U.S. patent number 3,921,177 [Application Number 05/352,005] was granted by the patent office on 1975-11-18 for microstrip antenna structures and arrays.
This patent grant is currently assigned to Ball Brothers Research Corporation. Invention is credited to Robert E. Munson.
United States Patent |
3,921,177 |
Munson |
November 18, 1975 |
Microstrip antenna structures and arrays
Abstract
A microstrip antenna structure formed from a unitary conducting
surface separated from a ground plane by a dielectric film where
the r.f. radiator and feedlines form a generally planar arrangement
of unitary integrally formed electrical conductors. The r.f.
radiators are fed from an outside edge to selectively produce
linearly and/or circularly polarized radiation at a selected
resonant frequency(s). Necessary fixed phase shifting circuits are
integrally formed by printed circuit techniques in the generally
planar arrangement of electrical conductors for the circularly
polarized radiators. A plurality of such antenna elements are also
formed into a phased antenna array to achieve substantially ideal
array gain thus producing an extremely high gain antenna with
inexpensive printed circuit board construction techniques.
Furthermore, appropriately controlled phase shifting networks may
be integrally formed within the generally planar array of
electrical conductors in combination with switchable diode elements
to achieve any desired relative phase shifts between the array
elements and thus to steer the array beam in a desired
direction.
Inventors: |
Munson; Robert E. (Boulder,
CO) |
Assignee: |
Ball Brothers Research
Corporation (Boulder, CO)
|
Family
ID: |
23383388 |
Appl.
No.: |
05/352,005 |
Filed: |
April 17, 1973 |
Current U.S.
Class: |
343/846; 333/238;
342/371 |
Current CPC
Class: |
H01Q
21/065 (20130101); H01Q 9/0435 (20130101) |
Current International
Class: |
H01Q
9/04 (20060101); H01Q 21/06 (20060101); H01Q
001/38 (); H01Q 003/26 () |
Field of
Search: |
;343/769,846,854
;333/84M |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Lieberman; Eli
Attorney, Agent or Firm: Haynes; James D.
Claims
What is claimed is:
1. An antenna structure comprising:
an electrically conducting ground surface,
a single layer electrically conducting surface comprising both an
r.f. radiator conducting area and an r.f. feedline conducting area
integrally connected thereto,
a dielectric sheet disposed between said ground surface and the
single layer electrically conducting surface, and
said r.f. feedline being connected at the outside edge of said r.f.
radiator conducting area to at least one predetermined point on the
periphery of said radiator conducting area to achieve an r.f.
radiation pattern having predetermined polarization characteristics
from said radiator,
said r.f. feedline conducting area including:
a first section connected to only a first point on the outside edge
of said r.f. radiator conducting area,
a second section connected to only a second point on the outside
edge of said r.f. radiator conducting area,
said first and second points being separated by a predetermined
amount along the outside edge of said r.f. conducting area to
define two intersecting axes of current flow, each such axis
passing through a corresponding one of said first and second
points, and
phase shifting means connected between said first and second
sections and a common r.f. input/output point whereby the relative
phases of r.f. signals on said first and second sections are
controlled with respect to the phase at said common r.f.
input/output to produce an r.f. radiation pattern having circular
polarization characteristics.
2. An antenna structure as in claim 1 wherein said phase shifting
means comprises means for introducing a 90.degree. relative phase
shift thereby producing circular r.f. polarization.
3. An antenna structure as in claim 2 wherein said phase shifting
means comprises:
a closed rectilinear conductive path having four corners,
said first and second sections being connected to respectively
associated adjacent ones of said corners, and
said common r.f. input/output comprising one of the remaining two
corners for right-hand circular r.f. polarization and comprising
the other one of the remaining two corners for left-hand circular
r.f. polarization.
4. An antenna structure as in claim 2 wherein:
said r.f. radiator conducting area is a square shaped area.
5. An antenna structure as in claim 2 wherein:
said r.f. radiator conducting area is a circularly shaped area.
6. An antenna array comprising:
an electrically conducting ground surface,
a single layer arrangement of electrical conductors comprising both
a plurality of r.f. radiators and a corporate structure r.f.
feedline having a common input/output connected thereto,
a dielectric sheet disposed between said ground surface and the
single layer arrangement of electrical conductors,
each of said r.f. radiators comprising an elongated unitary
conducting area separate from said dielectric sheet, each of said
areas having a width substantially equal to one-half wavelength at
an anticipated operating frequency and a length of more than one
such wavelength with a plurality of spaced feed points along one of
the longer sides located at intervals of no more than one such
wavelength apart,
said plural r.f. radiators being spaced from one another by
substantially one-half such wavelength in a direction perpendicular
to the longer sides of the r.f. radiators, and
said corporate structure r.f. feedline being connected to said
spaced feed points at the outer edge of said r.f. radiators.
