U.S. patent number 3,710,338 [Application Number 05/102,983] was granted by the patent office on 1973-01-09 for cavity antenna mounted on a missile.
This patent grant is currently assigned to Ball Brothers Research Corporation. Invention is credited to Robert E. Munson.
United States Patent |
3,710,338 |
Munson |
January 9, 1973 |
CAVITY ANTENNA MOUNTED ON A MISSILE
Abstract
An antenna for use on a missile wherein a cylindrical conductor
is concentrically positioned about a metallic surface portion of
the missile so as to define a cavity of one-quarter wavelength
between one end of the cylindrical conductor, which is connected
with the missile surface, and a plurality of connecting positions
of energy transfer means. The energy transfer means is connected
about the periphery of the cylindrical conductor at intervals
substantially equal to a single wavelength of a signal so that the
cylindrical conductor and the missile surface adjacent an opposite
end of the cylindrical conductor form the elements of an asymmetric
dipole of high impedance.
Inventors: |
Munson; Robert E. (Boulder,
CO) |
Assignee: |
Ball Brothers Research
Corporation (Boulder, CO)
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Family
ID: |
22292739 |
Appl.
No.: |
05/102,983 |
Filed: |
December 30, 1970 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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787912 |
Dec 30, 1968 |
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Current U.S.
Class: |
343/708; 343/807;
343/769 |
Current CPC
Class: |
H01Q
1/286 (20130101) |
Current International
Class: |
H01Q
1/28 (20060101); H01Q 1/27 (20060101); H01q
001/28 () |
Field of
Search: |
;343/705,708,769,807 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lieberman; Eli
Parent Case Text
This application is a continuation of copending U. S. application
Ser. No. 787,912, filed Dec. 30, 1968, now abandoned, entitled
"Cavity Antenna" by Robert E. Munson.
Claims
What is claimed is:
1. In a propelled vehicle having a cylindrical body, such as a
missile, an antenna assembly comprising: an electrically conductive
cylindrical element positioned concentrically about and spaced from
a portion of said cylindrical body, said cylindrical element having
a diameter greater than the diameter of said portion of said
cylindrical body with which it is concentric and an axial length
substantially equal to one-quarter wavelength at an anticipated
operating frequency of said assembly; electrically conductive means
for connecting an entire circumferential edge of said cylindrical
element to said portion so as to define a coaxial cavity, one end
of which is completely opened and one end which is completely
closed; and electrical signal feed means connected to said element
and said portion at a point substantially adjacent the open end of
said cavity.
2. An assembly according to claim 1 wherein said cylindrical
element and at least a part of said cylindrical body form an
asymmetrical dipole of large diameter relative to one wavelength at
the anticipated operating frequency of said assembly.
3. In a propelled vehicle having a cylindrical body, such as a
missile, an antenna assembly comprising: an electrically conductive
cylindrical element positioned concentrically about and spaced from
a portion of said cylindrical body, said cylindrical element having
a diameter greater than the diameter of said portion of said
cylindrical body with which it is concentric; electrically
conductive means for connecting an entire circumferential edge of
said cylindrical element to said portion so as to define a coaxial
cavity, one end of which is completely opened and one end which is
completely closed; and electrical signal feed means connected to
said element and said portion, said feed means being connected to
said cylindrical element and said portion of said cylindrical body
at a plurality of substantially equally circumferential spaced
points, the circumferential distance between any two adjacent
points being approximately one wavelength at an anticipated
operating frequency of said assembly.
4. In a propelled vehicle having a cylindrical body, such as a
missile, an antenna assembly comprising: an electrically conductive
cylindrical element positioned concentrically about and spaced from
a portion of said cylindrical body, said element having a diameter
greater than the diameter of said portion of said cylindrical body
with which it is concentric and having an axial length
substantially equal to one-quarter wavelength at the anticipated
operating frequency of said assembly; electrically conductive means
for connecting an entire circumferential edge of said cylindrical
element to said portion so as to define a coaxial cavity, one end
of which is completely opened and one end of which is completely
closed; and electrical signal feed means including a plurality of
leads connected to said element and said portion of said
cylindrical body at points substantially adjacent the open end of
said cavity, said points being equally circumferentially spaced at
intervals approximately equal to one wavelength at said operating
frequency.
