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.

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.

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.