Note: Descriptions are shown in the official language in which they were submitted.
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1 CROSS REFERENCE TO RELATED APPLICATIONS
The present invention relates to antenna systems
radiating Doppler coded patterns using multiple beam
antennas, one form of which is described in Canadian
patentNo. 991,742, dated June 22nd, 1976, entitled "Antenna
System For Radiating Multiple Planar Beams", which is
assigned to the same assignee as the present application.
BACKGROUND OF THE INVENTION
This invention relates to systems for determin-
ing the angular position of a target with respect to a
reference location. In praticular this invention relates
to systems which use a frequency coded pattern to perform
angle measurement, also known as "Doppler" systems. In a
Doppler system an antenna radiates wave energy into a
region of space in a pattern wherein the frequency of
radiation varies with one of the angular components of
direction from the antenna. Frequency coded radiation has
in the past been achieved by radiating wave energy sequent-
ially from the individual antenna elements of an array.
This causes apparent motion of the radiation source,
resulting in a "Doppler" frequency shift which depends on
the relative angle of the target with respect to the antenna.
Some dificiencies associated with the sequentially-
excited array antenna for generating Doppler signals are
difficulty in controlling beam shape and complexity in
construction. A multiple beam antenna radiating a different
frequency on each beam would appear to be an attractive
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method for radiating a Doppler coded pattern. This
method could use a simpler antenna unit and have better
control over pattern shape and coding. An attempt to
continuously radiate different frequencies on the various
beams of a multiple beam antenna would result in random
interference between the radiated si~nals, resulting in
widely varyin~ signal amplitude and failure of coding.
SU~RY OF T~-~ INV~NTION
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It is an object of this invention, therefore, to
provide a new and improved antenna system for radiating
a Doppler pattern into a region of space from a multiple
beam antenna.
It is a further object of this invention to pro-
vide such a system wherein the radiated signal has a sub-
stantially constant amplitude versus time characteristic
during a time period. ~-
It is still a further ob~ect of this invention to
provide such a system wherein the radiated pattern can be
shaped to coincide with the desired region o~ space.
In accordance with the invention, there is provided
an antenna system for radiating wave energy into a desired
region of space in a desired radiation pattern during a selec-
ted time period. The desired pattern is one in which the
fre~uency of the radiated energy within the region of
space varies with at least one of the components of angular
direction from the antenna system~ The antenna system
includes an antenna unit capable of radiating a plurality
of beams in different directions within the region of
space from a common aperture, and having a plurality of
wave energy input ports, such that each of the ports
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corresponds to one of the beams. The antenna system
additionally includes means for sin~ltaneously supply-
ing a plurality of wave energy signals during the time
period, one to each of the ports of the antenna unit,
with each of the signals having a phase, measured with
respect to the phase o~ the wave energy signal supplied
to the port corresponding to an adjacent antenna beam,
which varies during said time period between a pre-
determined pair of values, the variation being less than
360 and the sense of the variation being alike for pairs
of antenna ports corresponding to similarl~ adjacent
beams. When these signals are supplied to the antenna
ports, the antenna radiates the desired radiation pattern.
For a better understanding of the present
invention, together with other and further objects thereof,
reference is had to the following description taken in
conjunction with the accompanying drawings~ and its scope
will be pointed out in the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is one embodiment of an antenna system
constructed in accordance with the present invention.
Fig. 2 is a diagram illustrating the phase of
wave energy signals used in conjunction with the Fig. 1
antenna.
Fig. 3 illustrates the operation of the Fig. 1
antenna.
Fig. 4 is an alternative embodiment of the
present invention.
DESCRIPTION AND OPERATING OF THE FIG. 1 ANTENNA S~STE~
The antenna system of Fig. 1 includes an antenna
unit consisting of a plurality of feedhorns lOa, b, c for
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1 illuminating a focusing reflector 11. The feedhorns 10
are located near the focal axis of the parabolic cvlindrical
reflector 11 and displaced from each other such that wave
energy from each feedhorn 10 illuminates the reflector 11
and causes a beam to be radiated at a different angle in
space with respect to the antenna system. This type of
antenna unit is more fully described and covered by the
above referenced patent.
Associated with each of the feedhorns 10 are corres-
ponding wave energy input ports 12a, b, c~ Each of these
input ports 12 are connected to a corresponding one of the
phase shifters 14a, b, c by suitable transmission lines
13a, b, c. An oscillator 15 supplies wave energy signals
to a power divider 16. The wave energy signals from the
outputs of the power divider 16 are supplied to the phase
shifters 14. Varying phase control signals are generated
by control unit 17 and supplied to control inputs of the
phase shifters 14. Thus, the wave energy signals supplied
to the phase shifters 14 have their phase shifted in relation
to each other in accordance with the phase control signals
such that signals with varying phase in relation to each
other are supplied by transmission lines 13 to the input
ports 12 of the feedhorns 10.