5. In a missile having a circumference greater than a plurality of
wavelengths, in a given dielectric, of a signal, an integrated
antenna comprising: a cylindrical conductor concentrically
positioned about a body portion of the missile and connected at one
end with the missile so as to form a continuous circumferential
cavity about the portion of the missile said cavity having one end
completely opened and the other end completely closed; and a
plurality of coaxial transmission lines each having an inner
conductor and an outer conductor, the inner conductors each being
connected with the cylindrical conductor at peripheral intervals
equal at least to substantially one wavelength of the signal, in
the given dielectric, and the outer conductors being connected with
the missile, both the inner and outer conductors of respective
coaxial lines being spaced from the one end at substantially
one-quarter wavelength of the signal, in the given dielectric, so
that said cylindrical conductor and missile form a high impedance
cavity, the impedance being in parallel with radiation impedance
existing substantially between said cylindrical conductor and
missile body.
6. An antenna assembly comprising: an inner electrically conductive
cylindrical element having an axial length at least substantially
equal to one-quarter wavelength at the anticipated operating
frequency of said assembly; an outer electrically conductive
cylindrical element positioned concentrically about and spaced from
at least a portion of said inner cylindrical element, said outer
cylindrical element having a diameter greater than the diameter of
said portion of said inner cylindrical element with which it is
concentric and an axial length substantially equal to one-quarter
wavelength at said anticipated operating frequency; electrically
conductive means for connecting an entire circumferential edge of
said outer cylindrical element to said portion of said inner
cylindrical element so as to define a coaxial cavity, one end of
which is completely opened and one end of which is completely
closed; and electrical signal feed means connected to said
cylindrical elements at a point substantially adjacent the open end
of said cavity.
7. An antenna assembly comprising: an inner electrically conductive
cylindrical element; an outer electrically conductive cylindrical
element positioned concentrically about and spaced from at least a
portion of said inner cylindrical element, said outer cylindrical
element having a diameter greater than the diameter of said portion
of said inner cylindrical element with which it is concentric and
an axial length substantially equal, at most, to the axial length
of said inner cylindrical conductor; electrically conductive means
for connecting an entire circumferential edge of said outer
cylindrical element to said portion of said inner cylindrical
element so as to define a coaxial cavity, one end of which is
completely opened and one end of which is completely closed; and
electrical signal feed means connected to said cylindrical
elements, said feed means being connected to said cylindrical
elements at a plurality of substantially equally circumferential
spaced points, the circumferential distance between any two
adjacent points being, at most, approximately one wavelength at an
anticipated operating frequency of said assembly.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to antennas and more particularly to an
improved cavity antenna particularly useful for integration with a
missile.
2. Description of the Prior Art
Presently, rockets are being utilized for carrying instrument
payloads for short term measurement of various high altitude
environmental data. The various sensor instrumentation often
operates in conjunction with a transmitting unit for instantaneous
and continuous data transmittal to one or more ground tracking
stations. However, constant monitoring has been difficult due to
signal nulls encountered as the vehicle assumes different roll and
aspect orientations.
Although automatic gain control amplifiers have to some extent
alleviated the problem of antenna deficiencies, the need to improve
the antennas so as to develop radiation patterns without signal
lobes of varying strength has nevertheless been recognized as the
fundamental problem. More particularly, an antenna should be
characterized by an isotropic antenna radiation coverage, that is,
a pattern of constant relative power for any orientation of the
antenna. As such, the pattern coverage is then relatively constant
regardless of the roll or aspect orientation of the rocket thereby
facilitating data monitoring at a tracking station. The avoidance
of signal nulls as a characteristic of the antenna precludes one
important factor that often previously has caused temporary loss of
signal information; and in an unrecovered rocket -- the permanent
loss.
It has most always been recognized that an antenna on supersonic
vehicles such as rockets must satisfy harsh aerodynamic design
requirements. For example, the antenna should not substantially
disrupt the mechanical structure of the rocket; otherwise, its
ruggedness is decreased. Further, the antenna design must not
substantially increase air drag.
Heretofore, attempts toward solving or obviating the aforementioned
problems have not been rapidly forthcoming due, at least in part,
to other concentrated effort of antenna designs for use as guided
missile type weapons. In such limited use, it usually is not
required that the missile antenna have a constant power level
relative to isotropic radiation but often only that a high
sensitivity threshold exist forwardly of the missile and in the
direction of the target.
Although some previous attempts have also been made to design
antennas having patterns more nearly isotropic for uses in rockets
carrying environmental data sensors and associated instruments for
telemetry purposes and the like, the antennas have been less than
satisfactory in either failing to satisfy the strict and
aforementioned aerodynamic design requirements or in exhibiting
intolerable signal variations in the aspect patterns (the signal
pattern measured about the missile in a plane containing the
missile), and/or in the roll patterns (the signal pattern measured
about the missile in a plane perpendicular to the missile axis).
