U.S. patent number 3,611,391 [Application Number 05/023,143] was granted by the patent office on 1971-10-05 for cassegrain antenna with dielectric guiding structure.
This patent grant is currently assigned to The United States of America as represented by the Secretary of the Army. Invention is credited to Homer Eugene Bartlett.
United States Patent |
3,611,391 |
Bartlett |
October 5, 1971 |
CASSEGRAIN ANTENNA WITH DIELECTRIC GUIDING STRUCTURE
Abstract
An end-fire antenna having a thick-wall dielectric tube
concentrically poioned about the principal axis of a cassegrain
antenna and seated on the main reflector functions as a guiding
structure for focusing the antenna radiation.
Inventors: |
Bartlett; Homer Eugene
(Melbourne, FL) |
Assignee: |
The United States of America as
represented by the Secretary of the Army (N/A)
|
Family
ID: |
21813356 |
Appl.
No.: |
05/023,143 |
Filed: |
March 27, 1970 |
Current U.S.
Class: |
343/755; 343/785;
343/781R |
Current CPC
Class: |
H01Q
13/24 (20130101); H01Q 19/193 (20130101); H01Q
19/10 (20130101) |
Current International
Class: |
H01Q
19/10 (20060101); H01Q 13/24 (20060101); H01Q
13/20 (20060101); H01Q 19/19 (20060101); H01g
019/10 () |
Field of
Search: |
;343/753,574,755,785,840,781 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Lieberman; Eli
Claims
It should be emphasized that the above disclosure was meant to be
exemplary of the possible applications of this invention and not in
any way, a limitation thereof. Various modifications and changes in
the specific details of construction and operation described may be
resorted to without departing from the spirit and scope of the
invention as defined by the appended claims.
1. A tapered-wall, dielectric tube guiding structure mounted on the
main reflector of a cassegrain antenna wherein the feed and
subreflector of said antenna extends into the tapered-wall tube
effectively providing an end-fire antenna such that energy from the
feed is effectively guided along and contained between the walls of
the tube and is directed along the axial length of the guiding
structure to produce at the opposite end of the tube a directive
electromagnetic signal for transmission through free space, said
tapered-wall dielectric tube has an inside taper with the
cross-sectional area progressively increasing along the axial
length of the tube from the end of the guide seated relative to the
feed toward the opposite end of the tube from which the radiating
signals emanate into free space.
2. The guiding structure as set forth in claim 1, wherein said
thick-wall dielectric tube is fabricated from a dielectric material
having a dielectric constant greater than one.
3. The guiding structure as set forth in claim 2, wherein the
material chosen for fabrication of the tube is selected from the
group of materials comprising, Rexolite, polypropylene,
polystyrene; Plexiglas and Styrofoam.
4. The guiding structure as set forth in claim 1, wherein the
outside diameter of the tube is determined by applying the
formula
where .lambda. is the desired wavelength expressed in inches and
.epsilon. is the dielectric constant of the material from which the
tube is fabricated, the thickness of the wall is approximately one
third the outside diameter of the tube and the length of the tube
is approximately equal to d.sup.2 /9.lambda., where d is the
outside diameter of the tube in inches and .lambda. is the desired
wavelength expressed in inches.
5. The guiding structure as set forth in claim 4, wherein the
inside taper of said dielectric tube is determined in accordance
with the formula 0.66(90.degree. -.theta..sub.cr) where
.theta..sub.cr defines the critical angle of the wall boundary and
is the angle of incidence of the electromagnetic waves generated by
the launcher on said boundary, above which total reflection of the
incident wave occurs.
Description
BACKGROUND OF THE INVENTION
This invention relates to directional antennas and more
specifically to end-fire antennas of the thick-wall dielectric-tube
type.
End-fire antennas are well known in the art and encompass the class
of antennas having a principal axis and so designed and excited
that the maximum intensity of radiation is directed along the
principal axis. Typical antennas within the end-fire classification
are the dielectric rod, disc on rod, yagi, and ferrite rod
antennas.
A considerable amount of time and effort has been expended
heretofore in attempts to develop an end-fire antenna which would
be more easily transported and would provide a specified required
gain without having to contend with the excessive length of the rod
or the excessive size of the reflecting dish. Prior attempts to
effect such an antenna had met with little success until the
present invention overcame the obstacles and made feasible a highly
transportable antenna array utilizing thick wall, end-fire,
dielectric-tube antennas.
