US3553705A

US3553705A - Parabolic reflector antenna

Info

Publication number: US3553705A
Application number: US751977A
Authority: US
Inventors: Albert A Ondrejka
Original assignee: Individual
Current assignee: Individual
Priority date: 1968-08-12
Filing date: 1968-08-12
Publication date: 1971-01-05
Anticipated expiration: 1988-01-05

Abstract

A PARABOLIC REFLECTOR ANTENNA HAVING A FEED SYSTEM WHICH IS AN INTEGRAL PART OF THE REFLECTOR CONTOUR WITH THE FOCAL LINES OR FOCAL CIRCLES BEING COINCIDENT WITH THE LOCUS OF POINTS THAT COMPRISE THE REFLECTOR CONTOUR.

Description

Jan. 5, 1971 A. A. ONDREJKA` '3,553,705

` yARABoLIC REFLEToR ANTENNA Filed Aug. 12,`196S 5 Sheets-Sheet. 1

k kwak M7 @A @g55/AZ.

Jan. 5,'1971 A. A. ONDREJKA 3,553,705

" PARABOLIC REFLECTOR ANTENNA Filed Aug. l2, 1968 v 5 Sheets-SheetI 4.

,wafer/4. mmm/w ram/Erf' Jan. 5, 1971 'N N yA.A oNnREJKA 3,553,705

` VPARABoLIc REFLECTOR ANTENNA Filed Aug.r 12, `1968 'xsneets-sheet 5 wlw/pferd@ United States Patent O Int. Cl. H01q 19/12 U.S. Cl. 343-771 5 Claims ABSTRACT OF THE DISCLOSURE A parabolic reflector antenna having a feed system whichis an integral part of the reflector contour with the focal lines or focal circles being coincident with the locus .of points that comprise the reflector contour.

BACKGROUND OF THE INVENTION :the intersection of two identical parabolic curves so that the focal points of the parabolic curves are coincident with the locus of points that compose the antenna crosssection. This antenna reflector cross-section is a parabola which isa unique configuration different from the ordinary parabolas with external focal points. This parabola can be projected or expanded into a cylindrical type revolution by rotation.

A type ofv communications or radar antenna used by the military and commercial agencies is known as the lparabolic reflector antenna. This type may be divided into two basic classes: `the parabolic cylindrical and the paraboloid of revolution. These antennas with their parabolic 4cross-sections have the property that electromagnetic rays lradiating from the feed, located at the focal point, are reflected from the antenna and are transformed into a `plane wave.

The present parabolic cylindrical antennas and paraboloids of revolution antennas use an external feed usually lllocated at lthe focal line or focal point. A parabolic cylindrical antenna uses what is called a straight line source feed consisting of a series of dipoles or a slotted waveguide located on the focal line of the parabolic reflector. Simple flared beam patterns with good directivity are obtained. The paraboloidal reflector antenna uses a ,point source fixed at the focal point so that the reflector takes the divergent rays from the point source and converts them into a beam of parallel rays. These rays producea highly directive pattern known as a pencil .fbeam. l

Since the present parabolic reflector antennas use external feeds that have to ybe located at an external focal point or focal line, they must be supported by some struc- ,tural means, such as struts, braces, or in some cases by Vthe inherent structural strength of the waveguide. Being l in front4 ofthe antenna reflector, the feed system consisting of dipole or horn, plus waveguide and support structures such as struts, or braces cause aperture blockage vwhich reduces antenna efficiency. The wind blowing lon the feed system may cause distortion of antenna reflector contour, or shift the feed from the focal plane Acausing large sidelobes and perturbations in the phase ofthe aperture distribution. The focal point fed parab- -oloid when'the pointed at high elevation angles, has a high level spillover of energy which illuminates a hot earth and results in an unusually large antenna noise contribution. Spillover around the paraboloid causes an increase in antenna by translation, or into a special paraboloid of 3,553,705 Patented Jan. 5, 1971 rice back lobe intensity to the detriment of the main lobe intensity. If the feed support structure sags due to gravity or ice, beam pointing errors will occur. Thus, the invention is superior to the present designs in that it will eliminate or greatly reduce the weaknesses of the existing parabolic antennas.