7. An antenna array as in claim 6 wherein said corporate structure
r.f. feedline comprises predetermined different r.f. transmission
lengths between the common input/output and the various r.f.
radiators to produce a tapered amplitude distribution over the
aperture of the array thereby reducing sidelobes in the r.f.
radiation pattern of the array.
8. A phased antenna array structure comprising:
an electrically conducting ground surface,
a single layer arrangement of electrical conductors comprising a
plurality of r.f. radiators and a respectively corresponding
plurality of r.f. feedline antenna structures respectively
connected thereto and also connected to a common r.f. input/output
point,
a dielectric sheet disposed between said ground surface and the
single layer arrangement of electrical conductors,
each of said r.f. feedlines being connected at the outer edge of
its correspondingly associated r.f. radiator to at least one
predetermined point to achieve an r.f. radiation pattern from each
r.f. radiator having predetermined polarization
characteristics,
controllable phase shifters being interposed in said r.f. feedlines
to control the relative phase of r.f. energy associated with each
r.f. radiator and thereby to control the beam direction of the
overall radiation pattern of said array,
said phase shifters being an integral part of said single layer
arrangement of electrical conductors, and
each of said phase shifters including switchable diodes for
controlling the phase shift to be produced thereby.
9. An antenna array comprising:
an electrically conducting ground surface,
a single layer electrically conducting surface comprising both a
plurality of r.f. radiator conducting areas and a plurality of r.f.
feedline conducting areas integrally connected thereto,
a dielectric sheet disposed between said ground surface and the
single layer electrically conducting surface,
each of said r.f. feedlines being connected at the outside edge of
its correspondingly associated r.f. radiator conducting area to at
least one predetermined point on the periphery of said radiator
conducting area to achieve an r.f. radiation pattern having
predetermined polarization characteristics from said radiator,
said plurality of separate r.f. radiators and respectively
corresponding r.f. feedlines being arranged in a phased array
including interconnections between said plurality of r.f. feedlines
to connect all of the plurality of r.f. radiators with a common
r.f. input/output point, and
wherein the interconnected r.f. feedlines comprise predetermined
different r.f. transmission lengths between the common input/output
and the various r.f. radiators to produce a tapered amplitude
distribution over the aperture of the array thereby reducing
sidelobes in the r.f. radiation pattern of the array.
10. An antenna structure comprising:
an electrically conducting ground surface,
a single layer electrically conducting surface comprising both an
r.f. radiator conducting area and an r.f. feedline conducting area
integrally connecting thereto,
a dielectric sheet disposed between said ground surface and the
layer electrically conducting surface, and
said r.f. feedline being connected at the outside edge of said r.f.
radiator conducting area to at least one predetermined point on the
periphery of said radiator conducting area to achieve an r.f.
radiation pattern having predetermined polarization characteristics
from said radiator,
said r.f. radiator being formed in a rectangular shaped area,
the longer side of the rectangular area being substantially equal
to but slightly less than one-half wavelength long at a first
anticipated operating frequency when r.f. is to be fed into a
shorter side of the area, and
the shorter side of the rectangular area being substantially equal
to but slightly less than one-half wavelength long at a second
anticipated operating frequency when r.f. is to be fed into a
longer side of the area.
11. An antenna structure comprising:
an electrically conducting ground surface,
a single layer electrically conducting surface comprising both an
r.f. radiator conducting area and an r.f. feedline conducting area
integrally connected thereto,
a dielectric sheet disposed between said ground surface and the
single layer electrically conducting surface,
said r.f. feedline being connected at the outside edge of said r.f.
radiator conducting area to at least one predetermined point on the
periphery of said radiator conducting area to achieve an r.f.
radiation pattern having predetermined polarization characteristics
from said radiator,
said r.f. feedline being connected to only one point on the outside
edge of said r.f. radiator conducting area to produce an r.f.
radiation pattern having circular polarization characteristics,
said r.f. radiator conducting area including means dimensioned
differently in two mutually orthogonal directions, said means
providing two corresponding complex-valued electrical impedances
along said directions at the intended r.f. operating frequency,
which two complex-valued impedances are complex conjugates of each
other thus facilitating the desired circular r.f. polarization
characteristic.
