U.S. patent number 3,587,106 [Application Number 04/745,014] was granted by the patent office on 1971-06-22 for broad band antennas having spiral windings.
This patent grant is currently assigned to General Dynamics Corporation. Invention is credited to James W. Crooks, Jr., Ray M. McIntyre, Jr..
United States Patent |
3,587,106 |
Crooks, Jr. , et
al. |
June 22, 1971 |
BROAD BAND ANTENNAS HAVING SPIRAL WINDINGS
Abstract
A broad band antenna or feed for a parabolic reflector having a
pair of spiral windings skewed with respect to the axis of a
conical support surface on which they are wound in alternating
relationship. Two such supports are rotatably mounted with their
axes offset at an angle from the reflector axis so as to conically
scan the target area. The feed achieves maximum illumination of the
control portion of the reflector and provides a constant squint
angle with respect to the reflector axis over a broad band of
frequencies.
Inventors: |
Crooks, Jr.; James W. (San
Diego, CA), McIntyre, Jr.; Ray M. (San Diego, CA) |
Assignee: |
General Dynamics Corporation
(N/A)
|
Family
ID: |
24994871 |
Appl.
No.: |
04/745,014 |
Filed: |
July 15, 1968 |
Current U.S.
Class: |
343/739; 343/840;
343/761; 343/895 |
Current CPC
Class: |
H01Q
3/18 (20130101); H01Q 11/083 (20130101) |
Current International
Class: |
H01Q
11/08 (20060101); H01Q 11/00 (20060101); H01Q
3/18 (20060101); H01Q 3/00 (20060101); H01q
001/36 () |
Field of
Search: |
;343/792.5,840,895,761 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Leiberman; Eli
Claims
What we claim is:
1. An antenna comprising
a. an electrically conductive element spirally wound to define a
substantially conical surface having an axis, and
b. at least those turns of said element having larger diameters
being skewed with respect to said axis so as to depart from
equiangular relationship with respect to the other turns of said
element which are not so skewed, adjacent ones of said larger
diameter turns which are disposed successively in a direction
towards the apex of said conical surface being spaced from each
other by progressively smaller distances.
2. The invention as set forth in claim 1 wherein the projection of
said turns having larger diameters on a plane perpendicular to said
axis is an elliptical spiral.
3. The invention as set forth in claim 1 including a reflector
having an axis and a focal point spaced from said reflector along
said reflector axis, said surface and said reflector being disposed
on opposite sides of said focal point with said reflector axis at a
predetermined angle to said surface axis whereby the direction of
the radiation pattern of said turns of spirally wound element is
substantially parallel to said reflector axis.
4. The invention as set forth in claim 3 including means for
supporting said elements on said surface for rotation about said
reflector axis.
5. The invention as set forth in claim 3 including a second
electrically conductive element spirally wound identically with
said first named conductive element except in an opposite sense to
define a second substantially conical surface, the axis of said
second surface being disposed in the same plane as said first
surface axis and said reflector axis, said second surface axis and
said first surface axis intersecting said reflector axis at the
same point and making the same angle with said reflector axis as
said first surface axis.
6. The invention as set forth in claim 5 including means for
supporting said elements on said first and second surfaces for
rotation about said reflector axis.
7. An antenna comprising first and second electrically conductive
elements spirally interwound to define a substantially conical
surface having an axis, at least those turns of said elements
having larger diameters being skewed with respect to said axis so
as to define a skew angle which is different from the skew angle of
the remaining turns, the skew angle being defined as the angle
formed by a first line connecting diametrically opposite points on
turns of first and second elements which are adjacent to each other
and a line perpendicular to the axis in the plane containing the
axis and said first line.
8. The invention as set forth in claim 7 including a transmission
line having two conductors, connected respectively to said first
and second elements at the apex of said conical surface.
9. The invention as set forth in claim 7 wherein all of the turns
of said first and said second conductive elements have the same
skewed relationship.
10. The invention as set forth in claim 7 wherein said first and
second elements are so wound that the intersection of a first plane
perpendicular to said axis and said surface defines a circle and a
second plane intersecting said axis at the same point as said first
plane and intersecting diametrically opposite points of adjacent
ones of said first and second elements and said surface defines an
ellipse.
11. A unidirectional antenna for providing a radiation pattern in a
certain direction comprising
a. a pair of windings spirally wound as to define a conical surface
having an axis lying at a predetermined angle to said certain
direction, and
b. at least a plurality of those turns of said windings having the
larger diameters being skewed with respect to said axis so that a
plane perpendicular to said certain direction and intersecting
diametrically opposite points on adjacent ones of said turns of
different ones of said windings defining a squint angle with
respect to said axis said squint angle being equal to the
difference between 90.degree. and said predetermined angle.
