WO1989001246A1

WO1989001246A1 - Phase shifting element, antenna comprising phase shifting elements and method of steering an antenna

Info

Publication number: WO1989001246A1
Application number: PCT/GB1988/000622
Authority: WO
Inventors: Kenneth Milne; Hugh Duncan Griffiths
Original assignee: Independent Broadcasting Authority
Priority date: 1987-07-28
Filing date: 1988-07-28
Publication date: 1989-02-09
Also published as: AU2087988A; GB8717845D0

Abstract

Substantially planar phase shifting elements, for example for use in antennas, may be constructed to deviate electromagnetic radiation in the frequency range 30 MHz-300 GHz. Phase shifting elements analogous to lenses, prisms and focussing reflectors impart a phase shift to radiation incident thereon which varies with the position of incidence. Multi-layer phase shifting elements comprise spaced thin dielectric sheets with a graded metallic coating providing a suspectance varying with position on the sheet. Antenna apparatus uses the phase shifting elements analogous to lenses and focussing reflectors as the antenna itself, and uses the phase shifting elements analogous to prisms for beam steering.

Description

Phase shifting element, antenna comprising phase shifting elements and method of steering an antenna.

This invention relates to phase shifting elements, for example for use as antennas or components of antennas. It has particular application to antennas for use with microwave radiation, though it is applicable to a wider range of radiation, most usefully from about 30 MHz (10 m) to about 300 GH z (1 mm).

Known microwave antennas use lenses of a number of different types, the principal ones being as follows:

(a) Dielectric lenses

These are analogous to refracting lenses used in optics. The thickness of the lens varies across its width in a predetermined manner. To reduce the size and weight of such a lens it may be zoned, that is to say, its thickness is stepped down in a succession of steps each of which provides a path change equal to one wavelength of the incident radiation, so that the average thickness of the lens remains more or less constant across its width.

(b) Metal-plate lenses

These use parallel-plate waveguides extending generally parallel to the direction of propagation of the incident radiation. Focussing is achieved by the plates being of non-uniform length. (c) Lenses with a non-uniform index of refraction

These use dielectric materials of which the index of refraction varies within the lens in a predetermined fashion. One such lens is known as the Luneburg lens.

Further information .on these lenses may be found, for example in "Introduction to Radar Systems" Merrill I. Skolnik, 2nd edition, pp 250-252. The above lenses have various advantages and disadvantages, but they all share the disadvantages of being bulky and heavy, and when a number of lenses are assembled together the resulting assembly is particularly bulky. Furthermore, they tend to suffer from mismatch effects, though attempts have been made to deal with these by the microwave equivalent of optical blooming.

Embodiments of the present invention provide phase shifting elements, for example for use in antennas, which are relatively thin and can be - concatenated without resulting in a bulky structure. The elements may provide the function of refracting elements such as lenses and prisms.

According to the present invention there is provided a phase shifting element, comprising: at least one sheet, of substantially constant thickness and adapted to impart to electromagnetic radiation incident thereon a phase shift varying with the location thereon at which incidence occurs.

In a preferred embodiment the phase shifting element is adapted to define a plurality of zones such that the phase shift imparted.by -the at least one sheet varies discontinuously across each zone boundary.

The overall range of phase shifts provided by the zones is preferably not more than 360° or not substantially more_, than 360°.

In the accompanying diagrammatic drawings: Figure 1a shows an assembly of three microwave-refracting elements;

Figure 1b shows, by way of explanation, the optical equivalent of Figure 1a;

Figures 2a to 2c show respectively the microwave equivalents of a prism, a focussing lens and a focussing reflector;

Figures 3a to 3c show three microwave- deviating devices, each comprising a plurality of phase shifting elements;

Figures 4a and 4b relate to the device shown in Figure 3a and are graphs showing in Figure 4a the relationship required between the susceptances of the component elements for zero reflection and in Figure 4b the insertion phase shift;

Figure 4c also relates to the device of Figure 3a and is a diagram showing the equivalent of Figure 3a in terms of a transmission line loaded at intervals with shunt susceptances;

Figures 5a and 5b each show the transmission line equivalents of a number of devices;

Figures 6a and 6b relate to the device shown in Figure 3c and are graphs showing in Figure 6a the relationship between the normalised susceptance of the two outer sheets and the insertion phase shift and in Figure 6b the electrical spacing between adjacent layers and the insertion phase shift;

Figure 6c also relates to the device of Figure 3c and is a diagram showing the equivalent of Figure 3c in terms of a transmission line loaded at intervals with shunt susceptances; and

Figure 7 shows part of a sheet intended to alter the polarization of the incident radiation.