The aspect and roll radiation patterns in antennas of even
relatively recent design have often fluctuated as much as 30 db
from isotropic radiation.
SUMMARY OF THE INVENTION
The present invention overcomes these and other disadvantages and
provides an antenna having an improved signal radiation
pattern.
The antenna, particularly useful in airborne telemetry vehicles
which assume many orientations relative to any given tracking
station, has a low profile to avoid substantial increase of air
drag and is integrated in the missile so as to maintain the
mechanical strength of the missile body.
A cylindrical conductor is connected as a short circuit at one end
to the skin of the missile and is concentrically positioned
therewith so as to define a cavity. A plurality of coaxial
transmission lines are connected to the cylindrical conductor and
missile at a distance from the shorted end of substantially
one-quarter wavelength of the radio frequency signal to be
propagated. In effect, a "fat" asymmetrical dipole (a dipole which
in cross section is large in physical size in terms of the
wavelength of the signal) is formed by the cylindrical conductor
and the skin of the missile, which dipole develops substantially
uniform aspect and roll radiation patterns.
It is accordingly an object of the invention to provide a novel
antenna having an improved signal radiation pattern.
It is another object of the present invention to provide an
improved antenna integrated in a missile.
Another object of the invention is to provide an antenna utilizing
the missile skin as a multi-wavelength asymmetric dipole.
A further object of the invention is to provide an antenna
integrated in a missile to form an asymmetric dipole without
appreciably disrupting the continuity of the missile surface and
support structure or appreciably increasing weight and/or air drag
on the missile.
Another object of the invention is to provide an antenna having a
reduced radiation pattern variation about the roll axis.
Another object of the invention is to provide an antenna having a
reduced radiation pattern variation about the aspect axis.
A further object of the invention is to provide an antenna
integrated in a rocket having a circumference substantially greater
than the wavelength of signals to be transmitted and/or received by
the antenna wherein an asymmetric dipole is formed by one portion
of the missile body, and a cylindrical conductor and another
portion of the missile body, the conductor being connected at one
end to the missile body and concentrically extending therewith.
These and other objects and advantages will be apparent to those
skilled in the art from the following description of a preferred
embodiment of the invention as shown in the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a missile having the antenna of
this invention thereon;
FIG. 2 is a broken-away partial side view of the missile and
antenna shown in FIG. 1, but greatly enlarged with respect
thereto;
FIG. 3 is a diagrammatic illustration of electromagnetic energy
radiation from the antenna integrated in the missile as shown in
FIGS. 1 and 2; and
FIG. 4 is a graphic representation showing an experimental antenna
gain radiation pattern utilizing the outer surface of a missile as
a "fat" asymmetric dipole.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to the embodiment of the invention as illustrated in FIG.
1, an antenna 10 is shown integrated in a missile 12 having a
metallic outer cylindrical body, or skin, 14 and a nose portion
15.
As best shown in FIG. 2, antenna 10 includes a cylindrical
conductor 16, preferably of brass, which is concentrically
positioned with respect to the missile and spaced outwardly
therefrom by a layer of dielectric material 18 characterized by low
electrical loss, for example, polytetrafluoro ethylene
(commercially available Teflon).
The cylindrical conductor 16 is curved inwardly toward the forward
portion of the missile to be received in an annular notch 20 of a
conductor 22. Conductor 22 is a ring concentrically mounted about
the missile and is secured thereto by a plurality of spaced screws,
such as screw 23. As shown in FIG. 2, conductor 22 is substantially
triangular in cross-section with notch 20 being cut at the edge of
one leg so that a continuous smooth outer surface is presented
along the entire missile protuberance formed by conductors 16 and
22, to thus minimize the effects of air drag in the widening
transition area.
The cylindrical conductor 16, conductor 22, and the forward portion
of the missile 10 form one element of a fat asymmetric dipole (as
explained hereinafter) with the aft portion of the vehicle body 14
as the other element. Conductor 16 also forms, along with a portion
of the missile body 14, an annular cavity 24 which contains
dielectric material 18. As hereinafter described, the axial length
of cavity 24, as corrected for the impedance producing effect of
the dielectric layer 18 therein, is approximately one-fourth of the
carrier signal wavelength at the anticipated frequency of
operation. The cavity may be excited by signal energy produced at a
source 25 within the missile 12 and which may be of conventional
type; for example, a unit having an appropriate power source and a
radio frequency oscillator modulated in accordance with data
signals from external environmental sensor devices.