12. An antenna structure comprising:
an electrically conducting ground surface,
a single layer electrically conducting surface comprising both an
r.f. radiator conducting area and an r.f. feedline conducting area
integrally connected thereto,
a dielectric sheet disposed between said ground surface and the
single layer electrically conducting surface,
said r.f. feedline being connected at the outside edge of said r.f.
radiator conducting area to at least one predetermined point on the
periphery of said radiator conducting area to achieve an r.f.
radiation pattern having predetermined polarization characteristics
from said radiator,
said r.f. feedline conducting area comprising:
a first feedline connected only to a first point on the outside
edge of said r.f. radiator conducting area to produce a linear r.f.
polarization characteristics; and
a second feedline connected only to a second point on the outside
edge of said r.f. radiator conducting area to produce a circular
r.f. polarization characteristic,
said r.f. radiator conducting area including means dimensioned
differently in two mutually perpendicular directions, said means
providing two corresponding complex-valued electrical impedances
along said directions at the intended r.f. operating frequency,
which two complex-valued impedances are complex conjugates of each
other thus facilitating the desired circular r.f. polarization
characteristic.
Description
The subject matter disclosed herein is related to my co-pending
commonly assigned application Ser. No. 352,034, now U.S. Pat. No.
3,811,128 filed concurrently herewith. It is also related to
earlier commonly assigned U.S. Pat. No. 3,713,162 issued Jan. 23,
1973 and to the commonly assigned co-pending patent application
Ser. No. 99,481 filed Dec. 18, 1970, now U.S. Pat. No.
3,810,183.
This invention generally relates to microstrip antenna structures
and to phased arrays formed from a plurality of such
structures.
In designing antenna structures it is attempted to make such
antennas perform a desired electrical function such as
transmitting/receiving linearly polarized, right-hand circularly
polarized, left-hand circuitry polarized, etc., r.f. signals with
appropriate gain, bandwidth, etc., electrical characteristics. Yet
it is also necessary for these structures to remain mechanically
light, simple, cheap and unobtrusive since such antennas are often
required to be mounted upon other structures such as high velocity
aircraft, missiles, and rockets which cannot tolerate excessive
deviations from aerodynamic shapes. Of course, it is also sometimes
desirable to hide the antenna structures so that its presence is
not readily apparent for aesthetic and/or security reasons.
Accordingly, the ideal electrical antenna should physically be
paper thin and protrude on neither side of a mounting surface (such
as an aircraft skin or the like) while yet still exhibiting all the
requisite electrical features.
A microstrip printed circuit board antenna formed by etching a
single side of a unitary metallically clad dielectric sheet or film
using conventional photo resistetching techniques potentially
presents the closest approximation to these ideal requirements.
Typically, the entire antenna structure may possibly be only 1/32
inch to 1/8 inch thick while minimizing cost and maximizing
manufacturing/operating reliability and reproducibility.
Furthermore, the cost to the customer is minimized since single
antenna elements and/or arrays of such elements together with
appropriate r.f. feedlines, phase shifting circuits and/or
impedance matching networks may all be manufactured as integrally
formed electrical circuits along using low cost photo
resist-etching processes commonly used to make electronic printed
circuit boards. This is to be compared with many complicated costly
prior art techniques for achieving polarized radiation patterns as,
for instance, a turnstile dipole array, the cavity backed turnstile
slot array, etc.
While the above referenced related application Ser. No. 99,481
and/or U.S. Pat. No. 3,713,162 disclose some elongated forms of
microstrip antenna radiators, it has now been discovered that other
microstrip antenna radiator structures are advantageously suited to
transmit/receive r.f. radiation having predetermined polarizations
such as linear polarization and left-hand or right-hand circular
polarization.
Furthermore, these newly discovered microstrip antenna structures
have been discovered to be especially well suited for use in an
overall array comprising a plurality of such individual elements
where the individual elements are phased relative to one another to
provide high gain fan beam or pencil beam radiation patterns when
disposed in a flat or even curved array of such microstrip antenna
structures.