Figure 8a shows the phase shift required to achieve the focussing lens element of Figure 2b; Figure 8b shows the corresponding susceptance values for the inner and outer sheets of the device of Figure 3a, needed to achieve the phase shift variation of Figure 8a;

Figure 9 is a scrap view of part of a sheet constituting an element according to the present invention;

Figure 10 shows an interior view of a room in which antenna apparatus according to the invention is arranged adjacent to a window;

Figure 11 shows an exterior view of a house on which further antenna apparatus according to the invention is mounted; and

Figures 12a and b illustrate a method for steering antenna apparatus according to the invention in, which:

Figure 12a shows an exterior view of a house on which antenna apparatus with a steering system according to the invention is mounted, and

Figure 12b shows an exploded view of the antenna apparatus with steering system of Figure 12a.

As indicated by Figure lb. Figure 1a shows the electrical equivalents of a thin lens 1 , which produces a focussed beam, followed by a pair of thin prisms, 2 and 3, which can be used to produce a variable angular deviation of the beam. When the prisms are arranged in the attitude illustrated, the deviation produced by the first prism is cancelled by that of the second prism and there is no net angular deviation. Rotation of both prisms about the longitudinal axis of the system by equal amounts and in opposite directions produces an angular deviation in the plane of the paper. A subsequently rotation of the pair of prisms in unison about the same axis rotates the beam out of the plane of the paper. Thus, by proper selection of the two prism rotation angles, the beam can be steered anywhere within a cone having a semi-vertical angle equal to twice the deviation produced by each prism. In addition the centre of the cone can be offset from the above mentioned longitudinal axis by displacing the feed horn 4 transversely with respect to the longitudinal axis.

To avoid excessive interaction between the local evanescent fields of the three components of Figure 1a the separation between the components is preferably at least of the order of one wavelength.

In Figure 1a the variation' in the phase shift imparted by each of the sheets is denoted in diagrammatic fashion by a pattern of plus and minus signs denoting respectively phase shifts greater and less than an intermediate value which is denoted by dots. The representation used in Figure 1a is able only to designate three discrete values of phase shift, but it is to be understood, and it will be explained further below, that the variation in phase shift may be continuous except at the junctions between adjacent zones.

Figures 2a, 2b and 2c illustrate respectively three microwave equivalents of refracting elements, namely a prism, a focussing lens and a focussing reflector. For the element illustrated in Figure 2a to achieve the effect of a prism, the phase advance at a distance y from the longitudinal axis must be given by the following equation:

^ (y) = 2Try sin D - 2 pT where D = angular deviation A ~\ = wavelength p = any integer (1 ) For the element shown in Figure 2b to achieve the effect of a focussing lens, the phase advance at y must be given by the following equation:

(y) - 2τr (_/(F² + y²> - F) 2pπ where F - focal length λ

(2)

For the element—shown in Figure 2c to achieve the effect of a focussing reflector, the phase advance at y must be given by the following equation:

ø(y) - 3L (7(F² + y²) - F) - p * (3) λ

Equations (1) to (3) above assume that the refracting elements are electrically thin and can be treated as simple phase shifters. In practice, some minor modifications to these equations may be needed if the ray paths are not exactly perpendicular to plane of the element concerned.

In order to maintain the electrically-thin property, the phase shifting elements use the zoning principle, i.e. the actual phase shift is limited to a range of 360*C, or not substantially more than 360^βC. The theoretical phase shift required, i.e. that required to give e-qual path lengths for all ray paths , is reduced or increased by a multiple of 360', as required to fit within the range available from the medium being used for the phase shifting elements. This zoning is represented by the last term in each of equations (1) to (3). Zoning of the phase snifting element causes some bandwidth limitation compared to an element without zoning. However, the fractional bandwidth of the zoned component is approximately 0.5/N, where N is the number of zones, so that reasonably wide signal bandwidths can be obtained at microwave frequencies, even with a comparatively large number of zones .

