Communications
Beam Shaping of Sectoral Horn Antennas by
Dielectric Flanges
K G
NAJR*
Department of Electrical & Electronic Engineering
University of Leeds, LS2 9JT, England
&
G MOHANACHANDRAN
Department
& K T MATHEW
of Physics, University of Cochin, Cochin 682022
Received 16 May 1976; in revised/arm
4 August 1976
Dielectric flanges are found to be effective in modifying the
radiation patterns of sectoral horn antennas. The flange parameters and the basic functioning are the same as that of
metal flanges. An asymmetric flange system consisting of a
metallic element on one side and an equivalent dielectric
element on the other is observed to have a very significant
tilting effect on the beam. Relevant radiation patterns are
presented and a theoretical explanation is given.
Shaping of radiation patterns of sectoral horn
antennas by metallic flanges has been a subject of
intense study in the past.":"
Nair and Srivastava!
and Nair et a/.2 pointed out that the radiation pattern
of a sectoral horn antenna can be narrowed down or
broadened
up by a symmetrical
flange system not
only by adjusting the flange-length
and the flangeangle as reported by Butson and Thompson," but
also by varying a third parameter involved.
This is
the relative position of the flange with respect to the
aperture
of the horn.
The flange-axis asymmetry
and the flange-length asymmetry
WIll cause beam tilt
of the system.
Another type of asymmetry
which
may be imposed on a flange system has been suggested
by Koshy et a/4 The amplitudes of excitation of the
secondary radiators, which are assumed to be situated
at the edges of the flanges need not be equal.
This
unequal amplitudes of excitation can be achieved by
taking flange-elements of different materials on both
sides of the flange. This suggestion has been taken
up for the present investigation.
The radiation patterns are plotted by an automatic
pattern recorder.
A heterodyne
receiver system is
arranged on the turn-table as shown in Fig. 1. The
local oscillator is a Gunn diode which can be tuned
from 8 to I I GHz.
The horn under test is used as a
receiver of CW signal transmitted
by a standard
·Present address:
Microwave Laboratory,
ment, Cochin University. Cochin 682022.
254
Physics Depart-
pyramidal horn radiator.
The flange system consists
of a rectangular frame to which the flange elements
can be conveniently attached.
Provision is made to
vary the flange-length
and the flange-angle
conveniently.
The lay-out of the flange system is the
same as described in an earlier paper."
As dielectric
elements, polythene plates of thickness 0.\5 ern are
used while the metal elements are of aluminium of
the same thickness. A schematic diagram of a flanged
sectoral horn is given in Fig. 2.
When a symmetric dielectric flange system is used,
it has been observed that the radiation pattern in the
E-plane of an H-plane sectoral horn will be changed
as in the case of metal flanges.
The on-axis power
of the horn fluctuates with the position of the flange
relative to the aperture of the horn, giving sharp
maximum and minimum as shown in Fig. 3. The
position of the flange corresponding to maximum onTRANSMITTING
~
HORN
~
..
"
....
AUTOMAT~C
PATTERN
RECORDER
TURN TABLE
rr---
~~~l
1
SYNC. CI RCUIT
Fig.
1- Experimental
arrangement
showing
the
antenna and the heterodyne receiver system
horn
,.
~'/AL
.p
&
(-~tf'~: ,
..~
.'
"HORN~"~'
,.:~·'·'e
--~.'j;~
,<" Z
--
- AX'S
"'J
FLANGE
Fig. 2- Schematic diagram of a flanged sectoral horn (The
flare of the horn is in the plane normal to the paper)
COMMUNICA
axis power is referred to as a-position, while that of
the minimum on-axis power as M-position.
When
there are many O-and i\-I-positions, we take the most
prominen tones.
The effect of the dielectric flanges on the E-plane
radiation patterns of H-plane sectoral horns is studied
by keeping the flange at the M and
positions. Some
representative radiation patterns are shown in Fig. 4.
For a comparison, the half-power beam width of the
natural patterns and that of the modified beams due
a
(a)
- '-;- - -"- -
..
x
o
E
0..
o
2
- \ .....
