U.S. patent number 3,852,762 [Application Number 05/415,634] was granted by the patent office on 1974-12-03 for scanning lens antenna.
This patent grant is currently assigned to The Singer Company. Invention is credited to George Henf, Leonard Schwartz.
United States Patent |
3,852,762 |
Henf , et al. |
December 3, 1974 |
SCANNING LENS ANTENNA
Abstract
An improved microwave antenna for use in aircraft guidance in
which respective azimuth and elevation antenna are fed through
dielectric lenses by respective rotating scanners to result in
scanned planar beams without physical antenna rotation. The
respective scanners are mechanically coupled to thereby insure
synchronization so that radiation is fed to only one antenna at a
time.
Inventors: |
Henf; George (Pleasantville,
NY), Schwartz; Leonard (Scarsdale, NY) |
Assignee: |
The Singer Company (Little
Falls, NJ)
|
Family
ID: |
23646523 |
Appl.
No.: |
05/415,634 |
Filed: |
November 14, 1973 |
Current U.S.
Class: |
343/756; 343/761;
343/780; 343/876; 343/779; 343/783 |
Current CPC
Class: |
H01Q
15/246 (20130101); H01Q 3/12 (20130101); H01Q
19/12 (20130101) |
Current International
Class: |
H01Q
19/12 (20060101); H01Q 15/00 (20060101); H01Q
15/24 (20060101); H01Q 3/12 (20060101); H01Q
19/10 (20060101); H01Q 3/00 (20060101); H01q
019/12 (); H01q 019/08 () |
Field of
Search: |
;343/757,761,839,854,756,779,780,783,876 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lieberman; Eli
Attorney, Agent or Firm: Kennedy; T. W.
Claims
What is claimed is:
1. A scanning lens microwave antenna comprising:
a. a fixed circular reflector;
b. a scanner comprising an annular shaped rectangular waveguide
split in half with the inner half containing a fixed inlet port and
forming a stator and the outer half containing a plurality of
rotatable outlet ports and forming the rotor and means in said
waveguide to cause only one output at a time to couple to said
input;
c. a parallel plate waveguide directing energy from said scanner to
said reflector whereby said scanner will scan energy across said
reflector; and
d. a parallel plate dielectric lens located at the end of said
parallel plate waveguide, said dielectric lens mounted within a
thin metal outer casing which is coupled to said waveguide and
having a tapered matching section extending into said
waveguide.
2. The invention according to claim 1, wherein said means to couple
one output at a time comprises a stationary director in the stator
having a mitered H-plane extending partially across the input port
and a plurality of rotating directors in said rotor one being
provided for each output port and having a mitered H-plane bend
extending partially across its associated output port.
3. The invention according to claim 2, wherein said stationary
director is extended around the major portion of said stator.
4. The invention according to claim 2, wherein said parallel plate
wave guide comprises a pillbox antenna with the end of said pillbox
providing said curved reflector.
5. The invention according to claim 4 and further including a
polarization rotator at the output of said pillbox antenna.
6. The invention according to claim 5, wherein said pillbox has
additional parallel plate bends and terminates in an output horn
which is perpendicular to the plane of the scanning lens within
said pillbox.
7. The invention according to claim 5, wherein said polarization
rotor comprises a plurality of printed circuits comprising a
pattern in copper on thin fiber glass and a plurality of low
density foam dielectric spacers of a thickness of one quarter wave
length sandwiched between said plurality of printed circuit
boards.
8. The invention according to claim 2, wherein said curved
reflector is a doubly curved surface having a circular shape in the
direction of scanning.
9. The invention according to claim 8, wherein said parallel plates
contain bend of essentially 90.degree. to direct energy to said
curved reflector.
10. The invention according to claim 9, wherein said parallel
plates have a circular feed aperture to produce a planar output
scan.
11. The invention according to claim 9, wherein said parallel
plates have a linear feed aperture to produce a conical scan.