The oscillator 15, power divider 16, phase shifters
14, transmission lines 13 and control unit 17 together comprise
means for simultaneously supplying a plurality of wave energy
signals, one to each of the ports 12 of the antenna unit,
with each of the wave energy signals having a predetermined
varying phase in relation to any other of said signals.
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Each of the feedhorns 10 in Fig. 1 is designed to
illuminate the re~lector 11, which forms a common aperture.
The antenna unit radiates a beam for each of the feedhorns
10 in a direction which is unique to each of the feedhorns
by reason of the displacement of the feedhorns 10 from each
other as explained more fully in the aforementioned co-
pending application. Each o~ the input ports 12 of the
feedhorns 10 is therefore associated with an antenna beam.
Those skilled in the art will recognize that other
types of multiple beam antennas may be substituted ~or the
antenna unit shown in Fig. 1. The antenna must be capable
of radiating a plurality of beams in different directions
within a desired region of space from a common aperture,
and have a plurality of wave energy input ports such that
each of the ports corresponds to one o~ the beams. Antennas
of this type may be conveniently re~erred to as "Beamport1'
antennas.
The transmission lines 13 may be any type appropriate
for use at the operating frequency chosen ~or the antenna
system. It is important, however, in the Fig. 1 embodiment
that these transmission lines have a phase length in relation
to each other which is appropriate for supplying the wave
energy signals to the ports 12 with the required varying
phase in relation to each other.
The phase shifters 14 may be any type which is
appropriate for the frequency of the wave energy signals.
Examples of suitable phase shifters are ferrite phase shifters
and diode phase shifters, both of which use phase control
signals to vary their apparent electrical length and thereby
phase shift the wave energy signals. The phase control
signals supplied by the control unit 17 should be signals
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appropriate for controlling the phase shifters 14
selected for use in the antenna system. These signals
may be digital logic signals or analog signals according
to the type of phase shifters selected.
The oscillator 15 may be any suitable generator
of wave energy signals at the chosen operating ~requency.
The power divider may be any of the commonly used types,
~ell known ~n the art, such as couplers, "T" junctions
or reactive dividers.
It will be evident that other means may be used
to supply the necessary wave energy signals Witll a varying
phase in relation to each other. For example, phase control
may be pèrformed at a different frequency than the radiated
frequency and using frequency converting devices, or by
performing a digital or analog frequency synthesis to
generate the required signals. Phase control may also
be achieved by using mixing devices rather than phase
shifters.
Fig. 2 illustrates typical varying phase of the
signals supplied to the input ports 12 of the Fig. 1
antenna. Phase is shown in relation to the phase of signal
"C," ~Jhich would be supplied to the input port 12c, for
example. As is evident from the diagram, the phase of the
signals "A" and "B," which would be supplied to input ports
12a and 12b, respectively, have a varying phase in relation
to the phase of the signal "C" and in relation to each
other. As shown in ~ig. 2, the sense of phase variation
for the signals supplied to each port with respect to an
adjacent port is alike for pairs of antenna ports corres-
~0 ponding to similarly adjacent beams. Consequently,
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signal "~", supplied to port lOa has a positive phase
variation ~rith respect to signal "B" supplied to port
lOb. Likewise, signal "B" has a positive variation with
respect to signal "C" supplied to port lOc. The phase of
the signals during a period nominally varies linearly from
a first predetermined phase point ~or each o~ the si~nals
to a second predetermined point ~or each of the signals.
The phase variation may depart ~rom a linear variation to
account for particular characteristics of various ~ntennas
such as defocus~ng or non-equal spacing of the feedhorns,
etc. The phase variation period may be continuously
repeated as shown in Fig. 2 to produce a substantially E
continuous frequency coding.
It should be noted that during any particular
period the effect of the linear phase variation is to
cause a frequency change in the corresponding wave energy
signal. However, it is not effective to continuously
supply wave energy signals of different frequency to the
input ports o~ the antenna to cause the desired radiation
pattern, because the phase relation necessary to prevent
interference of the signals in the various beams is only
present during a particular period. To prevent interfer-
ence between ad~acent beams it is necessary that the phase
between the signals supplied to ports corresponding to
adjacent beams never be such that the adjacent beams are
180 out o~ phase. Consequently, the total phase varia-
tion between adjacent ports can never exceed 360 and is
usually much less than 360~.