The signal source 25 is connected by a coaxial line 26 to an N-way
power divider 28 wherein N represents the numerical number of
output legs each of which is connected to a different one of a
plurality of coaxial lines, such as lines 30 and 32, as shown in
FIG. 2. As is known in the art, such dividers may be employed over
wide frequency bands to separate input R. F. signals into two or
more equal phase and amplitude output signals and to convey the
signals to the individual output legs with very little power loss.
A wide range of dividers of this type are commercially available
such as from the Microlab/FXR Co., 10 Microlab Road, Livingston,
New Jersey. The type of divider depends upon the particular power
requirements, frequency of operation, and the like. At a popular
telemetry transmission frequency of 2.2 GHz on a rocket
approximately 15 inches in diameter, for example, it would be
preferable to excite the cavity 24 at 14 or more "feed" positions
as more fully hereinafter described; therefore, the signal from
source 25 would be separated by divider 28 into a corresponding
number of equal phase and amplitude signal components.
The equal phase and amplitude signals are transferred from the
output legs of the divider 28 by respective coaxial lines each
having inner conductors such as conductors 34 and 36 and outer
conductors, such as conductors 35 and 37, Coaxial line 30 and
connections therewith are representative of the other coaxial lines
and respective connections. Near the termination of line 30, a ring
40 is fixed to the line. An internally threaded female connector 42
having a shoulder 43 is received over ring 40 and is connected to a
mounting jack 44 by means of external mating threads on the
mounting jack with shoulder 43 engaging ring 40 to hold the line in
position. As shown broken away in FIG. 2, inner conductor 34
terminates in a hollow sleeve portion 45 which extends into
mounting jack 44 when line 30 is held in position by connector 42
on jack 44.
The mounting jack is also of conventional type and includes an
integral rectangular flange 46 having holes to receive screws 48
for attachment of the jack to the missile body 14. Jack 44 has a
center conductor 50 one end of which is received into and is
tightly engaged with sleeve 45 when the coaxial line is held
positioned by connector 42 in jack 44. Body portion 52 of the jack
is formed by conductive material and has a recess 53 therein to
receive the outer conductor 35 of line 30 to thereby form an
electrical connection therewith when the line is positioned in the
jack by connector 42. Dielectric material 54, which is preferably
of the same material as the dielectric material in the coaxial line
30 fills the space between the center conductor 50 and jack body
portion 52.
As shown in FIG. 2, jack 44 also has an annular neck 55 which
extends from flange 46 through a hole provided in the missile body.
Center conductor 50 extends through neck 55 and dielectric material
18 to cylindrical conductor 16. Preferably, the cylindrical
conductor is provided with a hole 56 slightly larger than conductor
50 so as to permit conductor 50 to be received and fastened
therein, as for example, by soldering from a position outside the
antenna 10 in order that the dielectric layer 18 is not
disturbed.
In operation, it may be readily appreciated that the radio
frequency signal energy is transferred to the cavity 24 effectively
by a plurality of continuous coaxial lines from the power divider
28 with the cavity excited in the TEM mode. The cylindrical
conductor 16 and the portion of the missile body 14 forming cavity
24 has an impedance characteristic of effectively a one-quarter
wavelength coaxial transmission line which is short-circuited at
the end opposite the feed positions by conductor 22.
As is well known, the impedance produced by a cavity of a given
length with air as the dielectric in the cavity is different from
the effective impedance presented by the cavity when a dielectric
material other than air is contained in the cavity. The relation of
effective wavelength as a function of actual wavelength is:
.lambda..sub.c = .lambda./.sqroot. .epsilon..sub.R
wherein .lambda..sub.c is the corrected wavelength, .lambda. is the
actual wavelength in air at the frequency of operation and
.epsilon..sub.R is the dielectric constant, relative to a vacuum,
of the material in the cavity.
Thus, for operation at a carrier frequency of 2.2 GHz and utilizing
a dielectric such as polymerized tetrafluoro ethylene, as in the
aforenoted example, .lambda. is approximately 5.4 inches and
.epsilon..sub.R is approximately 2.5 inches. Solving the equation
according to the above yields:
.lambda..sub.c = 5.4 inches/.sqroot.2.5
= 3.4 inches
Therefore, the effective one-quarter wavelength cavity for the
exemplary parameters stated is slightly less than 1 inch in length
when polymerized tetrafluoro ethylene is the dielectric material in
the cavity. It may readily be appreciated that at the given carrier
frequency and for the rocket diameter of the example (i.e., 2.2 GHz
and 15 inches, respectively), the circumference of the rocket is
much greater than .lambda..sub.c ; namely, approximately 14 times
greater.