12. A scanning lens antenna for providing alternate orthagonally
scanned beams comprising:
a. a first scanning lens antenna comprising:
1. a pillbox antenna having its end formed to provide a curved
reflector;
2. a rotatable scanner including means to direct energy toward said
curved reflector within said pillbox; and
3. a first parallel plate dielectric lens interposed between said
directing means and said reflector to cause the reflected energy to
be collimated as it leaves said curved surface;
b. a second scanning lens antenna having its scanner mechanically
coupled to the scanner of said first scanning lens antenna such
that only one of said first and second lens antennae is able to
radiate at one time, said scanning lens comprising:
1. a double curved reflector having a circular shape in the
direction of scanning;
2. a rotatable scanner including means to direct energy toward said
double curved reflector;
3. a parallel plate wave guide enclosing said scanner; and
4. a second parallel plate dielectric lens interposed between said
directing means and said reflector to cause the reflected energy to
be collimated as it leaves said curved surface;
c. means to supply microwave energy to said first and second lens
antennae; and
d. means to rotate the scanners of said first and second lens
antennae.
13. The invention according to claim 12, wherein said means to
supply microwave energy comprises:
a. an RF source;
b. a first three port ferite circulator having one port coupled to
said source and a second opposite port coupled to a terminator;
c. a second ferite circulator having one port coupled to the input
of said first scanning lens antenna, a second opposite port coupled
to said second scanning lens antenna and a third port coupled to
the third port of said first circulator.
Description
BACKGROUND OF THE INVENTION
This invention relates to aircraft guidance in general and in
particular to a unique antenna mechanization which is capable of
generating radar guidance signals of the proper character suitable
for landing various types of aircraft, i.e., conventional fixed
wing, short take off and landing (STOL) and helicopter.
With the increase in air traffic, the need to expand instrumented
airports, and the variety and types of aircraft to be accommodated,
the single approach profile provided by conventional UHF-VHF
Instrument Landing Systems (ILS) in use today is not adequate. To
satisfy the spectrum of potential airborne users and the increasing
variety of airport ground facilities, a new type of scanning beam
landing system is required. A variety of requirements and signal
formats have been identified for various applications. Much of this
data has been summarized in the documentation of the Radio
Technical Committee for Aeronautic's subcommittee 117. DO-148
published by RTCA in November 1970 is typical of this data.
Scanning beam landing systems have for the most part employed
antennas wherein the entire antenna was physically rotated or
nutated to provide, within the approach airspace, the scanning beam
spatial motion necessary for landing guidance. This technique,
i.e., total antenna motion, has been primarily utilized since it
does provide, the most straight-forward means of minimizing any
variations to the radiated radar beam pattern as it scans the
approach airspace. This type of scanning antenna mechanization does
however have numerous limitations associated with it. Key
limitations are:
A. The antenna structural supports needed to maintain stable
operation in a rotational condition coupled with the high torque
mechanical drive subsystems necessary to start-up and rotate the
entire antenna result in bulky heavy ground station equipments.
B. Since the entire volume swept by the scanning antenna must be
enclosed to protect the antennas, the ground equipments are
generally large.
C. When an antenna is scanned, the time period during which
received guidance information is available to the approaching
aircraft is not continuous. Furthermore, as the number of antenna
scans per unit time is increased, the received data (dwell time)
per scan is correspondingly reduced.
D. To provide the necessary lateral and vertical landing guidance
data dictates that the scanning beams be swept in two orthogonal
directions. To provide orthogonal antenna scan requires two
antennas resulting in additional ground station volume to
accommodate each and additional electronic equipment to insure that
these radiated beams sequentially scan the approach volume in order
not to contaminate the received date e.g., beam
synchronization.