SUMMARY OF THE INVENTION
One of the primary purposes of this invention lies in the provision
for a more easily transportable satellite communications antenna
which is lightweight, compact and relatively inexpensive.
The present invention successfully accomplishes the above purpose
while maintaining all the advantages of the prior art without the
attendant disadvantages thereof. The improved results are
accomplished by utilizing a thick wall dielectric tube as a guiding
structure for a cassegrain antenna.
BRIEF DESCRIPTION OF DRAWINGS
The exact nature of this invention will be readily apparent from
consideration of the following specification relating to the
annexed drawings in which:
FIG. 1 illustrates the geometry of the tapered wall tube;
FIG. 2 shows a cassegrain antenna used as the launcher for the
end-fire antenna element;
FIG. 3 shows a thick-wall tube end-fire element with launcher;
and,
FIG. 4 shows a tapered-wall tube end-fire element with
launcher.
DESCRIPTION OF INVENTION
The thick-wall dielectric-tube end-fire antenna evolved from
research on cylindrical dielectric rod antennas, such as disclosed
by G. E. Mueller in U.S. Pat. Ser. No. 2,425,336. Generally, higher
gains are obtainable with dielectric rod antennas than with
conventional end-fire antennas and accordingly the first step in
providing the maximum gain of a dielectric rod antenna is to
calculate the optimum length and diameter of the rod.
The calculated diameter limitation for dielectric rods is expressed
by the formula:
where .lambda. designates wavelength and .epsilon. refers to the
dielectric constant of the rod material. The optimum length of the
dielectric rod is:
by the Hansen-Woodyard criterion, a well-known principle of
end-fire antennas.
It has been found that the optimum length of the dielectric rod is
very nearly approximated by the formula L=.lambda./.epsilon.-1
where the dielectric constant is low. Also, by experimental
verification, the maximum diameter of the rod for optimum
performance has been found to be closely approximated by
and therefore for maximum diameter rods the approximate length of
the antenna would be: L .apprxeq.d.sup.2 /.lambda. This formula
provides for a very effective highly transportable dielectric rod
antenna until the required gain results in excessive length for the
rod antenna. The requirement L .apprxeq.d.sup.2 /.lambda. results
in impractical length of cylindrical rods where the diameter of the
rod exceeds 1.5 feet at X-band. It has been found that a thick-wall
tube antenna and in particular the tapered-wall thick-wall tube
antenna has definite size advantages over a dielectric rod of the
same gain. From experimental results, the thick-wall tube appears
to afford considerable length reduction and models with wall
thicknesses of approximately one-third the tube outside diameter
appears to have the limitation:
and since
then L .apprxeq.d.sup.2 /9.lambda.. Thus, a thick-wall tube would
have one-ninth the length of a solid rod of the same diameter. It
has been determined that the thick-wall tube with a tapered inside
wall, actually conically shaped, has the highest effective aperture
of tubes tested. A thick-wall tube of optimum dimensions does not
have as large an effective area as the optimum solid rod, the
maximum being about 2.5 times the cross-sectional area for the
models investigated. Therefore, the thick-wall tube must have a
diameter of 1.3 times that of the rod for equivalent gain, which
results in a length of about 18.5 percent that of the rod.
FIG. 1 shows the end-fire element geometry used to experimentally
optimize the tapered-wall tube. The length, L, diameter, D, inner
dimensions, d.sub.1 and d.sub.2, inner flare angle, .theta. and
dielectric constant, .epsilon., were varied to determine the
optimum tapered-wall tube shape. Several models with various
dielectric constants were investigated and effective apertures as
high as 2.5 were attained.
As an example, the parameters of a tapered-wall tube with a
dielectric constant of 1.08 were optimized by varying .theta. and L
of FIG. 1 over wide ranges to obtain the maximum effective aperture
for a 16-inch diameter, D, tube. The dimension, d.sub.a, was equal
to zero. Angle .theta. was varied from zero degrees (a solid rod)
to approximately 18.degree.. The maximum effective aperture was
found to occur for an angle of 11.degree.. The length was varied
from 21 inches to 31 inches and the highest effective aperture was
observed at a length of 26 inches. The maximum effective aperture
was found to be 2.47 at 7.8 gc. and was broadband. In order to
further optimize the .epsilon.=1.08 tapered-wall tube, the angle
.theta. was varied on the model by varying the dimension d.sub.1
and holding d.sub.2 constant at 2 inches. The best performance was
obtained for an angle of approximately 9.5.degree. where maximum
effective aperture was 2.52, the highest obtained in the tests.