12. The invention as set forth in claim 11 including a transmission
line having two conductors connected respectively to different ones
of said pair of windings at ends thereof near the apex of said
surface, and resistive means connected between the ends of said
winding near the base of said surface.
13. The invention as set forth in claim 12 wherein said resistive
means includes a circular conductor disposed near the base of said
surface, a first plurality of resistive elements each connected
between the last turn of one of said pair of windings and said
circular conductor at positions spaced from each other around the
periphery of said circular conductor, and a second plurality of
resistive elements each connected between the last turn of the
other of said of windings and said circular conductor at positions
spaced from each other around the periphery of said circular
conductor.
14. The invention as set forth in claim 13 wherein the spacing of
said pluralities of resistive element is logarithmic.
15. The invention as set forth in claim 11 including a second pair
of windings spirally wound identically with said first pair of
windings except in the opposite sense so as to define a second
conical surface having a second axis, said second axis intersecting
the axis of said first conical surface and so disposed that the
same plane extends diametrically through both of said surfaces.
16. The invention as set forth in claim 15 including a reflector
dish having a focal point along the axis of said dish and said
surface being disposed on opposite sides of said focal point with
said dish axis bisecting the angle between the axes of said
surfaces.
17. The invention as set forth in claim 16 including means for
supporting said surfaces for rotation about the axis of said dish.
Description
The present invention relates to broad band antennas and
particularly to broad band antennas having conical spiral feed
windings.
The invention described herein was made in the performance of work
under a NASA contract and is subject to the provisions of Section
305 of National Aeronautics and Space Act of 1958, Public Law
85-568 (72 Stat. 435; 42 USC 2457).
The invention is especially suitable for use in a feed for
parabolic antennas which are adapted to track space vehicles by
responding to signals radiated therefrom. The invention however is
also generally useful in broad band antennas, including antennas of
a type which do not have a reflector dish as well as in directional
antennas where a desired radiation pattern in a direction having a
specified angle with respect to the angle at which the antenna is
mounted may be desired.
The conventional conical antenna having equiangular spiral windings
has a radiation pattern which is directed along the axis of the
cone on which the windings are disposed. It has been found that
this characteristic is detrimental to a scanning antenna where the
axis of the cone is offset from and makes an angle with the axis of
the reflector with which the antenna is used as a feed. The term
feed comprehends both reception and transmission of signals.
At certain lower frequencies in the band in which the antenna is
active although the feed is positioned for proper operation at the
highest frequency, instead of scanning conically in synchronism
with the rotation of the feed, the radiation pattern from the
reflector remains essentially along the reflector axis. This effect
is manifested particularly in the case of the plane of the Electric
Vector of linear polarized radiation. Since the antenna does not
scan with the requisite pattern on which the servosystem of the
tracking system depends, it becomes difficult to lock on to and to
track the source of incoming radiation, say a space vehicle or
missile. In the event that the ratio of focal length with respect
to the diameter of the reflector is small as for example (in deep
parabolic dishes) the side lobe energy or side lobe radiation
pattern of the feed impinges on the reflector and the feed pattern
does not effectively illuminate the whole reflecting surface of the
dish thereby compromising antenna performance.
It is therefore an object of the present invention to provide
improved scanning antennas.
It is a further object of the present invention to provide improved
broadband antennas.
It is a still further object of the present invention to provide an
improved conical feed for spiral antenna.
It is a still further object of the present invention to provide a
wideband conical spiral type feed which may be used in combination
with a dish or by itself as an independent antenna and whereby the
radiation pattern of the feed remains at a desired angle with
respect to the axis thereof over the entire band of frequencies
over which the antenna is active. The angle can be made
approximately constant or varied over the frequency range of the
antenna by controlling the angle of skew along the winding.
Briefly described, an antenna or feed embodying the invention
includes a support structure. A winding is spirally wound around
the support structure to define a substantially conical surface or
cone. A portion of the windings can be disposed at a predetermined
angle with respect to the axis of the cone so that the direction of
the radiation pattern remains at a constant angle with respect to
the cone axis. In the event that the aforementioned antenna
structure is sued as a feed for an antenna having a reflector, it
is desirably supported so that the cone axis is at a predetermined
angle to the axis of the reflector. This angle being selected to
achieve a desired squint angle. The squint angle of the antenna,
when this invention is used as a feed in a reflector, is the
direction of the radiation pattern from the reflector with respect
to the reflector axis. By virtue of the skewed windings on the
feed, the squint angle of the antenna can be made to be
approximately proportional to the antenna beam width over the
entire wide band of frequencies over which the antenna is active,
notwithstanding that the feed may be rotated about the axis of the
dish in order to provide scanning action.
