GB2121612A - Dipole array lens antenna - Google Patents

Abstract

Disclosed is a microwave lens of low weight and cost which may be used in communication satellites. The lens can be used with a simple feed, such as a horn, and achieves low cross-polarization levels. Flush mounting of the lens is possible. The lens comprises a plurality of parallel flat supports 12 of dielectric material, each carrying at least one array of commonly orientated conductive dipole elements 13 arranged in regularly spaced rows and columns. For circular polarization, crossed dipole elements can be provided. The supports may be formed of glass fibers and resin or plastic. Each dipole array can produce either positive or negative phase shift depending on the frequency of operation. By suitably scaling an array, the phase and phase versus frequency can be chosen at a given frequency. The phase, phase versus frequency, and attenuation characteristics can be controlled by choosing suitable scale factors, the required number of arrays, and their physical separation. <IMAGE>

Description

SPECIFICATION Dipole array lens antenna This invention relates to microwave lenses.

In communication satellites it is known to increase the utilization of allotted frequency channels by transmitting on orthogonally polarized carriers. The antennas used must therefore generate very low levels of crosspolarization to achieve the necessary isolation between the orthogonal modes of communication.

To generate a shaped beam, communication satellite antennas employ a large number of feeds with a common lens or reflector.

The feed arrays are complex and difficult to design. The antennas should be light weight, insensitive to mechanical deformation, and have adequate communications bandwidth with no significant loss of antenna gain.

Both reflecting antennas and lens antennas are currently used in spacecraft applications.

Known lens antennas often compare unfavorably with reflecting antennas in terms of bandwidth, efficiency, weight or complexity.

They are, however, less sensitive to mechanical inaccuracies, generate less cross-polarization, and have better wide angle scanning performance when the feed is displaced relative to the focal point of the lens.

The present invention provides a microwave lens of weight and cost which can be used with a single feed, such as a horn, and achieves low cross-polarization levels. The lens can be flush mounted.

Thus, in accordance with a broad aspect of the invention, there is provided a microwave lens comprising a plurality of parallel dielectric supports having arrays of conductive dipole elements secured thereto, said elements being commonly orientated and being arranged in regularly spaced rows and columns.

The invention will now be described in more detail in conjunction with the accompanying drawings, in which: Figure 1 is a plan view of a microwave lens according to the invention.

Figure 2 is a graph of phase shift vs frequency for a lens according to the invention, Figure 3 is a perspective view of a lens and associated feed system in an experimental set up, and Figure 4 is a perspective view of a zoned microwave lens in accordance with the invention, Figure 5 is a diagrammatic illustration of a lens which combines both positive and negative phase shifting elements, Figure 6 is a diagrammatic illustration of a zoned lens, Figure 7 is a diagram useful in explaining operation of a lens, and Figure 8 is a cross-sectional view of a lens according to the invention.

Referring to Fig. 1, a planar linear dipole array, generally indicated at 10, comprises a dielectric support 1 2 and a plurality of electrically conductive dipole elements 1 3 secured to the support. The dipoles can be cut from metal tape and secured to a plastic sheet, eg with adhesive. However, it is preferred to form the dipoles by etching techniques, eg by etching a copper clad epoxy glass laminate of, for example, 0.12 mm thickness.

The phase shifting characteristics of an array such as shown in Fig. 1 are shown in Fig.

2 for electromagnetic energy travelling normal to the array surface. Here it is assumed that the dipoles are resonant at a frequency fR of the order of 4.7 GHz. The array has a band stop region between the frequencies indicated by dashed lines 14 of about 4 to 5.5 GHz.

Frequencies in this range are highly attenuated.

In the frequency band of about 5.9-7.1 GHz, the array of Fig. 1 is considered to be a "long" dipole array, that is, the length of each dipole element is greater than half the resonance wavelength fR and, as shown by curve 21, signals in this range are given a positive phase shift upon passage through the array. However, in the frequency band of about 2 to 4 GHz, the array is considered to be a "short" dipole array and, as shown by curve 22 in Fig. 2, signals in this range are given a negative phase shift. By suitably scaling the dimensions of the arry, either positive or negative phase shift can be obtained over a desired frequency band with relatively low attenuation.

Two or more arrays can be placed parallel to each other but spaced apart, that is added in series. The total phase shift through such a combination of arrays very nearby equals the algebraic sum of the individual phase shifts and is insensitive to the spacing between arrays. Where two or more arrays are used, the array spacing is adjusted for minimum attenuation. The phase, phase versus frequency, and attenuation characteristics can therefore be controlled by choosing suitable scale factors, the required number of arrays, and their physical separation.

Fig. 3 shows an experimental set-up of an arrangement according to the invention comprising a microwave lens using linear dipole arrays of the type shown in Fig. 1. If the support 1 2 is thin and flexible it can be secured to a base member 1 5 by adhesive tapes 16. Naturally, the base member 15 and tapes 1 6 must be non-conductive so as not to interfere with microwaves beamed at the lens by a feed antenna, here shown as a horn type feed 1 8 on a support structure 20. Only the front array is shown but it should be appreciated that the lens comprises a plurality of parallel arrays.

