Surface Wave Antenna With Beam Tilt Angle Compensation

Abstract

A surface wave antenna for use on a finite asymmetric ground plane of length G in the propagation direction in which an end-fired antenna projects an electromagnetic beam of wavelength .lambda. polarized in the elevation plane and whose tilt angle varies inversely as G/.lambda.. A correcting dielectric film is mounted on the ground plane in front of the end-fired antenna for increasing the antenna length in the propagation direction.

Description

This invention was made pursuant to a contract with the U.S. Air Force.

BACKGROUND OF THE INVENTION

This invention relates to surface wave antennas and, more particularly, to such antennas mounted on finite asymmetrical ground planes, such as aircraft fuselages.

As is well known, a surface wave antenna radiates or propagates an electromagnetic wave of wavelength .lambda. along its axis. The wave or beam may be highly directed near the horizon, for example, in the case with the E vector being polarized in the elevation plane. The beam shape and its tilt angle .gamma. are influenced by the length of the ground plane G along the propagation path in front of the antenna.

Now, the beam tilt angle .gamma. is defined as the angle .gamma. in the elevation plane between the ground plane and the peak of the beam. It has been observed in the case of end-fired antennas and others that an electromagnetic wave of wavelength .lambda. for a long ground plane length G longitudinal to the antenna results in small beam tilt angles. Similarly, short ground lengths G produce larger tilt angles. These observations confirm that beam tilt angle .gamma. varies inversely as G/.lambda..

An aircraft fuselage represents a ground plane environment that appears finite in length cross-sectionally and semi-infinite in length longitudinally. In more cryptic language this type of ground plane may be expressed as finite asymmetrical ground plane.

These aforementioned factors play a significant part in the problem of communicating by radio from an aircraft to a ground station using directed electromagnetic waves or beams. As the aircraft changes its position relative to the ground station, the beam must be rotated to maintain alignment. At the same time, the antenna on the aircraft may rotate away from the longitudinal aircraft axis. When this occurs, the ground plane length G decreases and the beam tilt angle increases. Consequently, the beam tilt angle varies in azimuth due to changes in ground plane length of the aircraft fuselage.

It is accordingly an object of this invention to devise a surface wave antenna mounted on a finite asymmetrical ground plane which includes compensation for beam tilt angle variations.

It is related object of this invention that such beam tilt angle compensation be operative upon end-fired antennas flush mounted on aircraft fuselages.

It is still another related object that such compensation be operative upon surface wave antennas electrically or mechanically scanned in azimuth.

It is yet another object that the radiating beam pattern between an aircraft and a ground station maintain proper angle alignment independent of the rotational position of the surface wave antenna on the aircraft.

SUMMARY OF THE INVENTION

The above objects are satisfied by an embodiment of the invention in which a thin dielectric film is placed on the ground plane in the near field of the end-fired antenna along the propagation direction. This increases the antenna length and decreases the beam tilt angle. It has been observed that the dielectric length .DELTA.L approximates the difference between the ground plane length G and the length L.sub.g where L.sub.g is the distance between the far edge of the dielectric film and the edge of the ground plane. It furthermore has been found that the beam tilt angle .gamma. varies inversely as .DELTA.L/.lambda.. Thus, as the antenna is rotated in azimuth, the beam tilt angle can be made constant by varying the dielectric length .DELTA.L in each azimuth direction. Significantly, the dielectric film must be of a thickness less than the wavelength .lambda. of interest. Preferably, the thickness should be less than or equal to .lambda./16.

The present invention will be described by referring to the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a flared-slot antenna mounted on a ground plane according to the invention;

FIG. 2 graphically depicts the beam tilt angle characteristics as a function of ground plane conditions;

FIG. 3 shows the dielectric tilt angle compensator for a rotary surface wave antenna.

Referring now to FIG. 1 of the drawing there is shown a flared-slot antenna mounted on a ground plane 4 according to the invention. The flared slot 3 is formed from a dielectric having a permittivity approximately 2.7. One such antenna may be constructed to a width of 3.1 inches over a 4.5-inch length with a depth taper to 0.04 inches. Such an antenna is operable at, for example, 13.5 GHz. The flared slot is fed with a standard Ku band waveguide 2. This antenna provides a linear polarization in the elevation angle which is normal to the aircraft skin. Typically, such a shaped beam E plane or elevation pattern may be directed along the aircraft skin with a 20.5.degree. half power beam width. The symmetrical azimuth or H-plane pattern may have a 25.degree. half power beam width. The minimum system gain is in the order of 16db. over a bandwidth of 13.1 to 13.9 GHz. The bandwidth is limited by the rotary joint feed. However, this type of element produces a nearly constant pattern, gain, and impedance performance over the entire waveguide band. The antenna may be mounted on a 9-inch-diameter circular plate which rotates to provide 360.degree. coverage in azimuth. Microwave power may be transferred through a coaxial rotary joint (not shown) which, in turn, may be driven by a servomotor in a closed loop servosystem (also not shown). If the antenna can be made to rotate between 2 and 12 revolutions per minute (12.degree. to 72.degree. per second), then a maximum position error may be only in the order of .+-.1.5.degree.. Such a construction causes negligible signal loss because the azimuth pattern is nearly flat within 2.degree. of the beam peak.