It has been discovered that the necessary relative phase shifts for
such arrays can be economically achieved with phase shifting
circuitry that is integrally formed by printed circuit board
techniques wherein the r.f. feedline, impedance matching, etc.,
circuits are included within a generally planar arrangement of
electrical conductors comprising both r.f. radiators, r.f.
feedlines, etc. Of course, it will be appreciated throughout the
following discussion that the phrase "generally planar arrangement"
is to include the case where the integrally formed microstrip is
distorted from a purely planar structure to take on curved shapes
and the like. In such cases, the "generally planar arrangement"
would still constitute a single "layer" integral structure
conforming to some predetermined shape and is thus to be considered
as included in the following description (i.e. conformed
array).
The fan or pencil beam of radiation may also be controllably
steered by controlling switchable diodes or other controlled
elements mounted directly on the microstrip structure in
combination with appropriate integrally formed phase shifting
circuits, etc., as will be explained in more detail below.
Since the microstrip antenna structures described herein require
only one printed circuit board for an entire antenna radiator,
associated feedlines, impedance matching networks and
phase-shifting networks which printed circuit board is photo-etched
on only one side, there is no requirement for front to back
registration of plural photo-etched patterns nor are board
alignments required as when two or more separate printed circuit
boards are utilized.
It has been discovered that linearly polarized radiation may be
produced by simply feeding one point along one side of a square
shaped or rectangularly shaped microstrip radiator. The approximate
resonant frequency of this type of linearly polarized radiator is
determined by the radiator dimension perpendicular to the side on
which the r.f. energy is input. Accordingly, in the case of the
square radiator, the resonant frequency is determined by the length
of any one of the sides while in the rectangular radiator the
resonant frequency may be one of two frequencies. Namely, a first
frequency determined by the shorter dimension when r.f. energy is
fed into the longer dimensioned side and, correspondingly, a second
frequency determined by the longer dimension when r.f. enenrgy is
fed into the shorter side of the rectangular radiator.
The relevant dimension in both cases is substantially equal to
one-half wavelength of the anticipated operating or resonant
frequency when proper account is taken of the dielectric constant
for the dielectric material utilized in the microstrip structure.
That is, the relevant dimension should be approximately equal to
the relevant free space wavelength divided by two times the square
root of the relative permittivity for the dielectric material.
The necessary r.f. feedlines are preferably also formed using
integrated circuit or photo etching techniques to be included as a
part of the generally planar arrangement of electrical conductors
comprising r.f. feedline and r.f. radiators. Furthermore, the
dimensions of the r.f. feedline should be designed according to
conventional impedance matching techniques to match the antenna
impedance to the impedance of the anticipated coaxial cable or
other r.f. conduit connected to the r.f. feedlines on the
microstrip structure.
Circularly polarized radiation fields may be transmitted by driving
adjacent sides of a square microstrip radiator with signals having
relative phasing of 90.degree. to produce the required conjugate
phasing of the radiated fields. Either left-hand or right-hand
circularly polarized signals may be produced.
Circular polarization may also be achieved by driving the corner of
a square microstrip patch radiator. Furthermore, the microstrip
radiator does not need to be an exact square since it has also been
discovered that other shapes (for instance, a circularly shaped
microstrip radiator driven at points separated by 90.degree. about
its circumferential edge with signals having 90.degree. relative
phase angles) will also produce the desired circularly polarized
radiation.
The necessary r.f. feedlines, phase shifters and/or impedance
matching networks are also preferably integrally formed by the same
printed circuit board etching techniques with the microstrip
radiator(s) thus minimizing the cost and complexity of the overall
device.
It has been discovered that such microstrip radiators perform
exceptionally well known a plurality of radiators are utilized in a
linear or two-dimensional array to achieve a high gain fan or
pencil beam radiation pattern. Such arrays exceed the performance
of conventional arrays and very nearly approach the maximum
theoretical gain limits for such an array. In part, it is believed
that this unexpected and exceptional performance of microstrip
antenna arrays is due to the greatly increased uniformity of large
area sheet currents generated thereby.
It has further been discovered that such arrays of microstrip
radiators may be electronically steered using controllable phase
shift circuits that are also integrally formed with the r.f.
feedlines, impedance matching networks and microstrip radiators. In
one exemplary embodiment to be described in more detail below,
switchable diodes are connected into such printed circuit phase
shifters using conventional printed circuit board techniques
whereby such switchable diodes may be controlled by an
appropriately programmed mini-computer or other conventional
control means to achieve required relative phase shifts between the
driving currents supplied to the various elements of the microstrip
antenna array thus steering the fan and/or pencil beam to any
desired position as will be appreciated.
It is also possible to utilize the normal microstrip feedline
losses in such an integral array of microstrip radiators to achieve
an amplitude taper across the array aperture thus reducing
undesired sidelobes.