- -
6
4
1·0
0::"
075
,
-;' -
I
(b)
"
-,---'---,.-
i" - ',-
0·5
Zo "",
•. J
0'25
0
6
4
2
Z,cm
Fig. 3- Variation of on-axis power of the flanged horns with
the position of the flange with respect to the aperture [(a)
Horn HI; flange length 6'3 ems, flange angle 90°; (h) Horn
H2 ; flange length 5' 5 ems, flange angle 60°]
nONS
to metallic
and equivalent
dielectric
flanges are
tabulated in Table 1. It can be seen that the dielectric
flanges are not so effective as the metallic flanges in
shaping the beams from sectoral horns.
However,
they influence the beam shape considerably.
Keeping a dielectric flange element on one side
and an equivalent metallic element on the other, an
asymmetric
flange system has been set up. The
elements are of equal lengths and the flange-axis is
adjusted to be the same as the horn axis. By moving
the flange system back from the aperture, the on-axis
power variation is studied and the a-position is determined as explained earlier,
Plotting the radiation
pattern, it has been observed that the beams are tilted
from the common axis of the system.
The tilting is
found to be exclusively towards the metallic flange
element. In Fig. 4(c) and 4(d), the tilted patterns are
represented
for two H-plane sectoral horns
when
asymmetric flanges of different parameters
are used.
The results of the detailed investigation are tabulated
in Table 2, from which it can be seen that the beam
tilting effect becomes less prominent
and becomes
less and almost insignificant at larger flange-angles.
From the above experimental results, it may be
concluded that dielectric flanges are also effective in
controlling
the E-plane
radiation
pattern
of the
H-plane
sectoral horns.
According to the theory
suggested by Owen and Reynolds," the two edges of
the flange act as secondary radiators.
The resultant
pattern of the system can be obtained by combining
the radiations from the primary aperture of the horn
Table 1- Half-power Beam Widths of Horns with and
without Metallic and Dielectric Flange System
Beam widths of modified
E-plane radiation patterns
(in deg)
with metallic
with dielectric
flanges
flanges
····Horn
Horn
HI
HI
H2
H2
(')3 )
(82)
(91)
(82)
Flange
length
cm
Flange
angle
deg
6'3
45
60
90
120
42
22
20
16
21
20
18
16
64
40
38
38
68
52
56
64
5'5
45
60
90
120
34
23
26
30
38
28
24
20
62
45
52
54
64
55
40
42
Note:
The values in paranthesis
below the column
headings HI and H2 indicate the natural E-plane in deg.
Frequency:
9'98 GHz; Horn HI : Radial length 17'5 ern,
Bare angle 25°, H-plane aperture 10 em, E-plarce aperture
1'0 ern: Horn H2: Radial length 9,5 em, flare angle 60°,
H-plane aperture 12 ern, E-plane aperture 1'0 ern.
(a)
Fig, 4 - Natural and modified radiation patterns (A, patterns
for horn HI with flange elements of length 6'3 em and flange
angle 90°; B, patterns for horn H2 with flanges of length
5' 5 cm and flange angle 60°) of H-plane sectoral horns:
(a) natural pattern, (b) modified pattern with a symmetric
dielectric flange at a position; (c) focussed beam tilted by
an asymmetric
flange system when the dielectric flange
elements is on the right side, and (d) focussed beam tilted
by an asymmetric flange system when the dielectric element
is on the left side (The tilting is invariably towards the side
in which the metallic flange element is attached)
255
INDIAN
Table 2-Beam
J. RADIO
Left
Dielectric
Right
Left
Beam tilt (in
deg) L: 10wards left
R: towards
Flange
angle
dc:g
-------------
Metallic
VOL. 5, SEPTEMBER
J976
Tilt by Asymmetric Flange System
Flange length (in em)
.