SUMMARY OF THE INVENTION In accordance with the invention, a unique parabolic reflector antenna is provided that has a feed system which is an integral part of the reflector contour. The derived equations of special parabolic curves enable the focal lines or focal circles to be coincident with the locus of points that comprise the reflector contour. Since the focal lines or focal circle are an integral part of the reflector contour, there can be obtained :a cylindirical parabolic reflector antenna or a paraboloidl of revolution reflector antenna with no aperture blockage, increased efficiency, decreased spillover energy, and reduced side and back lobes.

The concept of having a parabolic reflector antenna with a focal line or focal circle that is an integral part of the reflector contour is a prime feature of this invention. This enables the feed system to be an integral part of the reflector contour, thus leaving a free unrestricted aperture for the reflection or impingement of electromagnetic energy. Therefore, all projections, outriggers, supports, feed horns, and waveguides that block the conventional parabolic reflector apertures are eliminated.

It is an object of this invention to provide a parabolic reflector antenna having a feed system which is an integral part of the reflector contour.

Another object of this invention is to provide a parabolic reflector antenna whose cross-section is created from the intersection of two identical parabolic curves so that the focal points of the parabolic curves are coincident with the locus of points that compose the antenna crosssection.

Still another object of this present invention is to provide a parabolic cylindrical reflector antenna whose antenna efliciency is increased by an unblocked aperture since the source feed and focal line are an integral part of the antenna reflector contour.

Yet another object of the invention is to provide a parabolic reflector antenna with reduced side lobes and back lobes since the source feed system is an integral part of the` antenna reflector and thus any reflections or scattering of rays resulting fromrimpingement on the feedrsystem are held to a minimum.` l

A further object of the invention is to provide a parabolic reflector antenna with reduced energy spillover since the feed system is inset in the antenna contour most of the energy is directed towards the reflector, thus reducing noise and blacklobe intensity.

A still further object of the invention is topprovide a parabolic reflector antenna with reduced reflector surface deviation and distortion since wind loads and ice loads will be reduced as the feed system is not exposed. The integral feed could serve as a lstructural member, actually stiffening and strengthening the reflector. The effects of gravity will be greatly nullifled, and defocusing will not -be a problem as in the standard parabolic antennas.

Still yet another object of Vthis invention is to provide a parabolic reflector antenna to obtain greater gain `and reduced side lobes and controlled directivity of beam patterns since in the paraboloidal antenna extremely sharp pencil beam patterns can be obtained.

Another object of this invention is to provide a para bolic reflector antenna design that can be built in different sizes dependent on the frequency desired.

These and other objects, features, and advantages, will become more apparent from the following description taken in conjunction with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1A is a view of the two identical intersecting parabolic curves, with parallel axes, and the creation of the unique parabola curve;

FIG. 1B shows pattern of rays in relationship to focal points;

FIG. 1C is a view of the two identical intersecting parabolic curves with perpendicular axes producing the unique parabolic curve;

FIG. 1D shows pattern of rays in relation to focal points;

FIG. 2 is a wiew of a parabolic cylindrical reflector antenna, with a series of dipoles in a straight line feed system;

FIG. 2A is a partial view of an inset slotted waveguide feed system;

FIG. 2B is a partial cross-sectional view of antenna reflector and an inset waveguide with another alternate feed system utilizing waveguide horns projecting from the waveguide;

FIG. 3 is a cross-sectional view of a paraboloidal antenna;

FIG. 3A is a front view of a paraboloidal antenna;

FIG. 3B is a partial view of the feed system for FIG. 3; and

FIG. 3C is a back view of a paraboloidal antenna.

Referring now to the FIGS. 1A-1C showing the essence of the invention, the special parabolic curve resulting from the intersection of two parabolas.

