Note: Descriptions are shown in the official language in which they were submitted.
108~845
This inyention relates ta antenna beam scanning and more
particularly to a nov~l method and apparatus os the non-mechanical
scanning of a microwave antenna beam using a frequency sensitive di-
electric loaded directive antenna system.
Scanning of antenna beams for many different purposes, is well
known in the art and employs many conventional mechanical and electronic
scanning techniques. Most mechanical techniques require relatively
heavy equipment which can only be operated to ~can at relatively slow
speeds and are therefore unsuitable for such purposes as continuous
airborne radar scanning. The scanning speed is too low to develop the
required raster and the inertial force~ developed in high speed aircraft
manoeuvreg tendto jam or even destroy the relatively delicate mechanical
scanner. Electronic techniques generally employ relatively expensive
devices such as phase shifters to accomplish a beam angle shift at a
single frequency or within a very narrow frequency band of the order
of + 20 MHz. Many well known designs have been developed, particularly
in the field of radar where 6ector scanning at a single frequency is
desirable.
However, in the field of microwave radiometry it is highly
desirable to trace the signature of the target, as a function of fre-
quency, in order to uniquely classify the target. For example, if the
rate of frequency sweep can be carefully chosen in relation to the time
constant of the radiometer and modern frequency stabilizing loops, then
the target may be spatially mapped by varying the look angle while
being uniquely classified at each look angle from the multi-frequency
tracking data. Such a system is particularly suitable for, for example,
use in a microwave radiometer scanning device for aircraft which may be
used, in an analogous manner to a ~canning radar beam, to locate and
identify ground installations fr~m the air even in bad weather conditions
such as fog or arctic white-out.
It has been found that a beam angle 6hift can be achieved by
the use of wedge shaped ~requency-sensitive dielectric inserts or coat~
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ings on the reflecting or quiding surface of a directive antenna. It
will be appreciated by those skilled in the art that substantially all
dielectric materials are frequency sensitlve in a general and very gradual
manner, but as used herein the term frequency sensitive dielectric
materials is intended to mean dielectric materials of fixed physical
dimensions and fixed relative dielectric constant ~r within the range of
frequency sweep. Thus as the frequency is swept the electrical dimensions
of the dielectric insert change accordingly as discussed in more detail
hereinafter. Surprisingly it has been found that the effect of the
dielectric material is to réduce the wave velocity thus increasing the
wave number by the square root of the relative dielectric constant and
hence increase the electrical size of the dielectric wedge region over the
physical size. For a fixed frequency, this is equivalent to altering the
angle of the reflecting surface by an amount which is almost equal to
the square root of the relative dielectric constant. The resulting
electrical dimensions, and hence the radiation pattern of the antenna
are sensitive to the frequency of operation within the passband, regardless
of the particular shape of the antenna selected. Thus the antenna may be
any design which can be made to reflect asymmetrically by placing one or
more dielectric wedges on the reflecting or guiding surface thereof. If,
at the centre of the frequency band the design dimensions are such that
the main lobe or beam is along the principal axis of the antenna, the
position of this lobe or beam will deviate from the axis and swing from
one side of the axis to the other as the frequency is swept from below
the centre of the band to above. In effect, from a radiation pattern
point of view, the antenna is only electrically symmetrical at the centre
of the frequency band of operation and is progressively asymmetrical at
all other frequencies.
It is, therefore, an ob~ect of the present invention to
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provide a method for electronically scanning a microwave antenna beam in
a simple and inexpensive manner.
It is another ob~ect of the present invention to provide
apparatus for effecting a beam angle shift in a microwave antenna beam.
By one aspect of this invention there is provided a method
for non-mechanical scanning of a microwave beam which comprises providing
at least one reflecting surface of a directive antenna with at least one
wedge shaped layer of a frequency sensitive dielectric material and
directing a multi-frequency microwave beam thereagainst to thereby effect
a beam angle shift.
