Dual Waveguide Horn Antenna

Abstract

A dual waveguide horn antenna is provided with a rigid foam having a dielectric constant approximately equal to that of air and tapered to conform to the shape of a conventional horn antenna. Two tapered walls are corrugated and all four walls are covered with a thin metallic coating of sufficient thickness to carry the RF current produced by the propogation of an electromagnetic wave through the dielectric. A metallic septum divides the dielectric into first and second waveguide sections and individual coaxial inputs are provided to each of the sections. Energy coupling between the two sections of the waveguide are significantly reduced by means of a microwave resistive material secured to the septum at the enlarged end of the dielectric. The resistive strip may comprise a carbonized substrate or an insulating material coated with the resistive film. Additionally, a thin film of paint may be applied directly to the metallic septum by various techniques including vacuum deposition.

Description

RIGHTS OF THE GOVERNMENT

The invention described herein may be manufactured, used, and licensed by or for the United States Government for governmental purposes without the payment to me of any royalty thereon.

BACKGROUND OF THE INVENTION

This invention relates generally to electromagnetic horn antennas, and more particularly to a dual waveguide horn antenna utilizing a lightweight foam material whose dielectric constant approximates that of air.

Conventional electromagnetic horn designs are usually constructed from sheet metal such s brass or aluminum and suitably shaped to achieve a desired radiation pattern. The disadvantages inherent in such systems are that they tend to be very heavy and quite expensive. Furthermore, any attempt at modification of the basic design in order to attain a change in performance results in still more complex and bulky structures, thereby adding further to their expense. Additionally, the E-plane radiation pattern of horns designed and constructed in the conventional manner have inherently high side lobes, a high degree of energy coupling between adjacent waveguide sections as well as other undesirable radiation pattern characteristics.

It is, therefore, a primary object of this invention to provide a dual waveguide horn antenna which is extremely lightweight, inexpensive, highly efficient and capable of performing a number of functions in electronic and radar systems.

Another object of the invention is to provide a horn antenna which is easy and inexpensive to construct as well as to modify.

Yet another object is to provide a dual waveguide horn antenna having high gain and low side lobes.

Still another object is to provide a dual waveguide horn antenna in which the coupling of energy between adjacent waveguide sections in significantly reduced.

SUMMARY OF THE INVENTION

Briefly, in accordance with this invention, a dual waveguide antenna is provided with a rigid foam material whose dielectric constant approximates that of air. The dielectric is tapered to conform to the shape of a conventional horn antenna and its walls are provided with a thin metallic coating of sufficient thickness to carry the RF current produced by the propagation of electromagnetic waves through the dielectric. First and second waveguide sections are provided by means of a metallic septum for dividing the dielectric material. Energy coupling between the adjacent waveguide sections is significantly reduced by means of a microwave resistive material secured to the edge of the dividing septum. Side lobe patterns are significantly reduced by applying corrugations to the two tapered walls to achieve a slow wave structure.

BRIEF DESCRIPTION OF THE DRAWINGS

The specific nature of the invention as well as other objects, aspects, uses, and advantages thereof will clearly appear from the following description and from the accompanying drawings, in which:

FIG. 1 is a perspective view, partially cut away, of a dual horn antenna in accordance with this invention.

FIG. 2 illustrates the sum and difference patterns of a dual waveguide horn antenna in accordance with one embodiment of the invention.

FIG. 3 illustrates the E-plane pattern in a single section of a dual waveguide horn antenna in accordance with this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, the dual waveguide horn antenna comprises a rigid foam dielectric material 10 whose dielectric constant approximates that of air. The foam 10 is shaped as a conventional horn antenna having two tapered walls 11 and 12, and two parallel walls 13 and 14. Each of these walls is coated with a thin metallic copper plating of sufficient thickness to carry the RF current produced by the propagation of the electromagnetic waves through the dielectric. Electromagnetic energy is applied to the antenna at its tapered end by means of a pair of coaxial inputs 20 and 21, and an electromagnetic window 22 is located at the enlarged end of the horn antenna. The window may comprise material which is transparent to the electromagnetic energy, or it may simply be a portion of unplated dielectric.

Corrugations 15 and 16 are applied to tapered walls 11 and 12 in order to achieve a slow wave structure. These corrugations have the effect of substantially reducing diffraction and scattering at the edges of the horn as well as significantly reducing the side lobes and back radiation in the E-plane pattern.