Basic lens theory was used to design the lens antenna. Referring to Fig. 7, there is shown a feed horn 1 8 at the focal point of a microwave lens 30 and producing a spherical wavefront 32 which impinges on the flat front surface of the lens 30. The path length between the feed horn 1 8 and the center of the lens 30 is equal to the focal length F but it increases out to the edges of lens. Portions of the wavefront at the edges of the lens thus lag in phase with respect to the wavefront at the center of the lens. That is, there is a phase error which, in degrees, depends on the wavelength. In other words, the phase error is frequency dependent.

To produce a plane wave, the lens has to compensate for this error over the required frequency band. A dielectric lens with a refractive index greater than one introduces a compensating phase lag or negative phase shift, resulting in a lens with the familiar convex cross section. As the refractive index is relatively independent of frequency, the lens is inherently broadband. A metal plate waveguide lens has a refractive index less than one and introduces phase lead or positive phase shift resulting in a concave cross-section. The lens operates normally close to the waveguide cutoff frequency in order to minimize lens thickness and weight. The refractive index is frequency dependent and this results in relatively narrow bandwidth.

A lens antenna using dipole arrays can introduce either positive or negative phase shift. By combining the arrays, the resulting phase shift and phase versus frequency characteristics can be independently controlled.

The lens can take the form shown in Fig. 5.

To reduce the lens thickness it can be designed in two sections as shown in Fig. 6.

The first section is zoned and is designed to correct the phase at the midband frequency.

The second section provides the required phase versus frequency characteristics for broadband operation. At the midband frequency it introduces zero phase shift.

It will be appreciated that Figs. 5 and 6 are only diagrammatic and not illustrative of the physical appearance of the lens structure. A simplified cross-sectional view of a Fig. 5 type of lens is shown in Fig. 8 as comprising 6 arrays 50-55. The dipole elements (not shown) to the right of curve 60 are short dipoles, resulting in negative phase shift, while those to the left are long dipoles, resulting in positive phase shift. The curve 60 is merely shown for illustrative purposes; it has no physical reality in the lens.

Portions of arrays 52 and 54 have been shown as dashed lines; these portions are free of dipoles. This is because the spacing for positive arrays is greater, eg twice as great, as for negative arrays, for minimum attenuation.

As shown in Figs. 4 and 6, the lens may be zoned, Fig. 4 showing three zones. The required lens phase shift ss is given by the equation

where 8 is the angle between the axis and the ray under consideration, as shown in Fig. 7, F is the focal length, and A is wavelength.

When the required phase shift exceedsir rad, the lens can be stepped from positive to negative as shown in the left portion of Fig.

6.

The sheets containing the dipoles may be supported at their periphery by a carbon fiber ring and lightly tensioned to provide parallel plane surfaces. One design which has been built consists of seven parallel sheets of dipoles with a total complement of 6700 dipoles.

Zoning the lens enables its thickness to be reduced by shifting the phase sr rad on one side of the zone boundary and - II rad on the other. A region is thereby created, albeit very narrow, centered on the zone boundary where the mean phase shift is 0 degrees and the energy is in antiphase with the energy in the rest of the lens. There is a consequent loss in antenna gain and an increase in sidelobe level in the forward direction.

A zoned microwave lens has been designed and constructed using the phase shifting properties of planar arrays of linear dipoles. The lens generates very low levels of cross polarization and has relatively wide angle scanning properties. It is inherently lightweight, mechanically simple, and should be inexpensive to manufacture.

Figs. 1, 3 and 4 show vertically orientated dipoles but obviously they could be horizontally orientated. Furthermore, both vertical and horizontal dipoles may be used, resulting in cross-shaped elements on the support sheets.

The dipole arrays have bandpass and bandstop filter characteristics and may be used to construct frequency selective lenses.

The dipole arrays can be added to existing antennas to modify their radiation patterns.

Claims (9)

1. A microwave lens comprising a plurality of parallel dielectric supports having arrays of conductive dipole elements secured thereto, said elements being commonly orientated and being arranged in regularly spaced rows and columns.

2. A microwave lens as claimed in claim 1 wherein the dipole elements are of thin metal.

3. A microwave lens as claimed in claim 2 wherein said metal is copper.

4. A microwave lens as claimed in claim 2 or 3 wherein said dipole elements are formed by printed circuit techniques.

5. A microwave lens as claimed in claim 2 or 3 wherein said dipole elements are formed by printed circuit techniques and said dielectric support is formed of glass fibers and resin.

6. A microwave lens as claimed in claim 1 wherein some dipoles cause positive phase shifting of incident radiation and others cause negative phase shifting, the combined positive and negative phase shifting being such as to cause an incident spherical wavefront to be converted to a substantially planar wavefront.

7. A microwave lens as claimed in claim 6 wherein the dipoles are arranged in zones to cause positive and negative phase shifting.

8. A microwave lens substantially as hereinbefore described with reference to Figs. 1, 2 and 3.

9. A microwave lens substantially as hereinbefore described with reference to Figs. 4, 5, 6, 7 and 8.