Radiating Cone Antenna

Abstract

A radiating cone antenna having a conical reflecting surface and a parabolic reflecting surface in spacial relationship and surrounding said conical reflecting surface. A linear directive radiator is positioned at the focus of the parabolic reflecting surface and energy from the radiator is reflected from the parabolic reflecting surface to the conical reflecting surface which, in turn, reflects the energy in a constant phase front.

Description

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION

The present invention relates to a radio frequency antenna and more particularly to an antenna having first and second reflecting surfaces for transmitting energy in a constant phase front.

Antennas having parabolic reflectors are widely used for directing beams of radio frequency energy. Generally, a beam of energy from a radiator is projected at a small angle to the axis of a parabolic reflector and the radiator is located on the axis at substantially the focal point of the reflector. The radiating system including the parabolic reflector is arranged to be oriented both in elevation and in azimuth by suitable mechanical means so that the axis of the paraboloid can be directed at a target. One such antenna system is shown and described in U.S. Pat. No. 2,541,806, entitled, "Beam Antenna System," which issued Feb. 13, 1951, to Burton P. Brown, Jr. In this patented device, the antenna member is a dipole which is located at or near the focus of a paraboloid reflector and the axis of the reflector passes through the dipole at or near the midpoint thereof. A source of signal energy is provided for exciting the dipole and this source produces pulses of oscillations of high frequency energy. Means is provided for shifting the antenna beam pattern from coincidence with the axis of the reflector and for rotating the antenna assembly so that a rotating beam will be projected.

SUMMARY OF THE INVENTION

The present invention relates to a radio frequency antenna for reflecting energy along a constant phase front. A conical reflecting surface is provided and a parabolic reflecting surface is spaced from and surrounds the conical reflecting surface. A feed mechanism is positioned at the focus of the parabolic reflector and energy from the feed mechanism is reflected from the parabolic reflector to the conical reflector. The conical reflector, in turn, reflects energy along a line of constant phase, which is the basic criteria for maximum antenna gain.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view illustrating the use of a parabolic antenna of the prior art;

FIG. 2 is a diagrammatic view illustrating the use of a parabolic antenna of the present invention;

FIG. 3 is a side view showing a parabolic antenna of the present invention in an annular configuration;

FIG. 4 is a top view of the parabolic antenna shown in FIG. 3 of the drawing; and

FIG. 5 is a sectional view of a preferred embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring first to FIG. 1 of the drawing which illustrates a prior art configuration of a parabolic reflector, the point of feed, A, is located at the focus of the parabolic curve 11 and waves which are reflected from the parabola parallel with the focal axis arrive at line C with equal phase. The path lengths of AB.sub.1 C.sub.1, AB.sub.2 C.sub.2, AB.sub.3 C.sub.3, and AB.sub.4 C.sub.4 are equal. The use of parabolic reflectors in the antenna art is more fully described on pages 336-350 of the text, "Antennas," by John D. Kraus, McGraw-Hill Book Company, Inc. (1950).

Referring now to FIG. 2 of the drawing, the present invention utilizes a parabolic reflector wherein the reflector is positioned so that its focal axis is parallel to the line C, which represents a constant phase front. Electromagnetic energy emitted from point source A, which is located at the focus of the parabolic curve 11, is reflected from a reflector represented by curve 11 to a second reflector represented by straight line 12. In FIG. 2 of the drawing, the path lengths of AB.sub.1 B'.sub.1 C.sub.1, AB.sub.2 B'.sub.2 C.sub.2, and AB.sub.3 B'.sub.3 C.sub.3 are equal.

By rotation of curve 11 and line 12 about axis D, parabolic surface 13 is generated which has point A as a radiating feed point, as shown in FIGS. 3 and 4 of the drawing. Feed point A is located as the focus of surface 13. Surface 14, which is generated by the rotation of line 12, is a cone. By way of example, angle .theta. might be 90.degree., and surface 14 would be a right angle cone. It can be seen that the altitude of the cone is perpendicular to the focal axis of parabolic surface 13.

Referring now to FIG. 5 of the drawing, there is shown a feed element 15 which is energized from circular waveguide 16. It is desirable to locate the feed point at the apex of the cone surface 14, with the apex also being the focus for parabolic surface 13. Feed element 15 might be any conventional rear feed primary antenna and, by way of example, might be crossed dipoles, spirals, horns, or reflectors fed by waveguides.

Numerous advantages can be obtained by using the antenna system of the present invention. In many feeds, the rays of electromagnetic energy which are more rearwardly directed are at greater field strength. For example, referring to FIG. 5 of the drawing, ray F would have a greater field strength than ray G. When ray F is reflected from conical surface 14, however, it will be further from feed 15 than ray G. Thus, rays reflecting from surface 14 will be at a lower intensity as the feed is approached. This permits larger feed structures to be used without having the usual severe shadowing which results in lower gain and also pattern distortion is conventional parabolic reflector antennas. Also the impedance mismatch resulting from energy reflecting back into the feed is minimized, or eliminated and, therefore, larger RF bandwidths can be achieved.

The conical shape of surface 14 provides a convenient mechanical support of large feed structure and either all or part of a transmitting and receiving RF system can be packaged in the inner cone space. The advantages to be gained from this arrangement are the elimination of RF rotary couplings, and shortening of transmit and receiver RF lines thereby resulting in minimum energy loss and improved noise figure. If desired, the outer surface of reflector 17 can be made spherical and positioned in a socket type of gimbal for direction purposes.

As shown in FIG. 5 of the drawings, the forward edge of reflector 17 can extend beyond the end of feed mechanism 15. Thus reflector 17 prevents unwanted electromagnetic radiation from either being transmitted or received to the side of the antenna. The ordinary "spillover" effect common to a conventional parabolic antenna is avoided thereby providing a very "low noise" antenna having high efficiency. Additionally, a flat sheet of dielectric material 18 can be fitted to the front of reflector 17 to serve as a radome and to permit a simple means of pressurization, if desired.

The embodiment shown in FIGS. 3, 4, and 5 of the drawing can be elliptical in shape or of other configuration without departing from the scope of the invention. Also this invention could apply equally well to a visible spectrum and to other RF spectrums which use a parabolic reflector in receiving or transmitting energy. By way of example, the present invention could be used with a carbon-arc search light whereby energy would not be redirected back to the lamp thereby providing for cooler operation.

It can thus be seen that the present invention provides an improved parabolic reflector antenna which does not have many of the inherent disadvantages common to presently used reflector antennas.

Claims

I claim:

1. A parabolic reflector antenna comprising:

a conical reflector,

an annular parabolic reflector surrounding said conical reflector with the focal axis of said parabolic reflector being perpendicular to the altitude of said conical reflector, and

an antenna feed positioned at the focus of said parabolic reflector whereby energy radiating from said antenna feed is reflected from said parabolic reflector to said conical reflector which reflects energy along a line of constant phase.

2. A parabolic reflector antenna as set forth in claim 1 wherein the configuration of said conical reflector is a right angle cone.

3. A parabolic reflector antenna as set forth in claim 1 wherein the apex of said conical reflector is located on the focal axis of said parabolic reflector.

4. A parabolic reflector antenna as set forth in claim 1 wherein the forward edge of said parabolic antenna extends beyond said antenna feed.

5. A parabolic reflector antenna as set forth in claim 4 wherein the forward edge of said parabolic antenna is covered with a flat radome.