It has been realized that the most favorable radiation patterns are
developed when the cavity 24 is excited in the TEM mode with
signals of uniform phase and amplitude provided at feed positions
which are separated about the periphery of the cylindrical
conductor 16 at distances corresponding to substantially one
wavelength as corrected for the dielectric material 18 in the
cavity. Therefore, in the example, since the rocket 12 is
approximately 14 effective wavelengths in circumference, a 14-way
power divider 28, or combination of power dividers to obtain 14
equal phase and amplitude outputs, as already suggested, would be
utilized and 14 inner conductors, such as inner conductor 50 of
respective jacks, such as jack 44, would be connected to the
cylindrical conductor 16 at positions equally spaced one from the
other about the periphery of the body portion 14.
Since the corrected wavelength (.lambda..sub.c) is much less than
the circumference of the rocket 12, the cylindrical conductor 16
and the rocket body 14 form a fat asymmetric dipole. If the rocket
had a small circumference relative to the corrected wavelength,
deep nulls would exist in the aspect radiation pattern; that is, a
plurality of lobes would appear in the pattern with deep nulls
therebetween. Since this undesirable condition is equivalent to
exciting the cavity 24 with signal energy at a lower frequency, it
has been found that the antenna of the present invention operates
better as frequency is increased, instead of worse as is the case
in utilizing discrete radiators.
It should be apparent that the short circuit at the forward end of
the cavity presents a negligible impedance which effectively
transforms back as an open circuit at the feed positions; namely,
at the positions of the conductors such as inner conductor 50 which
conductors are each connected to the cylindrical conductor 16 at
peripheral position substantially equal to .lambda..sub.c /4
rearwardly from conductor 22. The apparent open circuit is
effectively in parallel with the radiation impedance of the fat
asymmetrical dipole. Since the antenna 10 is large in diameter in
terms of wavelength, the impedance across the asymmetrical dipole
is real and divides between the feed positions.
FIG. 3 shows a broken-away, reduced portion of the antenna 10 of
FIG. 1 integrated in rocket 12 wherein the dotted lines between the
cylindrical conductor 16 and the aft portion of the missile body 14
illustrate the signal energy emanating from the antenna 10 and more
particularly the approximate direction of the electric field
existing between the elements of the dipole at a particular instant
in time. The direction of the field is reversed (not shown) at
every half wavelength from the rearward end of the cavity 24 in
both the fore and aft directions since the polarity of cylindrical
conductor 16 relative to the aft portion of body 14 is constantly
changing in accordance with the frequency of the oscillatory
signal.
Referring to FIG. 4, the aspect radiation pattern developed by an
antenna utilizing the outer surface of a missile as a fat
asymmetric dipole is shown wherein the axis of the missile was
positioned substantially in the plane containing the pattern. The
pattern, representing the antenna gain relative to linear isotropic
radiation, is substantially representative within 1 db of the
infinite number of aspect radiation patterns which may be utilized
to define a figure of revolution about the missile axis. More
particularly, the radiation pattern of FIG. 4 may be substantially
represented by the pattern obtained in any plane defining a cross
section through the figure of revolution produced by all aspect
patterns and containing the axis of the rocket.
It may readily be appreciated that deep nulls exist only directly
forwardly (0.degree.) and rearwardly (180.degree.) from the tip and
tail of the rocket, respectively. As already noted, tip and tail
pattern nulls in a telemetry rocket are usually of little concern.
In the pattern shown, the area subtended due to the tip and tail
nulls is less than one-hundredth of one percent of a total
spherical surface enclosure about the aforementioned figure of
revolution.
As can also be seen from FIG. 4, the remainder of the signal
pattern displays an average strength variation between signal peaks
and nulls in the aspect plane of less than 5 db. Thus, the improved
antenna is highly favorable for the receipt or transmission of
electromagnetic signal energy to or from the missile without any
appreciable average loss in signal due to vehicle orientation.
Although only one embodiment of the invention has been shown and
described, various modifications as may appear to those skilled in
the art are intended to be within the contemplation of the
invention as defined in scope by the claims.
* * * * *