The horn antenna is divided into first and second waveguide sections, thereby providing a dual horn antenna, by means of a metallic septum 17 which is located halfway between the two corrugated walls 15 and 16 in the H-plane. The septum extends over the entire longitudinal area of the dielectric between the coaxial inputs 20 and 21 and the output window 22. The incorporation of such a septum into a horn antenna of the type shown has the effect of reducing the standard X-band waveguide to two half-height waveguides. Each of these waveguides is separately fed from one of the two miniature coaxial inputs 20 and 21. Electromagnetic energy flowing in each section of the dual waveguide horn antenna will cause electrical currents to be conducted at the two opposing surfaces of the septum 17. These currents, upon reaching the edge 23 of septum 17 will tend to cross over to the other side, thereby coupling energy from one section to the other. In order to prevent this cross-coupling, a microwave resistive material 18 is secured to septum 17 near its edge 23. Any electrical currents flowing along the surface of the septum will be dissipated into heat upon traversing the resistive material 18. Accordingly little or no energy will be coupled between the two sections of the antenna. Decoupling figures in excess of 30 db can be expected from the use of this resistive element. This represents a 10 to 15 db improvement over the use of a metallic septum without a resistive strip as herein described.

The microwave resistive strip may be applied in a number of different ways. One way would be to paint the edge surface of the septum with the resistive film or to apply the resistive film to the surface by means of vacuum deposition. Additionally, one may employ a carbonized substrate securely attached to the terminal portion of the septum. Finally, the resistive strip may comprise a thin insulating material which is secured to the terminal portion of the septum and a resistive coating applied to one or both sides of the insulating material. Suitable resistive materials may comprise nichrome or Synthane. The thickness of the coating as well as the area covered would depend upon a variety of factors such as the intensity of the radiation beam, the type of dielectric employed as well as other factors which would be well known to those persons skilled in the art.

The radiation patterns of the dual waveguide horn antenna used as a monopulse antenna is shown in FIG. 2. The sum and difference patterns are shown. It will be noted that the sum pattern has a high gain and essentially no side lobes. FIG. 3 illustrates the E-plane pattern of one section of the dual horn antenna.

The foam dielectric material for use in this invention may typically comprise expanded polystyrene and polyurethane. These materials typically have a dielectric constant of 1.05 and a loss tangent of 0.0004, thereby approximating the characteristics of air. Additionally, they are extremely lightweight, having a density of about 4 pounds per cubic foot. The walls of the dielectric waveguide as well as the septum are copper plated to a thickness of about 0.005 inches. The technique of copper plating is disclosed in copending application Ser. No. 739,578 filed Apr. 16, 1968.

I wish it to be understood that I do not desire to be limited to the exact details of construction shown and described, for obvious modifications will occur to a person skilled in the art.

Claims

I claim as my invention:

1. A dual waveguide horn antenna comprising:

a. a rigid foam dielectric material having a dielectric constant approximately equal to that of air and having tapered walls to conform to the shape of a conventional horn antenna;

b. a thin metallic coating covering the walls of the dielectric, said coating having sufficient thickness to carry the RF current produced by the propagation of an electromagnetic wave through said dielectric;

c. a metallic septum for dividing said dielectric into first and second waveguide sections;

d. an electromagnetic window located at the enlarged end of said tapered dielectric foam; and

e. means for applying electromagnetic energy to the tapered end of said dielectric.

2. The antenna of claim 1 wherein two of said tapered walls are corrugated to provide a slow wave structure.

3. The antenna of claim 1 wherein said thin metallic coating and said metallic septum comprise copper plating.

4. The antenna of claim 1 further comprising means for reducing energy coupling between said first and second waveguide sections.

5. The antenna of claim 4 wherein said means for reducing energy coupling comprises a microwave resistive material secured to said septum and located at the enlarged end of said dielectric.

6. The antenna of claim 5 wherein said resistive strip comprises a thin film resistive material secured to the surface of said septum by vacuum deposition.

7. The antenna of claim 5 wherein said resistive strip comprises a thin film paint on the surface of said septum.

8. The antenna of claim 5 wherein said resistive strip comprises a carbonized substrate.

9. The antenna of claim 5 wherein said resistive strip comprises an insulating strip secured to the edge of said septum and a microwave resistive coating on said insulating strip.

10. The antenna of claim 1 wherein said metallic septum equally divides said upper and lower waveguide sections.