in both planes. Although the
agreement for this horn is not as close as that for circular
IV. Conclusions
A design for a l-megawatt feedhorn has been presented,
including a review of the design method. Measured patterns for
a prototype horn are in good agreement with those predicted
by the theory. Before a final high-power version of this horn is
fabricated, work must be completed on the matching of the
combiner section and also on the specifics of the cooling pas-
sages, due to the high power levels inwHved.
References
[1 I A. M. Bhanji, D. J. Hoppe, B. L. Conroy, and A. J. Freiley, "Conceptual Design of a
1-MW CW X-Band Transmitter for Planetary Radar," TDA Progress Report 42-95,
vol. July-September 1988, Jet Propulsion Laboratory, Pasadena, California, pp.
97-111, November 15, 1988.
121 W. North, "Haystack Hill Long Range Imaging Transmitter," Proceedings oJ'the 13th
Pulse Power Modulator Symposium, pp. 247-253, 1978.
[31 S. B. Cohn, "Flare-angle Changes in a Horn as a Means of Pattern Control," Micro-
waveJ., w)l. 13, pp. 41-46, October 1970.
[4] D. J. Hoppe, "Modal Analysis Applied to Circular, Rectangular, and Coaxial Wave-
guides," TDA Progress Report 42-95, vol, July-September 1988, Jet Propulsion
Laboratory, Pasadena, California, pp. 89-96, November 15, 1988.
151
Table 1. Square waveguide cutoff dimension for lower order
modes at 8.51 GHz
8.51-GHz cutoff dimension,
M ode
in.
TElo,o I 0.69343
TEll' TM11 0.98066
TE20,O 2 1.38606
TE21,12, TM21,12 t.55056
TE22, TM22 1.96132
TE30,O 3 2.08029
TE31,13, TM31,13 2.19282
TE32,23, TM32,23 2.50020
TE33, TM33 2.94197
Table 2. Modes required in a 6.1-in. square waveguide to duplicate
22.37-dB corrugated horn pattern
Mode power, Mode phase,
Mode Mode amplitude
percent deg
TEIo 0.91 83.2 0
TE30 0.07 0.5 90
TEl2 0.16 2.6 180
TMt2 0.37 13.7 180
100.0
152
MONITOR
SIGNALS
[ I
I FEED HORN 'I
I IN UNE INPUTS I I ORTHO INPUTS _ I
_, ;_w ; _ _K1 _ 250 kW
I TRANSMIT ,u.W LAYOUT
AND SWITCH MATRIX
I C+D r "_
I I E÷F
I IG÷HI -'Y_
I I
A+B+C+D
;K
E + F +r.G_÷ H _L
J IRECE'VE_WPLOM_IN_ I _
Fig. 1. 1-MW radar transmission line system.
I
_R
I
__1
]
0.95 in.
SQUARE
0 OlO 0
D
0 0 0 0
0 0 0 0
D M
0 0 0 0
I
2.060 in.-
Fig. 2. I-MW horn combiner section.
I
17.300 in.
6.100 in. 5.000 in.
I
10.000 in.--_
Fig. 3. 1-MW multiflare horn dimensions.
153
<[
Lu
z
.J
Lu"
o
<
0
10
0.8
0.6
04
I TE10MODE I I
f
TM12 MODE _. t
f_
/
/ TE30 MODE
' TEl2 MODE "_t I I
10 20 30 40
z, in.
Fig. 4. Mode generation along the multiflare horn.
50
-5
-10
m -15
z
<
(3
w -20
>
)-
<
',J' -25
_c
-3O
-35
-40
I I 1
_
H-PLANE/
E PLANE I
J\,
-80 -60 -40 -20
I _ l I 1 I
f
1 I I _
0 20 40 60 80
THETA, deg
Fig. 5. Theoretical horn patterns.
Fig. 6. Prototype horn.
154 ,_,:;0 '_,'_.iI.rE pH_,,_,-];+,._p N
0-5
-10
-15
-20 -
-25 -
-30 -
-35 -
-40
I
{a)
I I I
I •
/--H-PLANE,
THEORY
/._H-PLANE,
/MEASURED
V
t
-5
10
15
z"
_ -20
d
e:: -25
-30
-35
40
I I I I _ I I I I
(b)
,...V.,_V_ 7,'\7\ :
-80 -60 -40 -20 0 20 40 60 80
THETA, deg
Fig. 7. Comparison between theoretical and measured patterns:
(a) H-plane; (b) E-plane.
155


A multiflare horn with 1-megawatt power handling capability

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server