In summary, the key requirements associated with the generation of
scan beam data for a scanning beam aircraft landing system are
to:
1. provide orthogonal beam scan in the vertical and horizontal
planes of the approach volume;
2. generate fan beam scanning data where the beam parameters are
invarient throughout the approach volume. This requirement can be
simplified as follows: provide planar beam scan;
3. synchronize the scan of the individual beams (vertical and
horizontal) to avoid contamination of the received data that would
occur if both were received simultaneously; and
4. generate beam data relatively free from distortions that may be
caused by terrain elements in proximity to the ground station. That
is, generate narrow main beam radiation and low level extraneous
beam radiation (sidelobes).
SUMMARY OF THE INVENTION
The objective of scanning lens antenna-microwave (SLAM) of the
present invention is to satisfy the above requirements and
eliminate the aforementioned difficulties. The SLAM mechanization
generates narrow fan beams which scan orthogonally using a single
light-weight, fixed antenna and circular lens scan technique.
The unique features that can be realized from this technique
are:
1. SLAM produces narrow fan beam scanning from a stationary antenna
eliminating the need for rugged structure and large enclosures
associated with non-stationary antennas;
2. the SLAM produces two orthogonal beam scans from a single flat
plate configuration reducing dramatically the enclosure
requirements;
3. the SLAM provides automatic self-synchronized scanning operation
without requiring electronic switches, mechanical linkages, or
electronic circuitry; and
4. the SLAM permits scan rate of the actual beam in space to be
independent of the rotational speed of the internal lens-feed
scanner by using multiple feed lens assemblies.
The SLAM consists of an azimuth and elevation antenna each of which
uses multiple rotating feeds to generate the planar beam patterns
required for landing system applications. The elevation section of
the SLAM antenna consists of a rotary scanner which has one fixed
input port which sequentially couples to continuously rotating
output ports. Connected to the rotating output ports are waveguide
fed, dielectric lenses. The scanner lens configuration is so
arranged that the excited lens illuminates a folded pill box type
microwave antenna which uses a cylindrical reflector-180.degree.
bend. The lens reflector combination results in a collimated
microwave beam which rotates with the movement of the feed system.
The output of the pillbox is a vertical circular horn which is used
to output the planar scanning elevation beam.
The azimuth section of the integral antenna employs a rotary
scanner similar to the elevation section. Each output of the
rotating section of the azimuth scanner is connected to a waveguide
fed lens. The lenses, which are coupled to the scanner, rotate
inside of a parallel plate transmission line. Lens energy directed
from the rotary scanner is fed into a 90.degree. bend. The output
of the bent parallel plate transmission line is a circular horn
which excites a doubly curved reflector. This feed-reflector system
produces a planar azimuth beam whose elevation pattern is
determined by the reflector cross section. As the lens rotates, the
planar azimuth beam scans the approach volume. The azimuth and
elevation rotary scanner are combined into a single mechanical unit
and as such both output sections (Az and El) rotate together. The
two antenna sections are electronically isolated so that each
antenna's pattern performance is completely independent of the
other.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view illustrating the nature of a planar
beam scan.
FIG. 2 is a plan view partially in cross section illustrating the
rotating scanner of the present invention.
FIGS. 3a - 3d are schematic illustrations of the operation of the
scanner of FIG. 2.
FIG. 4 is a perspective view of a pillbox antenna used in the
present invention.
FIGS. 5a and 5b illustrate details of the antenna of FIG. 4.
FIG. 5c illustrates the relationship between the pillbox antenna
and the scanner.
FIGS. 6a and 6b are schematic diagrams illustrating the manner in
which the dielectric lens of the present invention collimates
radiation.
FIG. 7a is a plan view illustrating the dielectric lens of the
present invention.
FIG. 7b is an elevation view in cross section of the lens of the
present invention.
FIG. 8 is a diagram illustrating the directions of the rays of
radiation in the antenna of the present invention.
FIGS. 9a and 9b are respectively cross sectional and plan views of
a polarizing arrangement used in the present invention.
FIG. 10 is a perspective view of the azimuth antenna of the present
invention.
FIG. 11a is a plan view and FIG. 11b an elevation view illustrating
the arrangement of the components within the antenna of FIG.