These and other advantages and objects of the invention will be
more fully appreciated by reading the following detailed
description of the invention in conjunction with the accompanying
drawings, of which:
FIG. 1 is a plan view of an exemplary embodiment of a linearly
and/or circularly polarized microstrip antenna element according to
this invention;
FIG. 2 is a plan view of another exemplary embodiment of a linearly
polarized microstrip antenna element according to this invention
having two resonant frequencies;
FIG. 3 is a plan view of an exemplary embodiment of a circularly
polarized microstrip antenna element according to this
invention;
FIG. 4 is a plan view of another embodiment of a circularly
polarized microstrip antenna element according to this
invention;
FIG. 5 is a plan view of schematic diagram of an exemplary linear
array of microstrip antenna elements according to this
invention;
FIG. 6 is a graph of theoretical maximum and experimentally
measured gains for arrays of microstrip antenna elements
constructed according to this invention;
FIG. 7 is a polar plot of the gain pattern for a flat microstrip
antenna array constructed according to this invention superimposed
upon a reference gain pattern for a standard gain horn antenna;
FIG. 8 is a schematic plan view of a two-dimensional microstrip
antenna array according to this invention;
FIG. 9 is a schematic diagram of an electrically scanable phased
array of microstrip antenna elements constructed according to this
invention; and
FIG. 10 is a schematic diagram of an array embodiment for achieving
amplitude taper across the array aperture and hence reduced
sidelobes.
FIG. 1 shows a plan view of one exemplary embodiment of linearly
polarized microstrip antenna element according to this invention. A
uniformly dimensioned r.f. feedline 20 is shown in FIG. 1 although
those in the art will appreciate that the dimensions of the feeline
20 should be appropriately designed to match the impedance of the
antenna or microstrip radiator 22 with the impedance of a coax or
other r.f. conduit which will be connected to the input of the r.f.
feedline 20 at 24 to provide a source of the r.f. energy or to
conduct r.f. energy that may have been received by the antenna
element to a receiver as will be appreciated by those in the art.
The r.f. feedline 20 and the r.f. radiator 22 are formed from a
unitary sheet of conductive material that has been selectively
etched away using conventional printed circuit board construction
techniques from a substrate of dielectric material 26. The bottom
side of the substrate 26 is then positioned over a conducting
ground plane surface 28 which may, in fact, be a copper (or other
conductor) surface clad onto the bottom side of the dielectric
substrate 26. Alternatively, the microstrip structure on dielectric
substrate 26 may be conformed to the electrically conducting skin
of a vehicle or other conducting ground plane 28 as will be
appreciated.
The square microstrip radiator 22 should be dimensioned such that
its sides are equal to approximately one-half wavelength (.lambda.
.sub.d /2) at the anticipated operating frequency when proper
corrections are made for the dielectric constant of the dielectric
substrate 26. Namely, when the free space wavelength has been
divided by the square root of the relative permittivity of the
dielectric substrate 26 as will be appreciated by those in the
art.
While it may be first thought that the dimensions should be exactly
one-half wavelength, in actuality, the dimensions should preferably
be slightly less than one-half wavelength to insure that the
radiator input impedance is approximately or substantially all
real. That is, to insure that the imaginary part of the slot
reactance reflected from the far edge of the radiator substantially
cancels out the imaginary part of the reactance from the slot
located at the near edge of the radiator. Typically, the square
radiator 22 should have sides equal to approximately 0.49 of the
free space wavelength divided by the square root of the relative
permittivity as should now be apparent. Generally speaking, it has
been found that acceptable dimensions may range around 0.47 to 0.49
of the half wavelength .lambda. .sub.d /2 thus being substantially
equal to the half wavelength but still slightly less as should now
be apparent. Hereater, when the dimensions of the radiators are
discussed in terms of half wavelengths it will be understood that
in reality the relevant dimension should preferably be slightly
less than one-half wavelength to insure that the antenna input
impedance is substantially resistive.
The microstrip radiator does not have to be square to produce
linearly polarized radiation. For instance, as shown in FIG. 2, a
rectangular microstrip radiator 30 may be fed either from r.f.
feedline 32 attached to the longer dimension .lambda. .sub.d2 /2 of
the rectangular area or from an r.f. feedline 34 attached to the
shorter dimension .lambda..sub.d1 /2 of the rectangular area. It
has been discovered that the electrical r.f. sheet currents passing
along the surface of the microstrip radiator 30 will be
substantially parallel to the corresponding r.f. feedline as shown
by arrows 36, 38 in FIG. 2. Although the r.f. feedlines are
preferably located in the center of the respectively corresponding
sides to achieve maximum uniformity of sheet current distribution,
it is also considered feasible to connect the r.f. feedlines at
other points along the same side of the radiator without seriously
affecting the linear polarization characteristics of the
element.
