1. A clutter fence for an antenna which fence reduces clutter energy from ground objects, said fence comprising, an elongated sheet-like member formed of material opaque to electromagnetic energy and positioned to at least partly encircle said antenna at a radius R with respect to the effective center of reception of said antenna, said member having one of the longer edges thereof lying substantially in a horizontal plane passing through said center and having a vertical dimension substantially equal to √Rλ where λ is the
2. A fence in accordance with claim 1 in which said material absorbs energy
3. A fence in accordance with claim 1 in which said material reflects
4. A clutter fence for an antenna which fence comprises, an elongated sheet-like member formed of material opaque to electromagnetic energy, and supporting means at least partly encircling said antenna and having said member attached thereto to position said member at a radius R with respect to the effective center of said antenna with one longer edge of said member lying substantially in a horizontal plane passing through said center, said member, when so attached, having a vertical dimension substantially equal to √Rλ, where λ substantially
5. A fence in accordance with claim 4 in which said material absorbs energy
6. A fence in accordance with claim 4 in which said material reflects energy impinging thereon.
The invention herein claimed was made in the course of or under a contract with the Department of the Army.
This invention relates to reducing the adverse effects of ground reflected energy and/or energy from active sources at low elevation angles.
Microwave antennas are subject to receiving ground reflected energy and/or energy from active sources at low elevation angles either or both of which interferes with desired received energy. Hereinafter this interference is referred to as ground clutter interference while the energy is referred to as ground clutter energy.
Since microwaves propagate principally along line of sight paths, various screen or fence structures using energy path blocking or phase shifting materials have been devised to reduce the adverse effects of ground clutter energy. At least one of these structures relies on Fresnel zone screening. Fresnel zones may be viewed as concentric annular areas into which a wavefront approaching or leaving an antenna may be divided. The relative phase of the resultant radiation field in each zone shifts 180° from zone to zone while the phase angle of the resultant field in the innermost or first zone is 180° with respect to the phase of the resultant field of all remaining zones. Furthermore, the total energy in the first zone equals twice the total energy in all the remaining zones.
The theory of Fresnel zones and a structure relying on it are disclosed in U.S. Pat. No. 2,763,001 issued to H. E. Bussey on Sept. 11, 1956. This patent teaches that reflected wave suppression in a radio system may be produced by an opaque screen which is erected at the geometrical reflection point and, furthermore, which has a size and shape to block one-half of the energy in the innermost, or first, Fresnel zone. A disclosed embodiment comprises a screen erected on the earth's surface at a point midway between transmitting and receiving antennas and having a shape conforming to a 90° sector of the first Fresnel zone of the wavefront at that point.
There are several apparent shortcomings to the Bussey arrangement. First, the screen must be physically large because the area of the first Fresnel zone at the point where the screen is located is large. Second, such a point of location is far removed from either antenna, and real estate may not be readily available for placement of the screen. Third, such placement assumes two fixed antenna locations, which is not possible when the real or apparent source of electromagnetic energy may move in position (as, for example, in radar tracking). Fourth, such placement relies on good alignment, which is not always achieved and maintained because atmospheric conditions cause angular deviations in the transmission path.
An object of the present invention is to suppress or cancel unwanted energy arriving at or radiating from an antenna in a given direction or directions while simultaneously permitting wanted energy to be received from all other directions.
This and other objects are achieved by a clutter fence formed from an elongated sheet-like member of electromagnetically opaque material. This member is positioned to at least partly encircle an antenna with one of the longer edges of the member lying substantially in a horizontal plane whereby a line passed through the center of the antenna and that edge defines the elevation angle of the energy to be canceled. Furthermore, the member is positioned so that the other of its longer edges lies in a horizontal plane which is tangent to the boundary of the first Fresnel zone of the energy to be canceled. Within the azimuth angle defined by the fence, the fence blocks electromagnetic energy in a horizontal band having a vertical height equal to one-half of the diameter of the first Fresnel zone of the energy at the aforementioned elevation angle. As the remaining energy in the first Fresnel zone is substantially equal to the total of the energy in all of the remaining zones and, furthermore, its field is 180° out of phase with the field of that energy, the unblocked energy algebraically adds to substantially eliminate or cancel itself.
In accordance with the invention, the vertical height (and therefore the overall size) of the fence is kept to a minimum by locating the fence as close to the antenna as possible while still permitting the antenna to be effective at predetermined low elevation angles.