10.
FIG. 12 is a perspective view partially cut away illustrating the
combined azimuth and elevation antennas.
FIG. 13 is a schematic view illustrating the manner in which
radiation is fed to the elevation antenna.
FIG. 14 is a similar view illustrating the manner in which
radiation is fed to the azimuth antenna.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
A key requirement for landing system antennas is that they generate
a fan beam shape as shown in FIG. 1. A fan or planar beam is a
broadside or flat beam which is formed when the direction of
radiation is perpendicular to the radiating aperture. In planar
beam scanning, the radiation pattern in the scan plane is a
broadside beam with no change in the beam shape. In typical feed
scan types of beam scanning, the phase front and peak direction
deviate from the normal to the radiating aperture as the array is
scanned, producing a beam which has a conical shape, where the
degree of coning is a function of the scan angle. To achieve planar
scanning requires a unique antenna configuration.
The beam of FIG. 1 illustrates the elevation scan. A similar beam
must also be generated in the azimuth direction, i.e., it would be
rotated 90.degree. from the beam shown on the Figure. In the
landing system the beams are scanned in sequence. That is, first an
elevation scan is performed and then an azimuth scan and then
another elevation scan and so on.
ELEVATION ANTENNA
The elevation antenna comprises a four output scanning device with
four wave guide feeds and lens assemblies, a folded pillbox and a
polarization rotator. The scanner has one wave guide input and four
outputs, each of which are terminated with a feed and dielectric
lens assembly. There are no wave guide transitions (i.e., one
linear to circular transition and one circular to linear transition
as required in the rotating joint). The scanner arrangement is
shown in FIG. 2. This device is based on the field theory of
standard rectangular wave guides operating in the TE10 mode. For
this mode the current density at the center of the two broad walls
will be zero. This means that the wave guide can be split in half
without affecting the propagation characteristics of the wave
guide, by cutting it along the longitudinal center line. In such a
split wave guide the upper half can be shifted with respect to the
lower half along the longitudinal axis without affecting the
propagation characteristics of the wave guide.
As illustrated by FIG. 2, the rectangular wave guide which has
sides 30 and 31, is formed into an annulus in the H plane. The
dividing line between the halves of the wave guide is the circle
32. In this way the outer half 33, having the wall 30, can be
rotated with respect to the inner half 35, having the wall 31
without affecting the RF fields inside the wave guide. In this
particular application, the inner half 35 serves as a stator and
the outer half 33 as a rotor. Energy is coupled into the scanner
via a mitered H-plane bend 34. As shown on the figure, the miter
extends halfway across the wave guide. In this application, the
mitered device is called a director. The director has a length
along the wave guide and, in this region the wave guide is below
cutoff because the wide dimension is significantly smaller than
half a free space wavelength. Four similar directors 36 are placed
in the rotor 33 at 90.degree. intervals so that the energy is
coupled from the scanner by only one director at a time, since the
other directors are separated from the active one by a section of
cutoff waveguides. Isolation is further enhanced by extending the
input director 37 around the complete annulus except for the region
corresponding to the desired scan sector.
Scanner operation is illustrated by FIG. 3. As shown thereon, when
an output director is located opposite the input director, a short
is presented at the input and the incident energy is reflected in
the direction of the transmitter. The series of diagrams presented
on FIG. 3 shows the various positions of the rotor 33 with respect
to the stator 35. In FIG. 3a, the relative positions of the upper
and lower halves of the waveguide are such that energy is coupled
from the input to output 1. Outputs 2, 3, and 4 are isolated from
the input due to the cutoff properties of the directors 36 and 37.
In b, the upper half of the waveguide has been displaced with
respect to the lower half such that shorting of the input waveguide
is achieved. In c, the director 36a for output 4 is overlaying the
input and the input waveguide is shorted. The input remains in the
shorted condition until the direction 36a is in the position shown
in d. The dimension d is approximately half a freespace wavelength.