In FIG. 1A the parabolas labeled as 11 and 12 are plotted on two different rectangular coordinate systems so that the parabola axes are parallel, and the ordinate separation is 2a; so that the focal point of parabola 12 falls on parabola 11 and the focal point of parabola 11 falls on parabola 12. The letter a is identified as the focal distance from the vertex of the parabola, and also the distance from the vertex of the parabola to the directrix measured on the parabola axis. It is to be noted that the x and x1 axes are the axes of the parabolas. The y and y1 axes are coincident. The equation for a parabola with its axis coincident with the x axis is where a is the focal distance from the vertex. We are concerned with the special parabolic curve (f"f2) resulting from the intersection of the upper arm or limb of parabola 11 and the lower arm or limb of parabola 12. Since the two coordinated systems are 2a apart on the ordinate axis, the equation for the upper limb of parabola 11 is To find the coordinates of the vertex of this special parabola, solve for x by eliminating y..

Therefore, the coordinates of the vertex of the parabola are x=a/ 4, y=a in the x, y coordinate system.

Using the x, y coordinate system, we can plot the upper limb of the parabola using the equation y=\/4ax, and the equation y=-\/4ax+2a for the lower limb. It is to be noted that in the x, y coordinate system that the coordinates of one focal point (f1) are a, O, and the coordinates of the other focal point (f2) are a, 2a.

On the other hand, if we desire to have one equation for the parabola by incorporating a new coordinate system x", y so that the x axis coincides with the axis of the parabola and the y axis goes through the vertex 0, we come up with the equation 5": i wm'- a] and thus the coordinates of focal point 1 (f1) are (a-a/ 4), -a and the coordinates for focal point 2 are (fa-le) In FIG. 1B, there is shown the parabola taken from FIG. 1A. It shows that the axes of parabola 11 and parabola 12 are parallel to each other and they are spaced a distance 2a apart. Note that the upper limb of parabola 11 and the lower limb of parabola 12 are mirror images of each other. The axis of the parabola is equidistant from identical mirror points of each limb. Note the way the parallel rays reflect from the limbs of the parabola and cross the parabola axis and impinge on the focal points, when the rays are approaching the parabola. Rays emitted from each focal point cross the axis and are reflected from the parabola into parallel rays. It must be remembered that in a cylindrical reflector antenna that there are two focal lines, coplanar, and located equidistant a on each side of the axis. This cylindrical reflector antenna configuration is obtained by translating the parabola in a direction perpendicular to the plane of the parabola so that the axis becomes a plane.

To obtain a paraboloid of revolution, rotate the parabola around the axis as per (Theorem of Pappius or Guldinius). The focal points of the parabola are then a part of a focal circle. The rays produce a highly directive pattern known as a pencil beam. v

FIG. 1C shows another form of the parabola where the intersecting parabolas have their axes perpendicular to each other. The basic equation used to derive the parabola is .x1/2 -iy`1/2=l1`1/2 where b is a constant, which gives you a parabola whose axis is inclined 45 to the x, y axis, and where the focal point coordinates are (Ia/2, b/2). Y

This is shown as parabola 11. Its mirror image, Whose equation is y=2\/I2x, is moved down a distance bV-b, so that the focal point of parabola 12 falls on parabola 11, and the focal point of parabola 11 falls on parabola 12. By transferring the coordinate axes so that the origin is at the vertex of the parabola, we have the axis (x) going through the vertex at a distance c from. the x axis. The y axis is at a distance D from the y axis. The equation for this parabola is where hngt/Mazda b==a constant In FIG. 1D there is shown the parabola taken from FIG. 1C. It shows that the axis of parabola 11 and parabola 12 are perpendicular to each other so that the axis is the bisector of the angle between the axes. Note the way the rays emitting from the focal points are reflected from opposite limbs of the parabola and become parallel. These parallel rays combine and become beams. Note that the beams are perpendicular to each other.

` It must be remembered that in a cylindrical reflector antenna that there are two focal lines, coplanar, and located equidistant from the axis. This cylindrical reflector antenna configuration is obtained by translating the parabola in a direction perpendicular to the plane of the parabola so that the axis becomes a plane. The pattern of this antenna will be two flared directive beams perpendicular to each other.