Doppler frequency coding is most often associated
with an antenna which radiates energy from a moving radia-
tion source. Fig. 3 illustrates a sectional view of the
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antenna unit used in the Fig. 1 antenna system. At the
beginning o~ a period the phase of the wave ener~y signals
supplied to the feèdhorns 10 combine when radiated from
the feedhorns to form a radiation phase front l~a which
proceeds in the direction l9a, to illuminate an area
around the point 20a on the reflector 11. During the
period the phase of the wave energy supplied to the feed-
horns 10 varies, as shown in Fig. 2, causing the illuminated
area to move vertically across the reflector. ~t the end
of the period the phase of the wave energy signals supplied
to the feedhor~s 10 form the phase front l~b, which proceeds
in a direction l9b, to illuminate an area around point 20b
on the reflector. This process may be repeated for several
periods, causing the illuminated area on the re~lector 11
to repeatedly move from the vicinity around the point 20a
to the vicinity around the point 20b. Points 20a and 20b
are sho~n by way o* example in Fig. 3. The illuminated
area may center around any points on the section of the
reflector. This motion of the illuminated area on the
reflector causes the antenna system to radiate a pattern
similar to a sequentially excited array wherein the frequen-
cy of radiation varies with one of the angular components
of direction from the antenna.
The ~roup of feedhorns 10 may be considered to
be a phased array for illuminating the reflector 11 and
array design principles are therefore applicable. The
spacing between the feedhorns should be chosen such that
there will be no "grating lobes" on the reflector when
the feedhorns are excited by any of the phase relations
associated with a period. The number of feedhorns required
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is a ~unction of the angular region of space within which
it is desired to radiate the frequency coded pattern. A
larger number of feedhorns would cause a narrower illumin-
ated area and hence a larger angular region in which the
frequency coded pattern would be radiated. Other trade-
offs will be evident to those skilled in the art. For
example, the time durat~on of the phase variation period
is dependent on the amount of fre~uency shift desired in
the radiated pattern. me shape and size of the reflector
11 and feedhorns 10 are dependent on the region of coverage
and beamshape desired. The use o~ other ~eed elements in
place of feedhorns and other means ~or focusing wave
energy in place o~ a parabolic reflector will be evident
to those skilled in the art.
DESCRIPTION AND OP~RaTION OF THE FIG. 4 ANTENNA SYSTEM
Fig. 4 illustrates another embodiment of an antenna `-
system constructed in accordance with the present invention.
In the Fig. 4 system, wave energy signals are supplied to the
antenna ports 21a, b~ C3 d by similar devices 14-17 as in
the Fig. 1 antenna. The principal difference is that the
antenna unit in the Fig. 4 embodiment comprises an array
of antenna elements 22 which are coupled to the antenna
ports 21 by a Butler Matrix 23. The properties of a Butler
Matrix are well known in the art. Basically, each of the
input ports 21 is coupled to the antenna elements 22 by the
Butler Matrix 23 such that wave energy signals supplied to
each of the ports 21 will be radiated by the elements 22 in
a beam which is in a direction unique to that port. Thus,
the antenna unit in the Fig. 4 embodiment has the same
general characteristics as the antenna unit in the Fig. 1
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embodiment, that is, they are both "Beamport" antennas,
although different in form. ~;
Wave energy signals having varying phase in
relation to each other, ~hen simultaneousl~ supplied to
the antenna ports 21 in Fig. 4, will cause wave energy
signals to be sequentially supplied to the elements 22 of the
aperture in a manner resulting in an apparent motion o~
the radiation source. mis operation is evident because
o~ the nature of the transformation performed by the Butler
Matrix 23.
Other variations in antenna systems which embody
the present invention will be evident to those skilled in
the art. Other matrices can be used to provide the nec- -
essary multiple-beam, mutlitple-port antenna function,
including those which operate at a di~ferent ~requency than
the desired frequency of radiation in conjunction with
devices for frequency conversion. Also, devices which
are not matrices of themselves, sùch as enclosed lenses,
but have the same properties by reason of transmission
characteristics can be used in an antenna system con-
structed in accordance with the present invention.
In describing the various embodiments above,
reference has been made to transmitting antenna systems,
but it will be recognized by those skilled in the art
that the principles of the present invention can also be
applied to receiving antenna systems. Accordingly, the
appended claims shall be construed as covering both trans-
mitting and receiving antenna systems regardless of the
descriptive terms actually used therein.
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