The invention itself, both as to its organization and method of
operation, as well as additional objects and advantages thereof
will become more readily apparent from a reading of the following
description in connection with the accompanying drawings in
which:
FIG. 1 is a schematic diagram of a conventional antenna of the type
having a dish illuminated by a feed on which equiangular conical
spiral windings are disposed;
FIG. 2 is a schematic diagram similar to FIG. 1 of an antenna which
embodies the invention:
FIG. 3 is a plan view of a conventional equiangular or logarithmic
conical feed or antenna;
FIG. 4 is a view similar to FIG. 3 of an antenna or feed which
embodies the invention;
FIG. 5 is a sectional view taken along the line 5-5 in FIG. 4;
FIG. 6 is a section view taken along the line 6-6 in FIG. 4;
and
FIG. 7 is a plan view of a conical spiral antenna or feed in
accordance with another embodiment of the invention.
FIG. 1 illustrates a conventional telemetry antenna which may be
used as a receiving antenna for tracking and receiving signals from
space vehicles and missiles. The antenna may also be used for
transmitting. The antenna includes a dish reflector which is
parabolic in form and has a focal point along the axis thereof. The
dish is relatively deep, that is to say the ratio of its focal
distance (viz. the distance from the focal point of the dish to its
surface (F) to the diameter of the dish (d)) is relatively low say
of the order of 0.3. The antenna includes a feed 17 which is shown
in greater detail in FIG. 3. This feed includes two conical members
12 and 14, which may be made of insulating material. Each of these
members carries a pair of windings 16a and 16b and 18a and 18b. The
windings 16a and 16b on the conical member 12 alternate and are
wound in the form of equiangular spirals. Windings 18a and 18b on
the conical member 14 are similarly wound as equiangular spiral
windings but in a sense opposite to the windings 16a and 16b. Thus
the windings 16a and 16b will be responsive to one sense of
circular polarized radiation whereas the windings 18a and 18b will
be responsive to the opposite sense of circular polarized
radiation. The conical members are supported on a yoke 20 which may
be journaled for rotation about the axis of the dish.
Transmission lines 19 and 21 are respectively provided in the
cylindrical members 12 and 14 and are connected at their front ends
to the ends of the windings near the apex of their respective
conical members. The other end of the windings 16 and 18 are
terminated, say by resistors 22 and 24, respectively, each being
connected between the opposite ends of their associated windings.
The ends of the transmission lines 19 and 21, which extend from the
base of the conical members may be connected by way of cables to a
rotating joint for deriving signals form the respective windings 16
and 18. A two wire transmission line may be used for 19 and 21 or
alternately a coaxial line with a balun to couple from the coaxial
line to the balanced transmission line at the tip of the cone.
Alternately a coaxial transmission line can be used as the spiral
winding with a coaxial transmission line bringing the signal in (or
out) connected to one of the two spirals in the place of the
resistor termination. The coaxial transmission line center
conductor can be connected from one spiral to the center conductor
of the opposite spiral on the same cone at the small end (apex) of
the spiral with a small gap between the outer conductor of one
winding and the outer conductor of the second winding. The outer
conductor of the second spiral winding is shorter to the inner
conductor at the end of the gap. This is a conventional method of
feeding equal angle spiral antennas from a coaxial transmission
line and is sometimes referred to as the use of an infinite balun.
The terminating resistor is optional. The antenna, by virtue of the
fact that the windings 16 and 18 have smaller diameter portions
which spiral outwardly to larger diameter portions is operative
over a broad band of frequencies, say from the S band through the
UHF band. The effective center of radiation (viz. the point on the
feed 17 where the antenna is responsive to different frequencies of
radiation) varies with the distance along the axes of the conical
members 12 and 14. The shorter wavelengths will have their centers
of radiation near the apex of the cone and for longer wavelengths
the center of radiation will move along the axis of the conical
members away from the apex thereof.
As shown in FIG. 1, the apex of the conical members and the center
of radiation at short wavelengths is close to the focal point of
the dish. A typical E-plane radiation pattern at lower frequencies
and longer wavelengths is also shown spaced at its center further
away from the dish. In order that the antenna be responsive to
signals at the longer wavelengths, it is necessary that the
antennas scan a relatively wide field of view at lower frequencies.