In this position energy is now coupled from the input to output
4.
Higher-order modes are excited within the waveguide in the region
of the directors. These higher-order modes excite currents at the
centerline 32 of the waveguide wall that could result in a small
level of energy being leaked from the split in the waveguide.
Standard, half-wave, folded chokes are located at the separation
between the rotor 33 and stator 35 on both the upper and lower
sides of the waveguide. These chokes are continuous around the full
waveguide annulus and create an electrical short at the dividing
line and these prevent any leakage of RF energy from the
waveguide.
The diameter of the waveguide annulus is selected such that the
time periods during which energy is coupled to an output and during
which the input is shorted, are consistent with the elevation
radiation intervals.
The rotating scanner is a waveguide device and is capable of
handling, without breakdown, power levels approaching that of a
straight section of waveguide. The device also has a frequency
bandwidth capacity of greater than 20 percent. The VSWR over the
band of frequencies from 15.4 to 15.7 is less than 1.05:1 and the
insertion loss is less than 0.5 dB.
FIG. 4 illustrates a typical pillbox antenna 40 which is found in
the prior art.
The pillbox antenna illustrated is a parallel plate microwave
system in which the radiation is confined to two dimensions between
conducting sheets 41 and 43 and a conducting backwall 45 acts as a
reflector collimating the microwave energy. In a simple pillbox
such as this the feed system 47 is in the path of the radiated
collimated energy from the reflector. This results in feed blockage
and pattern deterioration. To avoid this blockage a folded pillbox
is used. Folded pillboxes have been described in the literature
such as the article by W. Rotman entitled "Wide Angle Scanning with
Double Layer Pillboxes" Trans IEEE PGAP Jan. 1958.
The folded pillbox used in the present devices and shown on FIG. 5
uses two sets of parallel plates, one indicated as set 49
containing the incident field from the feed and the other the
reflected-collimated-field from the reflector. Each set of parallel
plates forms a transmission line. The sections are connected by a
180.degree. bend 53 whose back wall forms a circular reflector in
the elevation plane.
FIG. 5 also shows some details of the relative location of the
scanner with respect to the waveguide. The scanner 55 is placed
within the plates 49 and rotated therein by the drive motor 57 in
the manner described above. As can be seen more clearly from FIG.
5c, as the scanner 57 rotates each of the output ports in
succession will direct their energy toward the circular reflector
of the 180.degree. bend 53 to the set of plates 51 and directed out
of the antenna aperture 59.
To minimize the tolerance requirements and reduce the production
costs of the parallel plate pillbox spacing, a TEM propagation mode
is excited by the waveguide feed. This mode has its electric field
perpendicular to the plates. The wavelength for the TEM mode is
independent of the plate spacing and the tolerance is based on
impedance mismatch considerations. Propagation of other modes is
prevented by holding the plate separation to a dimension less than
half a free space wavelength.
Small variations in the separation of the plates are not critical
as long as this half-wavelength maximum dimension is not exceeded.
Since a sandwich type of construction is utilized, and since the
pillbox surfaces are planar, the parallel plate spacing need not be
maintained to better than .020 inch throughout the structure for an
operating frequency of 15.5GHz.
The use of a TEM mode in the pillbox results in a horizontally
polarized radiation field which requires a polarization rotator to
change the horizontal polarization in the parallel plate line to a
vertically polarized radiating field.
Both a parabolic and circular reflector can be used with the
pillbox antenna of FIG. 5. The circular reflector is preferred due
to its superior off axis scanning properties. FIG. 6a shows that if
the parabolic reflector is fed from its point focus, an in phase
condition occurs at a plane AZ which is perpendicular to the
reflector axis. However, as the feed is scanned off the point
focus, the plane AZ tilts but an on focus condition no longer
exists on it. This manifests itself in unacceptable sidelobe
levels, beam-broadening due to coma and non-planar beam shape
(conical). On the other hand a properly fed circular reflector, as
shown in FIG. 6b, will overcome this condition since the center of
rotation for the feed and reflector are the same point. Thus the
physical relationship between the two, remain constant as a
function of rotation. At every feed angular position during the
scan, the same broadside aperture is obtained. This constant
illumination with scan motion results in a scan pattern having a
planar beam shape with constant beamwidth and side lobes.