SPACE PHYS.,
Right
+
2 {[cos ~: {I
+ [sin-~:
right
{I
+
e
cos (ex -
+ cos
(ex -
+
~)}l
(kl+k2COSP)
e -1- o)} ] (k2 sinp) } ... (2)
Tforn--HI
6·3
45
45
60
('0
90
90
120
120
45
45
00
60
90
90
120
120
6.3
6·3
6·3
6·3
6·3
6·3
6·3
6·3
0·3
6·3
6·3
6·3
6·3
6·3
0·3
5"5
S'5
5"5
5·5
5·5
5.5
5·5
5·5
5·5
5·5
5·5
5'5
5·5
5·5
5·5
5.5
H2
J8 L
20 R
14 L
12 R
8 L
5 R
0
0
20 L
17 R
15 L
14 R
4 L
6 R
2 L
0
21 L
18 R
:4 L
10 R
9 L
6R
0
3 R
17 L
15 R
12 L
10 R
9 L
6 R
4 L
3 R
and the two secondary radiators.
For a metallic
flange
system
of equal
elements,
Butson and
Thompson" have derived expression for the radiation
pattern using this method. For an asymmetric system,
a more general expression has been derived by Koshy
et al.4 In this case, the amplitudes of excitation of
the two secondary sources situated at the edges of
the flange system are assumed to be different, say kl
and k~. Let the flange axis be at an angle 8 with respect to the horn axis. B, and B2 are the flange lengths
and Z is the distance of the foot of the flange from
the aperture of the horn.
The resultant power Pe at
a distant point of bearing angle 6 is given by
Pe
= I
+
k~
+ ki+
+ 2{[COS
x (kl
+
+ cos
2klkzcosp
1
7T:
k2COSp)
(ex -
(j
(I + cos (ex -+
[Sin
+ ~»
7T~2_
}k2
6
+ 0»]
(I
sinp) }
.. (1)
where p is the phase difference between the two
sources, which depends on the values of B1, B2 and
Z, 2ex, the flange angle and A the wavelength of
radiation.
The secondary amplitudes ki and k2 are
with respect to that of the primary aperture, which
is taken as a linear source of amplitude unity.
For elements of equal lengths, B, = B2 and then
Eq. (I) will reduce to
256
Here, the phase difference
p
. e ~)
= 27TB.
~ sin ex SIl1(
+u
... (3)
Using these equations, the radiation patterns of offaxis flange systems are computed.
For symmetric
flange systems, ~ = 0, and in these cases, Eq. (2)
will be further reduced to
=
+
2{[COS·~A~{1
+ [sin
I
+ k~ +
PB
7TAB {I
COS P
k ~ +2klk2
+ cos
+ cas
(ex-G)}]
(ex -
6}]
(kJ ~k2COSP)
(h sinp) }
.. (4)
The value of kl has been estimated
by Butson and
Using the same method and
substituting
for k i, k ; is extrapolated
to be 0.026.
The normalized power patterns are computed using
the Leeds 1906A computer.
The theoretical patterns
are found to be agreeing
with the experimental
patterns in Fig. 4. There were noticeable differences
in the side-lobe levels, which an." presumably due to
unavoidable wall reflections.
Thompson" as 0.077.
One of the authors (K G N) expresses his thanks
to the Commonwealth
Scholarship Commission
in
U K for providing an academic
staff fellowship
which enabled him to work in the University of
Leeds, England.
Thanks are due to Prof. P A
Matthews,
University of Leeds, for helpful discussions and encouragement.
A part of the work was
conducted at the University of Cochin, Cochin.
The
authors express their acknowledgement
to Dr Joy
George for providing the necessary laboratory
facilities.
References
I. Nair KG & Srivastava G P, J. Ills tn, Telccommun
Engrs, New Delhi, 13 (1967),76.
2. Nair K G, Srivastava G P & Singh S B, J. Inst n, retecommun. Engrs , New Delhi, 14 (1968), 352.
3. Butson PC & Thompson G T, Proc. Instn. elect. Engrs,
8106 (1959), 422.
4. Koshy V K, Singh S B, Nair KG & Srivastava G P,
Int. J. Electron., 25 (1968),289.
5. Nair K G, Srivastava G P, Singh S B & Koshy V K,
lilt. J. Electron., 25 (1968),153.
6. Owen A R G & Reynolds L G, J. Instn, elect. Engrs ,
93 IlIA (1946), 1528.
7. Koshy V K, Nair K G & Srivastava G P, IEEE Trans.,
AP18 (1970),470.
8. Koshy V K, Nair K G & Srivastava G P, J. Instn, Tclecommun, Engrs , New Delhi, 17 (1971),344.