In the example shown in FIG. 2, the reasonant frequency of the
radiator 30 will be determined by the shorter dimension when it is
fed from the r.f. feedline 32 and it will be determined by the
longer dimension when it is fed by the r.f. feedline 34. Thus, the
same radiator may be used to operate at two different selected
frequencies. As indicated in FIG. 2, the same considerations apply
with respect to choosing the dimensions of the rectangular radiator
area 30 as with the radiator area 22 shown in FIG. 1. Namely: the
shorter dimension is approximately one-half wavelength of the
desired resonant frequency when r.f. feedline 32 is used while the
longer dimension is approximately one-half wavelength of the
desired resonant frequency when the r.f. feedline 34 is
utilized.
The square radiator 22 of FIG. 1 may also be used as a circularly
polarized radiator when fed from a corner as shown at 21 in FIG. 1.
Here, as shown in FIG. 1, the left-to-right dimension of the square
22 should be slightly less than one-half wavelength while the
top-to-bottom dimension should be slightly greater than one-half
wavelength as necessary to obtain two orthogonal admittances such
as 0.01 + j.01 and 0.01-j.0.1 across the square patch 22. Then,
when fed at the corner from feedline 21, the radiated fields will
have conjugate phases or, in other words, the total radiated field
will be circularly polarized. Left or right-hand circular
polarization can be achieved by choosing the r.f. input/output
corner. As just described and shown in FIG. 1, right-hand circular
polarization would result while left-hand circular polarization
would result if feedline 21 were moved to one of the adjacent
corners of square 22.
The square shaped r.f. radiator 40 shown in FIG. 3 also constitutes
a circularly polarized microstrip antenna element when driven on
the two adjacent sides 42 and 44 by r.f. currents having relative
phase differences of 90.degree.. As shown in FIG. 3, side 42 is fed
from r.f. feedline 46 while side 44 is fed from the r.f. feedline
48 where both feedlines emanate from the integrally formed printed
circuit phase-shifting arrangement 50 having an r.f. input/output
52 corresponding to left-hand circular polarization and r.f.
input/output 54 corresponding to right-hand circular polarization.
When input 52 is utilized, the r.f. signals propagating to and
along the r.f. feedline section 48 are 90.degree. out of phase with
similar r.f. signals propagated to and along the r.f. feedline
section 46. The same consideration apply when the input is at 54
except that the roles of the two r.f. signals are reversed and the
one that was leading by 90.degree. is now lagging by
90.degree..
Assume for the moment that the r.f. signals presented to side 44 of
the radiator 40 are represented by cos wt and that those signals
being input to side 42 are represented by cos (wt- .pi./2). In this
case, at t= o, the electric sheet current on the radiator 40 would
be directed substantially away from side 44 and parallel to side
42. Later, when wt = .pi./2, the radiating electrical sheet
currents would effectively have been rotated by 90.degree. to pass
parallel to side 44 and away from side 42. Still later, when wt =
.pi., the electric sheet currents would be effectively shifted by
another 90.degree. to be generally parallel to side 42 and directed
towards side 44. Finally, when wt = 3.pi./2, the electric sheet
current would be further rotated by another 90.degree. generally
parallel to side 44 and directed towards side 42. Accordingly, it
will now be appreciated that the radiator 40 will generate
circularly polarized radiation, the effective direction of circular
polarization being determined by side 42 or 44 being fed by
currents leading or lagging respectively by 90.degree..
FIG. 4 shows another form of circularly polarized microstrip
antenna element according to this invention wherein the radiator 66
is not square or diamond shaped as was the case in FIG. 3. A square
shaped phase shifting circuit 60 similar to the phase shifting
circuit 50 previously described in FIG. 3 is here utilized together
with r.f. feedline sections 62 and 64 conducting r.f. having
relative phase angles of .pi./2 to the feed points 68 and 70, which
feed points are located at a 90.degree. interval about the
circumference of the circular radiating element 66. As should now
be apparent. the same kind of left-hand and right-hand circularly
polarized radiation patterns may be obtained using this
arrangement.