In order to feed a circular reflector it is necessary to use a lens
to compensate for its inherent spherical abberation. This is
illustrated by FIG. 8 which shows that the reflected rays of a
circular reflector are uncollimated if it is fed from a point
source. To overcome this problem, a dielectrically loaded, parallel
plate feed lens is used on the output of each of the scanner feed
horns. The lens is shaped such that it refracts the rays so that
they strike the reflector at an angle which will allow them to be
reflected parallel to the antenna axis. The shape of this lens is
determined by applying two conditions upon the rays emitted from
the lens:
1. that they be reflected off the reflector parallel to the antenna
axis; and
2. that in a plane perpendicular to the antenna axis, the rays all
have the same electrical path length. If these two conditions are
satisfied, energy radiated from the reflector will be collimated in
a plane wave of uniform phase which provides a radiation pattern
with a 2.degree. halfpower beamwidth. Since the centers of rotation
of the scanner and reflector are coincident, the relative
configuration of the antenna is invariant as a function of rotation
and ture planar-beam scanning is achieved. FIG. 6b shows that if
the feed lens 61 is moved along the arc AB not only do the rays
remain collimated but the beam is always perpendicular to
generating source which in this case is the circular reflector.
This is extremely important if planar beam shapes are to be
maintained.
A typical feed lens 61 is shown on FIG. 7. Energy will be provided
out of each of the outputs 1, 2, 3, and 4 of FIGS. 2 and 3 through
a waveguide 63 to a lens 61. This lens 61 will comprise a parallel
plate dielectrically loaded feed lens. The dielectric material 65
is contained between two plates 67 and 69. The input end of the
lens has a tapered matching section 71. Once the dielectric
constant of the lens medium and the position of the feed horn
required to obtain the required output aperture size is determined,
a lens shape may be found that satisfies the condition noted above.
For example, at 15 GHz, a 2.degree. 3DB elevation antenna, the
reflector would be a cylinder with the radius of curvature of 20.9
inches. The point of the lens closest to the reflector is located
at a radius of 15 inches. The lens thickness is 0.261 inches, less
than half a free space wavelength, to prevent higher order mode
propagation and the relative dielectric constant of the lens
material 1.50.
Since the energy in the folded-pillbox design is propagated in the
TEM mode the polarization of the radiated energy is linear in the
horizontal direction. However, if vertically polarized energy is
required, a polarization rotator can be placed across the aperture
to provide vertical linear polarization.
Several techniques for accomplishing the polarization rotation are
possible. These include a double array of parallel strips, an array
of twisted waveguides, and a double array of printed-circuit
inductive and capacitive elements. Any of these methods are
suitable for the SLAM antenna. All but the technique incorporating
twisted waveguides involve conversion of polarization from
horizontal linear to circular and from circular to vertical linear.
The first two techniques, arrays of parallel metal strips and a
combination of parallel metal and dielectric strips, are both quite
heavy and difficult to manufacture. In addition, it is necessary to
adequately support the individual strips since their orientation
must be carefully maintained under shock and vibration conditions.
The method involving the twisted waveguide is a straightforward
conversion technique. However, it is difficult to manufacture and
even heavier than the first two methods.
The fourth method of conversion provides a very lightweight,
reproducible, structurally-sound unit. Such an arrangement is shown
on FIGS. 9a and b. The device consists of six layers 75 of metal
circuits photoetched on thin fibreglass sheets. A
quarter-wave-thick sheet of very low density polyester foam 77
separates each of the fibreglass sheets.