To obtain a paraboloid of revolution, rotate the parabola around the axis as per Theorem of Pappius or Guldinius. The focal points of the parabola are then a part of a focal circle. The rays produce a hollow cone beam.

FIG. 2 is a view of a parabolic cylindrical reflector antenna, and since its cross-section is a parabola, may be referred to as a parabolic cylindrical reflector antenna. Waveguide is set below the surface of antenna contour 21 so that the series of dipoles 22 are on the focal lines 23 of the antenna. Parasitic dipoles 24 shape and direct the rays as they are emitted or impinge on the dipoles 22. Plastic strip 25 is inset into the antenna 26 so that the parabolic contour is smooth and it protects the dipoles from Wind and ice loads. The strip has cutouts that allow the dipoles to come through so that the top of the strip 25 and the tops of the

dipoles

22 and 24 are in the same curved plane of the parabolic surface. Structural framework 27 is used to hold the reflector antenna on a pedestal not shown.

FIG. 2A shows a partial view of another feed system for the cylindrical parabolic reflector antenna. Waveguide 30 includes slots 31 that are cut through one surface of the waveguide 30. Length of slots, spacing, and angularity depend on the frequency band that the antenna is designed for. Plastic strip 34 is bonded to the waveguide 30 and keeps out the dirt and allows pressurization of waveguide. Plastic may be a transparent type, or may be made of reinforced plastic such as fiberglass. Waveguide 30 is a straight run without any joints, except for flanges at its extremities. Waveguide 30 is inset into the antenna reflector 36 that is properly rigidized by braces, channels, and angles not shown. Right angle flanged joint 35 fastened to the end of waveguide 30 on an end, and may have a flexible waveguide fastened on the other end which leads to the associated transmitter and receiver.

FIG. 2B shows a partial cutaway view of the side of reflector 36 of FIG. 2A with another type of feed system. This feed system consists of a waveguide 40 that has a series of small horns 41 fastened through one side of waveguide 40 so that energy can be transmitted through these horns into or out of the waveguide. The dimensions of the horns and spacing depend on the antenna size, and frequency that the antenna is designed to operate at. Inset plastic strip 44 serves to protect the horns and `waveguide from the weather. Structural members 42 are angles or channels that support waveguide 40 and rigidize antenna 46.

Other types of feed systems used in present antenna designs may be adapted to work with the parabolic cylindrical reflector antenna. It will be apparent to one skilled in the art that various changes, alterations, modifications and substitutions can be made in the arrangement and location of the various elements without departing from the true spirit and scope of the invention.

FIG. 3 shows a cross-section of another paraboloid of revolution reflector antenna. Antenna 56 is shown lwith its structural frame 57 that is fastened to a pedestal not shown. Focal circle 58 locates the feed system. This feed system can be any of the types shown in FIGS. 2, 2A, and 2B, except that the waveguide is formed into a circular configuration (annular ring) as shown in FIG. 3C. Right angle waveguide joint 59 is the junction for all of the waveguide leading from the associated transmitter and receiver.

FIG. 3A is the front view of the paraboloid of revolution reflector antenna of FIG. 3, showing the circular configuration (annular ring) of the plastic protective strip 54 and the focal circle 58.

FIG. 3B shows a closeup of a partial View of the feed system. 50 is the waveguide, 52 is one of the dipoles inserted into waveguide 50, and 52A is one of the parasitic dipoles that shape and direct the emission or impingement of rays to or from the paraboloid of revolution. Plastic protective annular strip 54 protects the dipoles from the weather. 57 are the angles or channels that secure waveguide 50 and rigidize.` the antenna.