It may be noted in FIG. 4 that the radiator 66 is symmetric with
respect to each of two mutually perpendicular axes 72 and 74
intersecting at the center of the circular area 66 also generally
passing through the feed points 68 and 70 located at 90.degree.
apart about the circumference of the circular radiating element 66.
A similar observation could also have been made for the feed
point(s) of the square or rectangular radiating areas already
discussed wherein the two mutually perpendicular axes would have
been parallel to the sides of the squares or rectangular area as
should now be apparent.
As those in the art will appreciate, circular polarization is only
a special case of elliptical polarization and in actual practice,
truly exact circular polarization is usually obtained if at all
only in a portion of an antenna radiation pattern with the
remainder of the pattern actually comprising elliptical
polarization of an approximation of the desired circular
polarization radiation. It will be understood that the term
circular polarization is used here in that same conventional
sense.
The microstrip antenna structures previously described also make
exceptionally good performing arrays when a plurality of such
individual antennas are formed into a phased antenna array to
generate fan or pencil beam radiation patterns. One exemplary
embodiment of a steerable array of such radiators is depicted in
FIG. 5. It should be understood that the entire array may be formed
as an integral printed circuit together with any required phase
shifting circuits, etc., to provide an extremely simple and cheap
phased array having exceptional qualities.
The exemplary four element linear array is shown in FIG. 5
comprises microstrip r.f. radiators 80, 82, 84 and 86 on dielectric
sheet 88 over ground plane 90. Each of these r.f. radiators is fed
by respectively associated r.f. feedline segments 92, 94, 96 and 98
which receive the output of respectively associated controllable
phase shifters 100, 102, 104 and 106. Although these phase shifters
receive equal power and equal phase r.f. inputs from the symmetric
corporate structure r.f. feedline generally indicated by reference
numeral 108, the outputs on r.f. feedline segments 92-98 have
controlled relative phase differences as a function of the control
input on line 110 to result in a controllably steerable fan beam of
radiation. As will be appreciated, similar controlled phase shifts
could be incorporated in a two dimensional array to achieve a
steerable pencil beam radiation pattern.
The exceptional performance of these microstrip antenna arrays is
believed to be caused by the exceptionally uniform illumination of
the array aperture. The close approximation of expected and
experimentally measured antenna gain for such an array versus the
theoretical maximum gain is shown in FIG. 6 and it can be seen that
the expected/experimental results very nearly approaches the
theoretical maximum.
Apparently the only reason the theoretical maximum is not obtained
is that, in practice, the microstrip feedline subtracts from this
gain as a function of the frequency and relevant transmission line
lengths. More particularly, the theoretical maximum gain G for an
absolutely uniformly illuminated aperture is:
G = 4.pi.A/.lambda.2
However, in actual practice, the microstrip feedline attenuation
subtracts from this gain
G.sub.Actual = 10 Log (4.pi.A/.lambda.2) - .alpha.
Where
.alpha..sub.Line = .alpha./inch .times. L"
Thus, the attenuation is dependent on frequency and line length. In
the X-band, for a 1/32-inch microstrip line, .alpha. equals about
0.12 dB/in. Since for an equal power, equal phase feedline network
the length of microstrip feedline is half of the height plus half
of the width, therefore for such an arrangement
.alpha. = 1/2.alpha./inch (W" + H")
Thus, in the X-band for a 5 inches .times. 3 inches antenna,
.alpha.= 0.48 dB and it should now be apparent how such losses will
affect any given array structure. An experimental model 3 inches
.times. 5 inches .times. 1/32 inches has been built and tested and
confirms a gain (FIG. 7) in excess of the theoretical predictions
as shown in FIG. 6. The error is within a .+-.1/2 dB expected error
in the antenna gain measurement.
The controlled microstrip phase shifters 100-106 as shown in FIG. 5
may, for instance, comprise conventional PIN diode(s) and printed
circuit phase shifting circuits where the PIN diodes are controlled
by a mini-computer or other appropriate control source to achieve a
desired relative phase difference between the r.f. energies being
fed to the several array elements as should now be apparent.
FIG. 7 reveals that experimentally measured plot of antenna gain
for a 3 inches .times. 5 inches .times. 1/32 inches flat microstrip
array at 9.92 GHz shows a gain of approximately 21 dB for the
maximum center lobe which compares favorably with the superimposed
(but rotated by 180.degree.) gain pattern of a standard gain
horn.