The major difficulty of all rotatable flared slot (or any other surface wave antenna) flush mounted on an asymmetrical ground plane is that the beam tilt angle in the elevation plane is a variable that depends on the azimuth angle. The effective ground plane lengths vary in front of the antenna as it rotates and thus produce the azimuth angle dependence. The finite ground plane prevents a true and fire condition since it raises the beam above the ground plane. The value of the beam tilt angle depends on the effective ground plane length in front of the flared slot.

Referring now to FIG. 2 of the drawing, there is shown a drawing of the flared-slot antenna-correcting dielectric with selected geometrical dimensions. This drawing is superimposed upon a graph showing that tilt angle .psi. changes when a correcting dielectric length .DELTA.L is used.

The flared-slot antenna directivity is determined by its longitudinal length L. The finite ground plane length G is the distance from the boundary of the ground plane to the point in the near field of the antenna along the propagation direction. .DELTA.L is the length of the correcting dielectric film along the propagation axis in the ground plane. L.sub.A is the sum of L+.DELTA.L.

For purposes of convenient measure the ground plane and correcting dielectric lengths reference the wavelength .lambda. of the electromagnetic beam. Thus, strictly speaking, distances are normalized in terms of the number of wavelengths i.e., G/.lambda. and .DELTA.L/.lambda..

In FIG. 2 the tilt angle as a function of G/.lambda. for .DELTA.L=0 is shown. An abscissa of correcting dielectric length .DELTA.L/.lambda. shows the length of the correcting dielectric necessary to alter the tilt angle. Illustratively, for an uncorrected ground plane length of G=15.lambda. and a beam tilt angle of .gamma.=11.5.degree., then .DELTA.L must equal 4.lambda. in order to change the angle to .lambda.=8.degree.. Consequently, as the ground plane lengths change in front of a rotating flared-slot antenna, varying lengths of correcting dielectric must be used to maintain the tilt angle constant. The correcting dielectric is a thin layer about .lambda./16 units in thickness and may be formed from polystyrene or a polycarbonate which has a higher impact strength and better thermal stability than the polystyrene.

Referring now to FIG. 3 of the drawing there is shown a semi-infinte ground plane 5 having a finite width. Flared-slot antenna 3 fed by waveguide 2 is rotatably mounted and substantially flush to the ground plane. The correcting dielectric 6 is spaced in front of the flared slot a distance .DELTA. L varying with the azimuth. The ground plane edge is illuminated by the antenna which provides a second source of radiation. This causes a rearrangement of energy near the peak of the beam as the ground plane length varies. Some improvement can be achieved in the horizon gain by substituting the wedge slots as shown in FIG. 1 for the flared slot to increase the azimuth aperture. The wedge slot profile is identical to the flared slot to maintain the same elevation pattern. However, the width is increased to provide a rectangular surface. This is effective in the presence of a circular aircraft fuselage cutout since both the length and width contribute to the gain rather than just the length as in the case of the flared slot. To minimize blockage it is required that a folded pillbox feed be used.

Claims

I claim:

1. In combination:

a conductive surface;

radiating means mounted substantially flush with said surface and having a radiation pattern that varies as a function of angular deviation about an axis perpendicular to said surface, with at least a portion of said variation resulting from the extent or contour of said surface; and

solid dielectric means extending along said surface in the region surrounding said radiating means.

2. The combination in accordance with claim 1, wherein said solid dielectric reduces the variation in said radiation pattern.

3. The combination in accordance with claim 2, wherein the extent of said solid dielectric along said surface varies as a function of angular deviation about said axis.

4. The combination in accordance with claim 3, wherein said radiating means has a directional radiation pattern which is rotatable about said axis.

5. The combination in accordance with claim 4, wherein said radiating means comprises a horn.

6. The combination in accordance with claim 5, wherein said horn has a portion thereof filled with dielectric.

7. The combination in accordance with claim 6, wherein said solid dielectric reduces variations in the radiation pattern in the plane of said axis.