GB1562866A - Antenn with performted metal plate angle filter - Google Patents

Description

(54) ANTENNA WITH PERFORATED METAL PLATE ANGLE FILTER (71) We, GENERAL DYNAMICS COR PORATION, a corporation of the State of Delaware, U.S.A., of 5011 Kearny Villa Road, San Diego, California 92138, U.S.A., do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: - This invention relates to antennas and is particularly directed to the reduction of the intensity of sidelobes transmitted or re ceived by such antennas.

The particular type of electronic an tenna to which the present invention is directed includes a feedline, such as a wave guide pipe, and a linear array of radiator elements such as waveguides coupled to the feed line. In an electronic scanning an tenna, phase shifting elements such as coil wound ferrite material or voltage driven PIN diodes may be positioned in the feed lines between the radiator elements and/ or positioned in the radiator elements for establishing the angle through which the main beam transmitted or received by the antenna is scanned. The width of the beam in a given dimension is determined by the length of a linear array of radiator ele ments in the given dimension. The array of radiator elements defines the aperture of the antenna. The radiator elements typi cally are spaced apart from each other by a distance of two-thirds wavelength. If a lesser number of radiator elements were to be included in the linear array in order to reduce the cost of the antenna, but the length of the array maintained so as not to reduce the beam width, the increased spacing between the radiator elements would result in intolerably intense sidelobes being .transmitted or received by the antenna.

According to the present invention there is provided an antenna for transmitting or receiving a signal of a given wavelength X.

comprising:- a feed line; an array of radi .ator elements coupled-to the feed line for refining the aperture of the antenna: and an angle filter positioned approximately parallel to the plane of the aperture of the antenna for reducing the intensity of sidelobes transmitted or received by the antenna; characterized by the angle filter comprising a plurality of parallel perforated metal plates, that are positioned approximately parallel to the plane of the aperture, each plate having a dielectric cover, wherein the plate of the angle filter that is closest to the aperture is positioned at a distance of (2n + 1) A/4 from the aperture, with "n" being an integer greater than 1, and wherein the distance between adjacent plates is approximately A/2, for causing low transmittance at high incidence angles and high transmittance at low incidence angles and in a frequency band about the given wavelength. The transmittance characteristics of the angle filter are such that the radiator elements can be spaced apart from each other by a distance greater than 2A/3 without the increased spacing resulting in intolerably intense sidelobes being transmitted or received by the an tenna.

The present invention thus provides the advantage of lower cost resulting from a reduction in the number of radiator ele ments necessary for providing a given beam width, without suffering from the disadvantage of intolerably large sidelobes, such as the grating lobes which occur with an electronic scanning antenna.

In another aspect of the present invention, wherein cost is not a factor, but wherein it is prime importance to reduce the intensity of sidelobes, the perforated metal plate angle filter is included in an electronic antenna wherein the spacing be tween radiator elements in an array on a feed line may be two-thirds wavelength or less.

The perforations in the metal plate preferably are circular holes having a diamether of approximately one-third wavelength, with the preferred center to center spacing between the holes being approximately twothirds wavelength. The preferred spacing between the radiator elements (center to center spacing) is approximately 0 9 wavelength.

There are two principal benefits derived from the angle filter having a plurality of perforated metal plates that are coated with a dielectric material. First, reflection of radiation between the radiator elements and the angle filter is reduced. Second, the angle filter can be stagger tuned to provide uniformly low transmittance at high incidence or scanning angles. In fact, it was discovered that unless the angle filter is stagger tuned there is a positive bump in the transmittance characteristic at high incidence or scanning angles as opposed to the decreasing relatively linear slope achieved with a stagger tuned angle filter as shown in Figure 4. The angle filter is stagger tuned by providing that the dielectric coated perforated metal plates have different transmittance characteristics. The transmitance characteristics of the individual plates preferably are varied by varying the center to center spacing between the apertures, and also can be varied by varying the diameter of the apertures, the thickness of the plates, the thickness of the dielectric material, and the dielectric constant of the dielectric material.