Figure 8a is a graph of phase shift versus radial distance from the lens centre (measures in wavelengths) corresponding to equation (2), i.e. a focussing lens. The graph shows the case of a lens having a focal length of 50λ.

A phase shifting element according to the invention preferably comprises a plurality of spaced sheets. The susceptances of the sheets and their spacings are chosen so as to provide substantially reflectionless transmission through the rϊiase shifting element, with the correct phase shift at each point. Figures 3a to 3c show, by way of example, three examples of the form which such a element may take, and these will now be described.

Figure 3a shows four parallel thin dielectric membranes 10 (there could be more or less than four) each with a graded metallic coating. The sheets are held at the required spacings by a support frame 11. The susceptance varires radially across - each sheet in such a manner as to define a plurality of concentric zones, as can be seen in the plan view which is included in Figure 3a. Figure 8b is a graph showing the variation in normalised susceptance with radial distance from the centre in order for a device constructed as in Figure 3a to achieve the phase variation indicated in Figure 8a and act as a focussing lens. The solid lines indicate the variation in susceptance for the two outer sheets and the broken lines indicate the variation in susceptance for the two inner sheets.

An inductive susceptance can be obtained with a thin metallic coating, at least a few skin depths thick, which is perforated at regular intervals by symmetrical holes, for example circles, squares or crosses. The value of the susceptance obtained depends on the pitch and size of the holes, increasing as the hole diameter decreases (to an infinite value when the hole vanishes) and also increasing as the pitch increases. Similarly, a capacitative susceptance can be obtained with a complementary screen, i.e. one comprising a coating of small symmetric thin metallic obstacles arranged to form a periodic lattice, similar to that often employed in artificial dielectric media. Design equations relating normalised susceptance to obstacle dimensions can be found in the literature, for example in

KOCK, W.E. , "Metallic delay lenses", Bell System Tech.

Journal 1948, 27, pp 58-82. Typically the holes or obstacles, as the case may be, have a diameter of up to about λ/4 and are at a pitch of about λ/4 - λ/2.

— An example of part of a sheet having a varying susceptance is shown in Figure 9. The drawing is a scrap view of the first two zones. Starting from the lens centre 0 the first half zone Zla comprises a series of concentric rings of holes which decrease in diameter with increasing distance from the centre 0. In this way an inductive susceptance is provided which increases from a low value mean the centre 0 to a high value in the radially outer portion of the half zone Zla. The second half zone Zlb comprises a series of concentric rings of obstacles which decrease in diameter with increasing distance from the centre 0. In this way a capacitative susceptance is provided which decreases from a high value adjacent the half zone Zla to a low value in the radially outer portion of the half zone Zlb. This pattern is repeated in the following half zones Z2a and Z2b, and the zones (not shown) which are radially outward from half zone Z2b. The radial width of successive half zones decreases with increasing distance from the centre 0. The design shown in Figure 9 could, for example, be that of the outer (or inner) sheets of Figure 3a, in which the inner (or outer) sheets would have zones of the same size but with the susceptance varying in different manner.

If the incident radiation is plane polarised the holes or obstacles need not be symmetrical. Continuous slots or wires (arranged either perpendicular or parallel to the electric field vector, depending on the sign of susceptance required) or holes or obstacles of virtually any shape may then be employed.

The embodiment of Figure 3b shows four susceptance sheets 20 supported in pairs on opposite sides of two sheets 21 of solid dielectric. The dielectric sheets 21 with their associated susceptance sheets 20 are spaced apart by a spacing ring 22. This embodiment could be manufactured along the lines of a sealed double-glazing window unit and, indeed, could form a replacement for a window where it is desired to be able to install the associated transmitting/receiving apparatus within a building.

While the phase- shif ing components shown in Figures 3a and 3b are substantially flat, curved components can also be used, for example where it is desired that the component conforms to the curved surface of the existing antenna or installation or, as in the embodiment of Figure 3c, where it is desired to use a variable spacing between a number of susceptance sheets as an additional design parameter. The embodiment shown in Figure 3c comprises three sheets 30, 31 and 32 supported by two dielectric members 33 and 34 made, for example of a foam material. The members 33 and 34 each have the shape of a zoned plano¬ concave lens, the members being reduced in thickness at a step such as 35 where a phase shift of exactly 360* Is required. The outer sheets 30 and 32 each conform to a respective one of the stepped surfaces of the members 33 and 34 and each have an inner and one or more outer segments. The theory underlying the embodiment of Figure 3c is discussed below with reference to Figures 6a and 6c.