Such a wide scanning field dictates a relatively large squint
angle. By a squint angle is also meant the angle between the axis
of the dish and the beam direction. The desired squint angle and
the desired beam direction is shown in FIG. 1. In order to obtain
this larger squint angle at longer wavelengths, it becomes
necessary to offset the axis of the conical members from the axis
of the dish. The foregoing offset is dictated by the characteristic
of equiangular spiral antennas of providing a radiation pattern
which is along the axis of the conical members, as shown in FIG. 1.
By virtue of this offset the angle of major illumination of the
dish at lower wavelengths is not sufficiently broad to evenly
illuminate the full dish, especially in the plane of the Electric
Vector (E-plane). In deep parabolas, this results in side lobe
energy impinging on the reflector surface; further, compromising
antenna performance. In addition, the effective squint angle may be
actually reduced to zero at some wave lengths. This characteristic
follows from the fact that the actual direction of the radiation
(viz. the direction of the beam) is perpendicular to the phase
front from the portion of the dish which is illuminated. The phase
front is the surface along which all of the energy is in phase. The
phase front of the radiation from the illuminated portion of the
dish at any frequency is defined by a surface generated by the
continum of points having equal distances along ray paths from the
center of radiation of waves at that frequency to the dish and then
outwardly from said dish. Two of such points 26 and 28 at the
limits of the angle of major illumination of the dish at a longer
wave length in the UHF band are shown in FIG. 1. These points are
generated by drawing ray paths from the center of radiation along a
straight line to the dish and then from the dish such that these
angles of incidence .theta..sub.1 and .theta..sub.3 equal these
angles of reflection .theta..sub.2 and .theta..sub.4, respectively.
The distance along the line between the center of radiation to the
dish and thence to the point 26 on the phase front is equal to the
distance along the lines from the center of radiation to the dish
and thence to the point 28. Since the actual direction of the beam
or the direction of the radiation pattern of the antenna is
perpendicular to the phase front which is illuminated, it may be
determined by construction that the actual direction of the beam is
approximately parallel to the axis of the dish making the actual
squint angle approach zero degrees (i.e. along the dish axis) for
radiation of lower frequency and longer wave lengths.
In accordance with the invention this deficiency is overcome by
providing a radiation pattern from the feed which is essentially in
a direction parallel to the axis of the dish so that the dish is
illuminated throughout the frequency range over which the antenna
is active. FIG. 2 shows a dish 30 which may be a relatively deep
parabolic of the type shown in FIG. 1. The antenna includes a feed
32 including two conical members 34 and 36. The conical member 34
having windings wound thereon in one sense to provide a left
circularly polarized feed element whereas the conical member 36 has
windings wound thereon in an opposite sense to provide a right
circularly polarized feed. The windings are skewed with respect to
the axis of their respective conical members 34 and 36. The
resulting E-plane radiation pattern is effectively parallel to the
axis of the dish so that the angle of principal illumination of
dish 30 is distributed over approximately the central portion of
the dish. The phase front resulting from this illumination may be
determined by construction of the same manner as explained in
connection with FIG. 1. It will be observed that two points 38 and
40 on the phase front along rays at the edges of the angle of
principal illumination are approximately at the same distance from
the dish axis.
Inasmuch as the actual beam direction is perpendicular to the phase
front from the illuminated portion it is apparent that the beam
direction will be at the desired squint angle and radiation at
longer wavelengths may readily be received by the antenna as it
scans a wider area. Accordingly, the invention provides a broad
band scanning antenna with adequate squint in that it makes
possible adequate scanning at wavelengths at which the squint is
inadequate with conventional feed to produce adequate tracking
error signals for proper servo performance. As shown in FIGS. 4, 5
and 6 the conical members 34 and 36 may be hollow cones of
fiberglass or other insulating material on which pairs of windings
40 and 42 are wound; the windings 40a and 40b being wound on the
cone 34 while the windings 42a and 42b are wound on the cone 36.
The cones are supported on a yoke 44 which may be journaled for
rotation about the axis of the dish, as shown in FIG. 2. The
windings 40a and 40b as observed on one side of the conical support
alternate with each other and are connected at their ends near the
apex of the conical member 34 to a transmission line (not shown)
similar to the transmission line shown in FIG. 3. The windings 42a
and 42b are wound in a sense opposite to the windings 40a and 40b
and are also alternated with each other. Another transmission line
may be connected to the apex ends of the windings 42a and 42b. The
other ends of the windings 40a and 40b near the base of the cone 34
may be terminated by a resistor 46 while the ends of the windings
42a and 42b near the base of the cone 36 may be terminated by a
resistor 48. This resistive termination is not essential and may be
omitted if desired. A termination is, however, preferred.