FIG. 3C is the backside of the paraboloid of revolution reflective antenna. Waveguide 60 is formed into an annular ring so that it follows the path of the focal circle. Dipole antennas 62 are inserted into the side of waveguide fitting 69 connects the circular arm of waveguide dipoles are located on the hidden side of waveguide 60, they are indicated in phantom lines. Right angle waveguide fitting 69 connects the circular arm of waveguide 60 in one plane on one end, and joins flexible waveguide 70 in another plane on the other end. The rest of the waveguide plumbing lea-ding to the associated transmitter and receiver (that are not shown) is connected to the other end of the flexible waveguide. The spacing, shape, and size of the dipoles depend on the frequency desired. Flanges 7l are bolted or clamped together to make the annular shape for the large antennas. On small antennas these flange joints might be eliminated and the annular shape of the waveguide may be obtained by electro-forming it in one piece. Special termination fitting 72 is closed at one end by plate 73 and that is filled with material 74 that absorbs electromagnetic energy, and thus eliminates disturbing internal reflections. If special shaped beams are desired, or if multiple phased. beams are desired, the annular waveguide could be divided into sectors with individual feeds that could have various polarizations, be fed in phase, sequence, or in a cyclic manner. It is to be noted that emphasis has been placed on the two special cases for the formation of the special parabola, first by the intersection of two parabolas with axes parallel to each other, and the second case of the parabolas with axes perpendicular to each other forming the parabola. The special parabola can be formed where the parabola axes are inclined at different angles to each other, as long as the focal points of each parabolic curve fall on the opposite parabolic curve. From the foregoing, it will be seen that the invention has been presented with particular emphasis on certain preferred embodiments. It will be apparent to one skilled in the art that various changes, alterations, modifications, and substitutions can be made in the arrangement and location of the various elements without departing from the true spirit and scope of the invention as defined in the claims.

What is claimed is:

1. A parabolic reflector antenna comprising a p-arabolic reflector formed by intersecting the upper limb of a first parabola and the lower limb of a second parabola so that the focal point of said second parabola falls on said upper limb ofsaid first parabola and the focal point y,of Said first parabola falls on said lower limb of said second parabola with the axes of said flrst and second parabolas being parallel and separated a preselected distance, said eXas being focal lines for said parabolic rellector, and electromagnetic feed means positioned at said focal lines and being an integral part of the reflecting surface of said parabolic reflector.

2. A parabolic reflector antenna as described in claim 1 wherein said electromagnetic feed means is comprised of a slotted waveguide for each of said focal lines.

3. A parabolic reflector antenna comprising a parabolic reflector whose contour is formed by intersecting the limb of a rst parabola with the limb of a, second parabola so that the focal point of said second parabola falls on said limb of said first parabola and the focal point of said first parabola falls on said limb of said second parabola with the axes of said first and second parabolas being perpendicular to each other to form focal lines, and electromagnetic feed means located on said focal lines and being an integral part of the reflecting surface of said parabolic reflector.

4, A paraboloid of revolution reflector antenna comprising a paraboloid of revolution reflector whose contour is formed by intersecting the limb of a first parabola and the limb of a second parabola so that the focal point of said second parabola falls on said limb of said rst parabola and the focal point of said first parabola falls on the said limb of said second parabola with the axes of said first and second parabolas being parallel to each other and separated a preselected distance from each other to provide a resultant Vparabola and then rotating the axis of the resultant parabola to provide a focal circle for said paraboloid of revolution reflector, and electromagnetic feed means positioned at said focal circle and being an integral part of the reflecting surface of said paraboloid of revolution reflector.

5. A paraboloid of revolution reflector antenna as described in claim 4 wherein said electromagnetic feed means is comprised of a slotted waveguide curved into a circle.

References Cited UNITED STATES PATENTS 1,341,674 6/1920 Rhodin 24U-41,37 1,739,800 12/1929 Patten et al. 240-41.35 2,160,853 6/1939 Gerhard et al. 343-840 2,471,284- 5/1949 Rea 343-840 3,039,098 6/1962 Bickmore 343-771 3,365,720 1/1968 Kelleher 343-914 FOREIGN PATENTS 612,756 11/1948 Great Britain 24U-41.37

ELI LIEBERMAN, Primary Examiner I U.S. Cl. X.R.