The printed circuits consist of inductive and capacitive shunt
susceptance elements such as copper squares 79 and strips 78
arranged, as shown on FIG. 9b, so that an incident linearly
polarized signal is split into two orthogonal components and the
phase of one element is delayed with respect to the other. The
first, third, fourth and sixth sheets are of the same design and
the second and fifth sheets are the same. The combination of the
first three sheets divides the incident linearly-polarized energy.
The combination of the remaining three sheets provides the reverse
of this operation because of a 90.degree. physical rotation of the
three sheets. In this way the circularly-polarized energy is
converted to vertical linear, the desired orientation.
Polarization convertors of this type have been used in many
applications. The mismatch of these devices is very small, less
than 1.1:1, and the insertion loss is less than 0.5 dB. Frequency
bandwidths in excess of 10 percent are readily attained with this
approach.
AZIMUTH ANTENNA
The azimuth antenna shown on FIG. 10 utilizes the same scanner as
the elevation antenna with similar lens but does not require the
folded pillbox. In this case the lenses rotating about a scanner
axis 80 feed a section of parallel plate transmission line 81 which
is used as the feed for a doubly curved reflector 83 which is the
antenna radiating aperture.
As in the case of the elevation antenna the radiating aperture must
be curved, in the scan plane, to maintain the planar beam shape.
The reflector and the feed both have a circular shape in the
azimuth plane whose center of rotation is coincident with the
scanner center of rotation as shown on FIG. 11A.
As shown thereon, there is a scanner 85 such as that described
above, with one waveguide 87 shown terminating in a lens 89. The
lens will be within the parallel plate feed 81 shown on FIG. 10
which terminates in a circular shape as indicated by the circle 91.
Energy is then radiated to the circular reflector 93.
In order to keep the structure compact, a right angle bend 95,
shown on FIG. 11B, is included in the parallel plate feed 81. In
the elevation plane the reflector 93 is shaped so as to generate a
csc.sup.2 .theta. pattern which minimizes the energy on the ground
while increasing the signal level above the beam peak.
The actual reflector shape is designed using an existing computer
program which provides an analytic solution based on geometric
optics. This method of calculation has been described in the
literature by A. S. Dunbar, "Calculation of Double Curved Reflector
for Shaped Beams," Oct. 1948. Proceedings of the IRE-Wave And
Electron Section, and consists of transforming the feed radiation
into the described reflector radiation pattern using geometric
optics equations.
Side lobe control is maintained in a number of ways. In the
elevation plane the actual radiation pattern of the parallel plate
feed is used to calculate the reflector's shape by the above
mentioned method. In the azimuth plane the scanner output feed and
lens are carefully designed and constructed in order to maintain
the proper phase and amplitude distribution on the reflecting
surface. Finally in the parallel plate feed itself, extreme care is
used to insure a homogeneous transmission path so that minimum
phase and amplitude distortions are introduced.
COMBINED ELEVATION/AZIMUTH ANTENNA
The elevation and azimuth antenna are combined in a single
synchronized unit as illustrated by the cutaway perspective view of
FIG. 12. Reference numerals used in the description of FIG. 12 will
be the same as those previously used where possible to aid in
collating the previously described figures with FIG. 12. Two
scanners, both the elevations scanner and the azimuth scanner will
be located on a common axis in the direction of arrow 98. The
elevation antenna is located in the front, with its scanner 55
feeding its four lens 61, one of which is shown, which then direct
the energy to the folded pillbox 99 from which it is directed
through a right angle bend to the elevation output horn and
polarizer 101. The resulting elevation scan is illustrated by the
energy patterns 103 shown as being emitted from the horn 101. The
azimuth scanner is not shown. However, one of its lenses 89 can be
seen which directs the energy through its folded pillbox 81 to the
reflector 93 as described above. The azimuth radiation pattern is
illustrated by the beams 105 from the reflector 93. To keep the
antenna unit as compact as possible, two right angle bends,
respectively, bend 107 and bend 109, have been added to the
elevation antennas folded pillbox resulting in the elevation beam
being radiated out normal to the plane of the scanner. Each antenna
section provides the required planar beam throughout its scan
sector. The scanners for the two antennas may be machined as a
common unit and driven by a single motor. Angular position
information is provided by a single, directly driven encoder.