The above discussion has dealt mainly with the focussing properties of the phase shifting elements-* However, as has also been mentioned above another important property is . their reflection behaviour, since it is desirable to obtain substantially reflectionless behaviour. A single sheet can be used if no account has to be taken of reflection but If reflection is to be dealt with a plurality of sheets is required. This aspect is discussed below with reference to Figures 4 to 6. The discussion takes advantage of the fact that the basic design of a phase-shifting element can be per ormedusing the conventicnal theory applicable to transmission lines loaded at intervals with shunt susceptances. For initial design purposes the assumption is made that although each sheet in fact has a susceptance which varies over its surface it will behave as though each part of the sheet were part of an infinite sheet having the same susceptance value throughout, and the assumption is also made that the propagation path is substantially normal to the sheets.

Figures 4a to 4c relate to the embodiment shown in Figure 3a. Figure 4c is a representation of Figure 3a in terms of a transmission line as discussed above. The four sheets are shown as being spaced at quarter wavelength (90*) intervals with the susceptance of the two outer sheets being equal to B^ and the susceptance of the two inner sheets being equal to B2- The representation assumes that the effect of the dielectric support membranes is negligible.

Figure 4a is a graph in which the normalised susceptances of the inner and outer sheets required to give zero reflection are plotted against one- another. Y₀ is the intrinsic admittance of free space. Two curves are plotted in Figure 4a, one as a solid line and one as a broken line, from which it will be understood that for any given value of the susceptance of the outer sheets there are two possible values of susceptances for the inner sheets which result in zero reflection. The values represented by the solid line will be referred to below as the principal values for B2 and those represented by the dotted line will be referred to below as the alternative values for B2.

Figure 4b shows the insertion phase shift resulting from the use of particular values of the normalised susceptance of the outer sheets and the corresponding values of normalised susceptance for the inner sheets which give rise to zero reflection. Once again there are two curves, the solid line curve representing the result of using the principal values of B2 and the broken line representing the use of the alternative values of B2. It can be seen that a phase variation of over 360^* can be obtained by employing susceptance values ranging from about minus 1.5 to plus 1.5.

It will be appreciated that the same phase shift can often be obtained with differing sheet spacings and corresponding susceptance values. Figures 5a and 5b illustrate just two of the many possibilities which will provide phase shifts of - 90^* , 180* and + 90* with zero reflection. Figure 5a is for the case where there are four sheets spaced at λ/4 from one another (90^*) and Figure 5b is for^' the case where there are four sheets spaced at λ/8 from o-ne another (45°). - The exact design chosen will depend on the intended application. For example, the design could be adjusted to optimize the bandwidth In the operating frequency range or could be selected so as to provide stop band characteristics at some specific frequency outside the operating range if interference at that particular frequency were anticipated. Additionally, it should be noted that although Figures 4 and 5 assume that the designs are symmetrical, this is not essential; three or more different spacings and correspondingly four or more different susceptances may be employed.

Figures 6a to 6c deal with the case, as in Figure 3c, where the sheets are not parallel to one another. In this particular case, as indicated by Figure 4c, the ratio of the susceptance of the inner layer to that of each of the two outer layers has been chosen to be 2:1 throughout the range of phase shifts. This ratio is chosen because it is known from transmission line theory that such a ratio generally provides rather broad-band properties. Figure 6a shows the normalised susceptance of the outer sheets which corresponds to a given insertion phase shift, and Figure 6b shows the spacing between the sheets, represented in degrees as the electrical spacing θ for given values of insertion phase shift. It will be appreciated that similar design curves can be produced for other susceptance ratios and that a very wide range of spacings is available to the designer.

The description given above implies that the susceptance values vary continuously over each sheet, except at the steps "between adjacent zones, so as to provide the requisite continuous change of phase shift while maintaining reflectionless transmission. With some manufacturing techniques it may be more convenient or economical to employ a small number of fixed susceptance ratios, even though this may result . In minor phase discontinuities and/or reflections. Thus, referring back to Figure 5b it can be seen that only two susceptance values (B - 2 and B - 4) need be employed if phase steps of 90* are acceptable.