By another aspect of this invention there is provided an
apparatus for non-mechanical scanning of a microwave beam comprising:
directive antenna means having at least one guiding or reflecting
surface; and
at least one wedge shaped layer of a frequency~sensitive dielectric
material in planar contact with at least part of said surface to thereby
effect a beam angle shift of a multi-frequency microwave beam directed
thereagainst.
The invention will be described in more detail hereinafter
by reference to the accompanying drawings in which:
Figure 1 is a schematic view of a symmetrical 22 U-plane
sector horn antenna of conventional design;
Figure 2 is a schematic view of a horn of Figure 1 modified
to be asymmetrical about its axis and loaded with a dielectric wedge
on the asymmetrical narrow dimension wall;
Figure 3 is a schematic view of a horn of Figure 1 with a
dielectric wedge loading on both narrow dimension walls;
Figure 4 is a graph showing beam angle deflection for a
horn of Figure 2 with a dielectric wedge angle of 7.5 at 9 GHz as a
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1081845
function of power (db);
Figure 5 is a graph showing beam angle deflection for a horn
of Figure 2 with a dielectric wedge angle of 7.5 at 10 GHz as a function
of power ~db); and
Figure 6 is a graph showing beam angle deflection for a horn
of Figure 2 with a dlelectric wedge angle of 7.5 at 10.88 GHz as a
function of power (db).
Although the present invention is applicable to antennas of
substantially any practical shape and dimension, the invention will be
described hereinafter with particular reference to an H-plane sector
horn structure desfgned to operate in the X-band (i.e. 8-12.4 GHz), as
such a structure has particular suitability to a radiometer system.
A basic H-plane sectoral horn 1 is shown in Figure 1, with
a flare angle ~ of 22, as measured between the flared walls 2 and 3 and
symmetrical about the axis 4 of the horn. Horn 1 is also provided with
a wavegulde structure 5 in known manner. A modified horn 1 is shown in
Figure 2 in which the flare angle ~ is reduced by an amount ~ (approxi-
mately 5~ on one side, along wall 3 as shown in Figure 2, and measured
from corner A ad~acent the throat of the waveguide 5.
The asymmetrical horn is dielectrically loaded by inserting
a wedge 6 of a frequency-sensitive dielectric material such as acrylic,
ertalon (Trademark for a nylon type thermoplastic material sold by
Miller Plastics Ltd., 19 Advance Road, Toronto, Canada~ or polyethylene
along the wall 3 with the apex of the wedge adiacent the throat A of the
waveguide. The angle (a) of the wedge 6 may be altered as required and
as explained in more detail hereinafter.
In an alternative embodiment, as shown in Figure 3 a symmetri-
cal horn as described with reference to Figure 1 is provided with two
dielectric wedges 7 and 8 adjacent walls 2 and 3 respectively. Wedges
.. . . . . . .
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108~845
7 and 8 may be of the same dielectric materials in which case the
wedge angles are different or alternatively the dielectric materials
may be different and the wedge angles the same.
In operation a microwave beam of a selected frequency range
within the general frequency range 1 GHz to 150 GHz and preferably of
the order of 8 - 15 GHz i9 generated within the waveguide and transmitted
from the horn antenna. Radiation pattern plots may be deve]oped as
required to show the effect of the dielectric loading.
Example 1
` A symmetrical horn as shown in Figure 1, an unloaded asymmetri-
cal horn as described with reference to Figure 2 and an asymmetrical horn
loaded with an acrylic wedge (dielectric constant ~= 2.59) on wall 3
were tested at frequencies of 8.0, 9.0, 10.0 and 10.88 GHz with 10 GHz
as the mid frequency of the X-band. The wedge angle a of the dielectric
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wedge wa~ vaxied between 1~5 and 7.5~ and xadi~ti~on pattern charts for
the various frequencie~; and wedge angles were, plotted. Repre~entative
charts are illustrated herein as Figures 4~6 respectively. The results
are shown hereinbelow in Table 1.