The antenna of the present invention is particularly useful in microwave communications systems; and it is also useful in other communications systems. It may be used for radar and also in combination with additional appropriate apparatus for sonar and imaging with acoustic and electromagnetic waves.

The invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a side elevation partially sectioned view of an electronic scanning antenna of the present invention; Figure 2 is a plan view on a reduced scale of a representative perforated metal plate included in the angle filter shown on edge in Figure 1, with a portion of dielectric cover removed; Figure 3 illustrates a beam pattern for the antenna of Figure 1 at an extreme scanning angle; and Figure 4 is a graph showing the transmittance characteristic of the perforated metal plate angle filter of Figure 1.

Referring to Figures 1 and 2, an electronic scanning antenna in accordance with present invention includes a feed line 10, a linear array of radiator elements 12 to 19 inclusive, an angle filter 20, absorbing material 21, and either phase shifting ele ments 22 to 28 inclusive and/or phase shifting elements 29 to 36 inclusive. Preferably only one set of phase shifting elements 22 to 28 or 29 to 36 is included in a single embodiment.

The feed line 10 has a terminal 38 for coupling the feed line 10 to a communications system. The feed line 10 is a waveguide pipe having openings to which the radiator elements 12 to 19 are coupled.

The radiator elements 12 to 19 are also waveguide pipe. The array of radiator elements 12 through 19 scans a main beam through an angle that is established by means of the phase shifting elements 22 to 28 or 29 to 36. The center to center spacing of the radiator elements 12 to 19 is 0 9 wavelength. The phase shifting elements are either coil wound ferrite material or voltage driven PIN diodes. The phase shifting elements 22 to 28 are positioned within the feed line 10 as shown in Figure 1; and the phase shifting elements 29 to 36 are respectively positioned within the radiator elements 12 to 19.

The angle filter 20 includes a plurality of parallel perforated metal plates 39, 40 and 41. Each of the perforated metal plates 39, 40, 41 includes a uniformly disposed array of circular holes having a diameter of approximately one-third wavelength. The center to center spacing between the holes is approximately two-thirds wavelength.

The lattice of the holes may be rectangular as shown in Figure 2, or it may be hexagonal.

Each of the perforated metal plates 39, 40 and 41 has layers of dielectric material coating 42 covering both sides of the plate.

The angle filter 20 is positioned at a distance of (2n + 1) X/4 from the aperture, with "n" being an integer greater than 1. That is, the distance between the lower plate 41 and the radiator elements 12 to 19 is (2n + 1) A/4. In the preferred embodiment shown in Figure 1, this distance is 5A/4. The distance between adjacent plates 39 and 40, and 40 and 41 is approximately k/2. The angle filter 20 is stagger tuned. The center to center spacing between the holes in the perforated metal plate 41 is less than such spacing in the plate 39 and greater than such spacing in the plate 40. The variation in such spacing is within 20 percent of the preferred spacing of 2A/3. The size of the holes in the plate 41 is the same as the size of the holes in the plates 40 and less than the size of the holes in the plate 39. The variation in such hole size in the different plates 39, 40 and 41 is within 25 percent of the referred hole size of x/3.

The plates 39, 40, 41 of the angle filter 20 may be curved when installed on the outside of an aircraft so as to reduce aerodynamic drag.

The absorbing material 21 is positioned at the ends of the linear array of radiator elements 12 through 19 and is disposed in relation to the end radiator elements 12 and 19 at the maximum angles through which the main beam is scanned. The absorbing material 21 absorbs energy from the grating lobes and other sidelobes reflected by the angle filter 20. The absorbing material 21 is rubber foam that is loaded with carbon.

Referring to Figure 3, the angle filter 20 causes little attenuation of the main beam 44 with in a scanning angle () of about to30". The grating lobe 46 and other sidelobes 48 and 50 which occur at angles between about +45 and +90 are reflected by the angle filter 20 and reflected into the absorbing material 21 and thereby eliminated.

A graph showing the transmittance characteristic of the angle filter 20 is shown in Figure 4, wherein the ordinate dimension is transmittance and the abscissa dimension is in degree from a line normal to the plane of the angle filter 20.