The optimum shape of a tapered-wall tube with dielectric constant
of 1.2 was also determined. The length and diameter were calculated
from the equations earlier presented. In these equations, letting
.lambda.=1.5 and .epsilon.=1.2, then L=8 inches and D=10 inches.
The dimension d.sub.2 was fixed at 1 inch and the angle .theta. was
allowed to vary by changing the dimension d.sub.1. The results
indicate optimum performance at an angle of 16.degree. compared to
9.5.degree. for the model with dielectric constant of 1.08.
An analysis of the optimum shapes of the .epsilon.=1.08 and
.epsilon.=1.2 models indicate that the design relations given by
the formulae earlier set forth are indeed valid. Also, when
compared to the complement of the critical angle in the case of
both .epsilon.=1.08 and .epsilon.=1.2 is approximately as follows:
.theta..sub.opt =0.66 (90-.theta..sub.cr).
In a transportable tapered-wall tube model, the end-fire elements
could be fabricated of spaced discs of high dielectric constant
material such as Rexolite, polypropylene, polystyrene, Plexiglas,
Styrofoam or any of several other dielectric materials having a
dielectric constant greater than one.
A launcher was designed to feed the optimized tapered-wall tube and
to match its focal plane pattern. The launcher consisted of a
dielectric guiding structure cassegrain feed and 16 inch
paraboloidal reflector with the end-fire element concentrically
mounted on the secondary reflector about the principal axis thereof
as shown in FIGS. 3 and 4. The particular launching arrangement of
FIG. 2 is disclosed in U.S. Pat. Ser. No. 3,430,244 which issued to
H. E. Bartlett et al. on 25 Feb. 1969. The launcher utilized for
the thick-wall tube tests is represented by the dual reflector
(cassegrain) embodiment shown in FIG. 2. The antenna system
comprises a feed 2, subreflector 4 and a main reflector 1. The feed
2 may, for example, be a horn, placed along the axis of reflectors
1 and 4. The location of the feed will depend almost entirely on
the shape of the subreflector surface. The dielectric guiding
structure 3 is arranged between the mouth of feed 2 and the convex
surface of subreflector 4, to guide substantially all of the energy
radiated by the feed which would otherwise fall outside the
subreflector surface (as ray OAX), toward the subreflector (as ray
OAB). Almost all of the energy is thus directed toward the
subreflector surface, reflected and passes through guide 3 at
angles less than the critical angle (as ray BCD). Upon striking
reflector 1, ray BCD is reflected along DE which is substantially
parallel to the principal axis of reflectors 1 and 4.
As shown in FIGS. 3 and 4, the thick-wall dielectric guiding
structure 5 is seated on main reflector 1 in concentric
relationship with the principal axis in a manner whereby
substantially all the rays emanating from main reflector 1 in the
direction of ray DE is captured by the guiding structure 5 and
focused along the principal axis to form an end-fire directive
antenna. As shown in previous calculations and experiments the use
of the dielectric guiding structure 5 allows one to obtain improved
results while reducing the size of the reflecting dish required and
shows remarkably improved results in comparison to a dielectric rod
antenna to the extent of reducing the length of the thick wall tube
to about one-ninth the length of a solid rod of the same
diameter.
FIGS. 3 and 4 disclose two separate embodiments of the invention in
the context which it was designed to be used. It is fairly obvious
that the exact design or shape of the dielectric tube will vary
according to ones specific need as dictated by the earlier
presented design formulas. It should also be noted here that the
guiding formulas. 5 of FIGS. 3 and 4 need not necessarily be of a
circular configuration as previously described, but may take other
forms and shapes.
The dielectric guiding structure 3 of FIGS. 3 and 4 should be
designed with scatter patterns having peaks approximately
40.degree. off -axis and a hull-on-axis, which is generally the
requirement for a launcher pattern.
In one particular instance a dielectric guiding structure
cassegrain feed was used having a 3.5-inch diameter choke horn with
a having a dielectric constant of 1.5 and an 8-inch diameter
subreflector. The dielectric guiding circuit flare angle was
approximately 14.degree. and the feed was used with a 16-inch
paraboloid main reflector.
* * * * *