A conventional circularly polarized antenna system often employs a separate circular polarizer located at the feed horn or at the aperture of the focussing objective. It is possible to incorporate a circularly-polarizing feature into an element according to the present invention. For example, extrapolating Figure 4, some 450* of phase change can be obtained by employing susceptance values ranging from about - 2.8 to + 2.8. Thus, at some point in a lens requiring a nominal phase shift of, say, X*, susceptance values appropriate to (X-45)* and to (X+45)* can be selected. Asymmetrical susceptance sheets can then be designed to provide the two sets of values for the two orthogonal linear polarizations .

Figure 7 illustrates one method of obtaining a susceptance sheet with differing susceptance values for, say, vertical and horizontal polarization. This is achieved by using apertures (or obstacles) which differ as viewed in the horizontal and vertical directions. In the example of Figure 7 there are apertures (or obstacles) which are in the form of crosses having horizontal arms longer than their vertical arms. The major (and hence the minor) axes of the apertures or obstacles of each sheet are aligned with the corresponding axes of each of the other sheets . The overall assembly of the sheets provides reflectionless transmission for either vertical or horizontal polarization, but with o an insertion phase difference of 90 . Hence, when fed by a linearly-polarized source at 45 inclination the output is a circularly-polarized wave. In the foregoing description phase shifting elements according to the invention have been treated in effect in isolation. Antenna apparatus incorporating elements according to the invention will now be described. Also, a method of steering antenna apparatus using such elements will be described.

Fig 10 shows an example of antenna apparatus incorporating a lens element according to the invention. This embodiment is illustrated in a special arrangment inside a window of a house eg. for domestic reception of direct broadcast by satellite. The antenna element 40 comprises a lens element according to the invention (see fig 2b) implemented in a substantially planar screen device, here shown mounted directly against a window plane 41. The lens element 40 is structured analogously to an offset paraboloid antenna and uses a conventional offset feed 43 located at its focus. A conventional low noise down convertor 44 is attached to the offset feed 43 and the rf signal from the down converter is fed through a cable 45 to a television receiver 46.

Fig 11 shows antenna apparatus incorporating a focusing reflector element according to the invention (see fig 2c). This antenna apparatus is also illustrated arranged to provide domestic reception of direct broadcast by satellite.

The focusing reflector antenna element 50 is shown in Fig 11 mounted on an outside wall of a house. The antenna element 5 could alternatively be mounted on the roof or a side of the chimney stack. In this embodiment the antenna element is arranged to operate in a reflection mode using a conventional feed (in this case an offset feed 53 located at the focus. A low noise down convertor 54 is connected to the feed 53 and the rf signal from the down convertor is passed through a cable into the house.

The antennas of figs 10 and 11 are shown mounted on or in south-facing walls. In order to receive transmissions from satellites in equitorial orbits dbs antennas located in the northern hemisphere should point in a generally southward direction (whereas antennas in the southern hemisphere should point .generally northward) .

Figs. 12a and b illustrate the use of prism elements according to the invention in a method for steering an antenna

Fig 12a shows an example of a flat plate antenna with steering system mounted on an outside wall of a house. Fig 12b shows the general structure of the antenna of fig 12a and is drawn with the elements spaced for greater clarity. Steering elements 6la and 6lb (each according to Fig 2a) are used to deviate the incoming beam so as to provide a substantially parallel beam incident on the antenna element 62. The antenna element 62 may be a conventional flat plate array (as shown) or some other focusing element, for example a lens element according to the invention (in which case this arrang ent conforms to fig la) . The steering elements (shown here with a multi-layer structure) could either be set manually so that the aerial points at one particular satellite or there could be provision for steering the aerial. As described in relation to figure la the prism elements 6la and b are rotated in order to achieve beam steering. While it would be possible to provide a mechanical arrangment for rotating the steering elements it is more convenient, particularly in a domestic setting, to provide means 5 for electronically steering the aerial. Electronic steering may be achieved using stepper.-s otors mounted on the aerial, with the user having an electronic control unit sending signals to the stepper motors.