As will become apparent during the following discussion of a specific embodiment of the invention, embodiments of the invention do not have the above-stated apparent shortcomings of the Bussey arrangement. Furthermore, embodiments of the antenna are applicable to both rotatable antennas and electronically-scanned phased arrays.
These and other objects and features of the invention will become apparent from a study of the disclosed embodiment.
BRIEF DESCRIPTION OF THE DRAWING
In the drawing:
FIG. 1 is a plan view of an embodiment of the invention;
FIG. 2 is a cross-sectional elevation view taken through FIG. 1 at the line 2--2;
FIG. 3 is a view looking into FIG. 1 at the line 3--3 with a representation of the first four Fresnel zones of a wavefront superimposed thereon; and
FIG. 4 and 5 are cross-sectional elevation views showing modifications of the embodiment of FIG. 1.
DESCRIPTION OF THE DISCLOSED EMBODIMENT
The embodiment of FIG. 1 and 2 comprises an elongated sheet-like member 10 which is mounted on a plurality of posts 11 so as to appear as a fence which at least partly encircles an antenna 12 at a radius R. Member 10 is formed from material opaque to electromagnetic energy in the sense that it either reflects or absorbs electromagnetic energy. This material may have either a mesh or a continuous surface consistency. Such materials are well known in the prior art and need not be further defined herein.
Arrows are shown in FIG. 1 and 2 to indicate the directions of travel of unwanted energy. Such energy may comprise ground clutter energy or any other unwanted energy having a substantially zero elevation angle. FIG. 3 shows the first four Fresnel zones of this energy at the point it encounters member 10. There are, of course, higher order Fresnel zones which for simplicity have not been shown. In accordance with the invention, the elevational positioning and the width of member 10 are chosen so that its upper edge lies in a horizontal plane which passes through the center of the first Fresnel zone while the lower edge lies in a horizontal plane tangent to the lower edge of the first Fresnel zone. In other words, the upper edge lies in a horizontal plane so that a line 13 passed through the center of antenna 12 and the upper edge of member 10 has the same elevation angle as the unwanted energy.
Member 10 blocks -- either by absorption or reflection -- one-half of the unwanted energy in the first Fresnel zone while blocking much smaller amounts of energy in the remaining zones. The field of the remaining energy in the first Fresnel zone and the summation of the fields of the remaining energy in all of the other zones are substantially equal to, but opposite in phase to, one another. These fields therefore substantially cancel one another at antenna 12 with the effect that the antenna receives highly suppressed unwanted energy.
As pointed out in the Bussey patent, the following formula (which is approximate in nature and whose derivation is well documented in the optical literature) may be used for determining the radius of the first Fresnel zone at any point between a receiving antenna and a source of energy:
r = √ ab λ/(a+ b) (1) where: r = radius of the first Fresnel zone; a = distance from source to point of interest; b = distance from receiving antenna to point of interest; and λ = wavelength of the unwanted energy.
In accordance with the present invention, when antenna 12 is functioning as a receiving antenna, the value of a in equation (1) is very large compared with the value of b, so the value of r becomes:
r = √bλ (2) On the other hand, when antenna 12 is functioning as a transmitting antenna, the value of r becomes:
r = √ aλ (3) Distances b and a of equations (2) and (3), respectively, are the radius R of member 10. The following expression is therefore applicable when antenna 12 is used for either transmitting or receiving:
r = √ Rλ (4)
From the above it is believed apparent that embodiments of the invention are useful for suppressing unwanted transmitted energy in addition to suppressing unwanted received energy. This is a desirable feature when it is necessary that low elevation angle energy be maintained at a low level. Furthermore, the dual nature of this feature is highly desirable for antennas used for both transmitting and receiving, as in the case of radar systems. In the latter case, received clutter energy due to transmission from the radar system undergoes suppression during both transmission and receiving, thus resulting in a very low clutter energy level.
From the above discussion, it follows that the width of member 10 is directly related to the square root of radius R of member 10. In other words, the smaller radius R is, the smaller the width of member 10. Radius R should, however, be sufficient to avoid beam blockage when scanning down to some preselected elevation angle.
Member 10 of FIG. 1 may have an electromagnetic energy absorbing material on the side from which the unwanted energy approaches, in which case substantially no energy is reflected. On the other hand, member 10 may have reflecting surfaces whereupon the energy is reflected in the opposite direction. Member 10 may also be sloped as shown in FIG. 4 and 5 so that energy is either directed upwardly or downwardly, respectively. When reflecting downwardly, the surface of the ground where the energy impinges may be made rough to scatter the energy or it may be coated with an energy absorbing material.