Another array of microstrip radiators is shown in FIG. 8 wherein
each of the microstrip radiators is as disclosed in the earlier
referenced related patent and/or application and has a plurality of
feed points fed from a corporate feed network designed to provide
equal phase power r.f. currents to all feed points of all of the
radiators. Preferably, the widths of the rectangularly shaped
radiators in such an array are equal to approximately one-half
wavelength at a desired operating frequency and they are also
spaced by approximately one-half wavelength. Of course, the
one-half wavelengths here discussed are considered to have been
corrected for the relative dielectric constant of the dielectric
sheet involved in the microstrip array and to include appropriate
allowances for making the actual dimensions slightly less than
one-half of such a wavelength to insure substantially resistive
input impedances for the several radiators involved.
While the individual microstrip radiators as shown in FIG. 8 are
similar to the elongated microstrip radiators previously disclosed
in the earlier referenced related copending applications and/or
patents having feed points at least once each wavelength along the
length thereof it has now been discovered that an array of these
elements as shown in FIG. 8 provides an unexpectedly high gain very
nearly equal to the maximum possible theoretical gain for an
aperture which is believed due to the extremely uniform sheet
currents produced by such an array. Of course, the array shown in
FIG. 8 could be made steerable by appropriately controlling the
relative phases of the driving signals to each of the radiator
elements. The array of FIG. 8 thus provides an extremely efficient
antenna with a very high gain approaching 100% of the theoretical
maximum aperture efficiency. It is very reliable and rugged while
at the same time being of minimum thickness and cost to provide a
virtually ideal antenna array structure.
Another electrically scanned phased array of microstrip antenna
elements is shown in FIG. 9. Here the exemplary array of four
radiators 150, 152, 154 and 156 are fed from a corporate network
structure having an input at 158 to provide equal power and equal
phase r.f. inputs to the four printed circuit microwave phase
shifters 160, 162, 164 and 166. As will be appreciated, the
relative phase of the output from these phase shifters (and hence
the input to the various radiators of the array) will depend upon
the location of the switchable diodes 168 in each of the various
phase shifters and the on-off condition of these diodes. That is,
for example, the diodes may be turned "on" or "off" by supplying a
control current and/or connection generated by an appropriately
programmed mini-computer or other conventional control means thus
controllably changing the relative phase delay of each hybrid phase
shifter 160-166 between 0.degree. and 180. Accordingly, by properly
controlling the diodes 168, the microstrip radiators may be excited
in any desired combination required to produce radiation patterns
in any desired direction. Of course, the number of diodes 168 may
be increased to refine the possible relative phase shifts that may
be achieved with such phase shifters 160-166 as should be
appreciated. Furthermore, the number of radiating elements can be
increased from the four shown in the exemplary embodiment of FIG. 9
to further reduce the bandwidth and increase the gain of the
overall array.
Undesirable array radiation pattern sidelobes may be reduced by
using an r.f. feedline arrangement as shown in FIG. 10. As
heretofore explained, the array elements have been excited with
equal power r.f. signals by a symmetrical corporate r.f. feedline
network as shown, for instance, in FIGS. 5, 8 and 9. The relative
phases have also been nominally equal except for the effects of
phase shifting circuits previously described.
However, in FIG. 10, the expected losses in the feedline network
have been utilized to vary the r.f. power levels supplied to the
various radiators. That is, the amplitude distribution has been
tapered to reduce undesirable sidelobes in the overall array
radiation pattern.
For instance, the feedline junction points 180, 182 have been
offset by one-half wavelength from their usual points 184, 186.
Thus, the difference in total feedline length from the common
input/output 188 to feed points 190, 192 and from input/output
point 188 to feed points 194, 196 is one whole wavelength whereas
it was previously zero. Thus, the relative phases of the r.f.
inputs to the array elements are unaffected. However, the longer
feedline lengths to points 190, 192 results in a reduced r.f.
amplitude relative to the r.f. amplitude at points 194, 196 thus
tapering the array aperture's amplitude distribution to reduce
undesired sidelobes. Of course, more detailed tapering or amplitude
shaping could be achieved by this same technique with an array
having larger numbers of elements.
Furthermore, this amplitude tapering can also be used with
elongated microstrip radiators as disclosed in the earlier
referenced related patent and applications.
While only a few embodiments of this invention have been
specifically described and discussed above, those in the art will
appreciate that there are many possible modifications and
variations of the exemplary embodiments without in any way
departing from the spirit and teaching of this invention.
Accordingly, all such modifications and/or variations are intended
to be included within the scope of this invention.
* * * * *