In order to enchance the performance of the feed 32, it may be
desirable to position a ground plane at the bases of each of the
cones 34 and 36. Alternatively, the ends of the windings 40a and
40b and 42a and 42b may be terminated by rings 50 (FIG. 7) of
conductive material such as wire which is disposed around
cylindrical portions 52 extending from the bases of the cones; only
one of the cones is illustrated in FIG. 7 to show this
construction. A first plurality of resistors 54 is connected
between the end of the winding 40a and the ring 50 while a second
plurality of resistors 56 is connected between the end of the
winding 40b and the ring 50. These resistors are parallel to the
axis of the cylindrical portion 52 and are spaced from each other.
The spacing between the resistor 54 is desirably logarithmic and
increases between resistors which are spaced further away from the
end of the winding 40a. A similar space relationship may be
provided between the resistors 56 which terminate the end of the
winding 40b.
The surface of the cones 34 and 36 is of course defined by the
conductive elements or windings 40 and 42 thereon. As shown in FIG.
5 this surface can be made elliptical in cross section for cross
sections taken perpendicular to the axis of the cones and the
surface be made conical for cross sections taken perpendicular to
the scanning axis (viz. the axis of the dish) as shown in FIG. 6.
This is a desirable but not essential feature of the design. The
windings 40 and 42 are also skewed with respect to the axis of the
respective cones 34 and 36 (viz. the conical surfaces which they
defined). As pointed out in connection with FIG. 2, it is desirable
that the direction of radiation at the longer wave length be
essentially parallel to the axis of the dish. This is accomplished
by providing the skew of the windings such that the windings are
tilted at a skew angle approximately equal to the angle which the
axis of the cones make with the dish axis. The skew angle is
identified in FIG. 4. The projection of the windings on a plane
perpendicular to the axis of the cones (viz. the conical surfaces
defined by the windings) can be beneficially modified to be an
elliptical spiral rather than circular spiral as is the case with
an equiangular spiral winding, such as is shown in FIG. 3. This
angle of skew as shown in FIG. 4 may be defined as the angle
between (1) line connecting diameterally opposite points on
adjacent turns of the windings 42a and 42b, in the plane containing
the center lines of the two cones 34 and 36 (2) another line
intersecting the axis of the cone 36 in the same place as the first
but perpendicular to the axis. In order to obtain an angle of skew
such that the desired squint angle is obtained the angle of skew is
made approximately equal to the angle which the center line of the
conical member 36 makes with the scanning axis, the diametrical
line should also be perpendicular to the dish axis. Inasmuch as the
reduction of the squint angle occurs principally at the longer
wavelength it may be desirable only to make those turns of the
windings of larger diameter tilt at the requisite angle of skew.
The smaller diameter of turns near the apex of the conical members
then may be equiangular. In a particular case wherein the windings
42a and 42b were approximately six turns each only the last three
turns of the windings were skewed.
The invention of course may be used in an antenna which does not
include a dish. In that event, the apex of the conical members 34
and 36 will point in the direction of radiation (viz. towards the
missile or space vehicle providing the source of signals to be
tracked rather than towards a reflector and the skewed winding
rotated about the center line of the cone to provide scanning. Even
if the antenna is not used as a scanning antenna, it may be
advantageously be employed in application where it is desired to
have a radiation pattern over a wide band of frequencies directed
at a predetermined squint angle. Thus it may be applicable where
ease of mounting is desired inasmuch as the axis of the conical
member may be referenced in a particular direction with respect to
the axis of the nose of an aircraft. The look angle or the
direction of the beam from the antenna or the direction of
radiation to which the antenna is responsive would then be
determined by the squint angle which is defined by the skew of the
windings with respect to the axis of the conical member. Instead of
rotating the antenna, the vehicle in which the antenna is mounted
may be rotated in order to obtain a scanning action.
From the foregoing description, it will be apparent that there has
been provided an improved antenna which is particularly adapted for
use as a wide band scanning antenna as in telemetry applications
and especially where a missile or space vehicle is to be tracked.
While illustrative embodiments of the antenna have been described
and other applications for the antenna have been suggested, various
other applications and modifications in the herein described
antenna will undoubtedly suggest themselves to those skilled in the
art. Accordingly, the foregoing description should be taken merely
as illustrative and not in any limiting sense.
* * * * *