Synchronization of the azimuth and elevation beams is ensured by
the inherent design of the switching section. It is unique in that
the normal power consuming active RF switch has been replaced with
a passive ferrite circulator. The circulator in conjunction with
the antenna scanner not only accomplishes the switching function
but presents a constant impedance to the RF source. The switching
function itself relies on the properties of the ferrite circulator
and the antenna scanner. This portion of the operation is
illustrated on FIGS. 13 and 14. RF energy is developed in
conventional manner from an RF source and is provided into the
system on a waveguide 111. From the waveguide 111 it enters a
ferrite circulator indicated generally as 113. The circulator 113
contains four ports labelled ports 1 to 4. Radiation enters through
port 1. A conventional termination 115 is coupled to port 4. Port 2
is coupled to the elevation scanner and port 3 to the azimuth
scanner via waveguides 117 and 119 respectively. On the drawing, as
indicated by the key, RF energy is a heavy solid arrow and low
level reflected energy by dashed arrows.
The scanner sequentially couples each antenna to the appropriate
terminals of the circulator. The combination of circulator/scanner
automatically directs the RF energy to the first available active
antenna. The nature of the scanner is such that either the azimuth
or the elevation antenna is capable of receiving this RF energy.
The scanner never permits both antennas to get RF energy
simultaneously. Note that both the elevation section 55 and azimuth
section 85 of the scanner have four radiating elements, represented
by the light areas 121, spaced 90.degree. apart. The shaded areas
123 separating these elements represents that portion of the scan
when a short circuit is presented at the antenna terminals. The
relative positions of the elevation and azimuth segments is fixed
when the single scanner is fabricated.
On FIG. 13, the scan cycle has been stopped with the azimuth
portion 85 of the scanner in a short circuit condition and the
elevation portion in a radiating condition. RF energy from the RF
source enters Port 1 of the circulator 113, rotates around the
first circulator junction 125 in a counterclockwise direction to
the second circulator junction 127 where it is rotated
counterclockwise to Port 2. At Port 2 it sees the elevation
antenna, through the "open" scanner 55 and is radiated. Any low
level energy reflected by the elevation antenna ia reflected back
into the circulator and rotated to Port 3 where the azimuth portion
85 of the scanner is short circuited. It is once again reflected
back into the circulator 113 and rotated to Port 4 where it exits
to the RF termination 115.
FIG. 14 shows the scanner advanced to the azimuth radiate position
and the elevation portion 55 of the scanner short circuited.
Azimuth RF energy enters the circulator 113 at Port 1, is rotated
to Port 2 where it encounters the short circuited elevation portion
55 of the scanner and is reflected back into the circulator. Here
it is rotated to Port 3 and passes through the open azimuth
antenna. Any energy reflected by the radiating antenna is reflected
back into the circulator 113 and rotated to and exits from Port 4
to the RF terminator.
When properly indexed, a single encoder 125 coupled to the shaft
127 driving the scanners provides antenna positional information
for the entire azimuth/elevation scan cycle. During the interval
when neither elevation nor azimuth information is required the
scanner automatically presents a short circuit to the RF source
which causes the RF to be reflected back through the circulator 113
and into the termination 115 thereby automatically isolating the RF
source from the antenna.
As described above, a circular feed aperture into the parallel
plates was disclosed which resulted in a planar output scan. It is
also possible to obtain a conical scan with the antenna of the
present invention by providing a linear feed aperture into the
parallel plates.
Thus, an improved scanning lens antenna which provides a planar
beam output useful in landing system applications has been shown.
Although a specific embodiment has been illustrated and described,
it will be obvious to those skilled in the art that various
modifications may be made without departing from the spirit of the
invention which is intended to be limited solely by the appended
claims.
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