WHAT WE CLAIM IS:- 1. An antenna for transmitting or receiving a signal of a given wavelength A comprising; a feed line; an array of radiator elements coupled to the feed line for defining the aperture of the antenna; and an angle filter positioned approximately parallel to the plane of the aperture of the antenna for reducing the intensity of sidelobes transmitted or received by the antenna; characterized by the angle filter comprising a plurality of parallel perforated metal plates that are positioned approximately parallel to the plane of the aperture, each plate having a dielectric cover, wherein the plate of the angle filter that is closest to the aperture is positioned at a distance of (2n + 1) A/4 from the aperture, with "n" being an integer greater than 1, and wherein the distance between adjacent plates is approximately A/2, for causing low transmittance at high incidence angles and high transmittance at low incidence angles and in a frequency band about the given wavelength.

2. An antenna according to Claim 1 wherein the perforations in the metal plates are circular holes having a diameter of approximately A/3 and the center to center spacing between the holes is approximately 2A/3.

3. An antenna according to Claim 1 or 2 wherein the angle filter is stagger tuned for providing uniformly low transmittance at high scanning angles.

4. An antenna according to Claims 1, 2 or 3, further comprising phase shifting elements positioned in relation to the radiator elements for enabling the main beam transmitted or received by the antenna to be electronically scanned.

5. An antenna according to Claim 1, 2, 3 or 4 wherein the radiator elements are spaced apart from each other by a distance greater than 2A/3.

6. An antenna according to Claim 5 wherein the radiator elements are spaced apart from each other by 0 9A.

7. An antenna for transmitting or receiving a signal of a given wavelength substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. relation to the end radiator elements 12 and 19 at the maximum angles through which the main beam is scanned. The absorbing material 21 absorbs energy from the grating lobes and other sidelobes reflected by the angle filter 20. The absorbing material 21 is rubber foam that is loaded with carbon. Referring to Figure 3, the angle filter 20 causes little attenuation of the main beam 44 with in a scanning angle () of about to30". The grating lobe 46 and other sidelobes 48 and 50 which occur at angles between about +45 and +90 are reflected by the angle filter 20 and reflected into the absorbing material 21 and thereby eliminated. A graph showing the transmittance characteristic of the angle filter 20 is shown in Figure 4, wherein the ordinate dimension is transmittance and the abscissa dimension is in degree from a line normal to the plane of the angle filter 20. WHAT WE CLAIM IS:-

1. An antenna for transmitting or receiving a signal of a given wavelength A comprising; a feed line; an array of radiator elements coupled to the feed line for defining the aperture of the antenna; and an angle filter positioned approximately parallel to the plane of the aperture of the antenna for reducing the intensity of sidelobes transmitted or received by the antenna; characterized by the angle filter comprising a plurality of parallel perforated metal plates that are positioned approximately parallel to the plane of the aperture, each plate having a dielectric cover, wherein the plate of the angle filter that is closest to the aperture is positioned at a distance of (2n + 1) A/4 from the aperture, with "n" being an integer greater than 1, and wherein the distance between adjacent plates is approximately A/2, for causing low transmittance at high incidence angles and high transmittance at low incidence angles and in a frequency band about the given wavelength.

2. An antenna according to Claim 1 wherein the perforations in the metal plates are circular holes having a diameter of approximately A/3 and the center to center spacing between the holes is approximately 2A/3.

3. An antenna according to Claim 1 or 2 wherein the angle filter is stagger tuned for providing uniformly low transmittance at high scanning angles.

4. An antenna according to Claims 1, 2 or 3, further comprising phase shifting elements positioned in relation to the radiator elements for enabling the main beam transmitted or received by the antenna to be electronically scanned.

5. An antenna according to Claim 1, 2, 3 or 4 wherein the radiator elements are spaced apart from each other by a distance greater than 2A/3.

6. An antenna according to Claim 5 wherein the radiator elements are spaced apart from each other by 0 9A.

7. An antenna for transmitting or receiving a signal of a given wavelength substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.