Table 1
8.0 GHz 9.0 GHz 10 GHz 10.88 GHz
O~ S!~ ~ llJ `p ~" G ¢~ ~" ¢ ~V . - . .
. .
1 1/2 lR 0 31/2R 31/2R1/2R 2R 1/2R 2R
2 1/4 1/2R 0 2R 4R 3L lR 2L 2R ,
3 1/4 lL 3R 11/2R 4R 5L lR 71/2L lR
_ .
4 1/4 7L 0 51/2L 4R llL 11/2R SB 2R
5 1/4 5L 4R SB 4R 18R 2R 13R lR
6 1/2 SB 0 llR 4R 6~ 2R 2L 11/2R
7 1/2 191~4R 7R 4R 3R 711. llL lR
e,l ~ 2.59 (~ r of acrylic)
o~-= Dielectric wedge angle (in tegrees~
t = Beam angle
R- Right of standard horn beam angle at
zero (in degrees) off axis
L = Left of standard horn beam angle at
zero (in degrees) off axis ~'
~ = Asymmetrical horn beam angle unloaded
SB ~ Split beam.
With reference to Table 1, the beam angles are indicated right
or left of the reference (symmetrical) horn zero or axis for each wedge
angle. It will also be appreciated that the wedge angles 3.25 and 7.5
inticate a falrly uniform rate of disperslon from left to right. Figures
4, 5 and 6 also inticate that, at a given tielectric wedge angle,
increasing frequency causes a significsnt beam angle shlft from right to
left.
Example 2
A ~ymlDetrical horn as shown in Figure 1, having a flare angle
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22~ was modified hy placing dielect~ic wed~e infie~ts 7 and 8 on both
of the narrow walls 2 and 3 as shown in Pigure 3. The inserts 7 and 8
were machinet from different dielectric materials, ertalon (dielectric
constant ~= 4) and polyethylene (dielectric congtant ~ 2.28) respective-
ly, and their respective wedge angles 2 and ô3 were different. Radia-
tion pattern charts, similar to those illustrated in Figures 4 to 6,
were prepared using a multi-frequency microwave beam in the X-band, as
in ~xample 1. The resulting beam angle ~hift at different frequencies is
summarized ln Table 2.
Table 2
~2 ¦3 ¦ 8.0 GHZ 9.0 GHz 10 GHz10.9 GHz
6 I2 1 12R 7R 3L 6L
R - Right of standard horn beam
angle at zero degrees off axis
L ~ Left of standard horn beam
angle at zero degrees off axis
It will be seen from Table 2 that beam scannlng as a function of frequency
is evident but that the rate of beam shift is asymmetrical within the
frequency bant. Because the shift i~ ~ubstantially uniform ln the
asymmetrical horn of Example 1, it i8 believed that an asymmetrical H-
plane sectoral horn with dielectric loading on the one wall clo~est to
the waveguide axis is to be preferred for beam scanning with frequency
sweep. In special circumstances, however, and for specially shaped
radiation patterns the use of a plurality of different wedges may be
more effective.
Many modifications may be made to the apparatuR described
herein without departing from the scope of the invention which may be
practised in many ways not 6pecificall~ described herein. For example
the shape and dimensions of the antenna are not critical to the invention
but depends merely upon the use, function and frequency of the de~ired
microwave beam scan. The dimensionsOf the antenna are primarily a
function of the wavelength of the microwave beam and must be selected
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depending upon the ulti~ate use cxitexia. Si~ilaxly the dielectric
material to be employed for the wedges is merely a matter of design and
suitability in a particular situation or environment. While the di-
electric material has generally been described herein as a solid,
generally thermoplastic material, it will be appreciated that it may
be any frequency sensitive dielectric material which term will include
liquids as well as solid materials. It will also be appreciated that a
plurality of superimposed or partially supeTimposed wedgé 6haped layers
of the same or different dlelectric materials may be employed.
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