The signals from the antenna element 62 pass along a cable to a low-noise down convertor 64 and the rf signals therefrom pass along a further cable into the house.

A polarizer screen ( if required ) may also be provided in this antennae in one of the 5 two positions 63 or 63'_* A weather proof enclosure 65 may also be provided and may protect the stepper motors in additional to the antenna assembly as shown.

It is to be understood that although the 0 invention has been described above inconnection with domestic dbs antennas the invention is fact of general application.

Also, the phase shifting elements according to the invention may be used in antennas in other ways ζ than those described above. For example, a single phase shifting element of the "prism" type may be used with a conventional antenna to steer the beam in a predetermined direction.

Claims

CLAIMS :

1. A phase shifting element, comprising: at least one sheet, of substantially constant thickness and adapted to impart to electromagnetic radiation incident thereon a phase shift varying with the location thereon at which incidence occurs.

2. A phase shifting element according to claim 1, wherein said at least one sheet is adapted to define a plurality of zones such that the phase shift imparted by the at least one sheet varies discontinuously across each zone boundary.

3. A phase shifting" element according to claim 1 or 2, wherein said at least one sheet is a plurality of sheets spaced one from the other and each sheet comprises a thin dielectric member bearing a metallic coating.

4- A phase shifting element according to claim 3_. wherein the susceptance of each sheet and the spacing of the sheets are adapted to provide substantially reflectionless transmission of electromagnetic radiation incident on the phase shifting element.

5. A phase shifting element according to claim 4 wherein each of said sheets has a susceptance varying with spatial location thereon.

6. A phase shifting element according to claim 5_. wherein the metallic coating of at least one of said sheets comprises a plurality of perforations, the size and pitch of said perforations varying over the surface of the sheet. 7. A phase shifting element according to claim 5_. wherein the metallic coating of at least one of said sheets comprises a plurality of obstacles, the size and pitch of said obstacles varying over the surface of the sheet .

8. A phase shifting element according to claim 6 or 7_. wherein said perforations or obstacles are of a uniform, asymmetric, shape.

9. A phase shifting element according to claim 2, wherein the at least one sheet is adapted to impart a phase shift varying discontinuously in said zones in addition to varying discontinuously at zone boundaries.

10. A phase shifting element according to claim 1, wherein said at least one sheet is adapted to impart; a phase shift varying in accordance with the following equation:

0 (_y) = 2TTy , sin D Λ

where 0(y) is the phase shift imparted to radiation of wavelength ^"λ incident at a distance y from the longitudinal axis of the phase shifting element, and

D is the angle of deviation produced by the phase shifting element.

11. A phase shifting element according to claim 2, wherein said at least one sheet is adapted to impart a phase shift varying in accordance with the following equation:

ø (y) = 2τr v sin - 2 _P τr where 0(y) is the phase shift imparted to radiation of wavelength A incident at a distance y from the longitudinal axis of the phase shifting element,

D is the angle of deviation produced by the phase shifting element, and p is an integer.

12. A phase shifting element according to claim 1 wherein said at least one sheet is adapted to impart a phase shift varying in accordance with the following equation:

0 (y) = _ J3L ( _F2_{+ y}2 __F) where 0(y) is the phase shift imparted to radiation of wavelength ~\ incident at a distance y from the longitudinal axis of the phase shifting element, and

F is the focal length of the phase shifting element.

13. A phase shifting element according to claim 2, wherein said at least one sheet is adapted to impart a phase shift varying in accordance with the following equation:

0 (y) = ² - (✓ _F2_{+ y}2 -F) - 2_P- where 0(y) is the phase shift imparted to radiation of wavelength ^ incident at a distance y from the longitudinal axis of the phase shifting element,

F is the focal length of the phase shifting elemen , and p is an integer.

1 . A phase shifting element according to claim 1, wherein said at least one sheet is adapted to impart a phase shift varying in accordance with the following equation:

where 0 (y) is the phase shift imparted to radiation of wavelength ~X incident at a distance y from the longitudinal axis of the phase shifting element, and F is the focal length of the phase shifting element.

15. A phase shifting element according to claim 2, wherein said at least one sheet is adapted to impart a phase shift varying in accordance with the following equation:

ø (y) = ~ - (/ T - F ⁾ - p-rr

where 0 (y) is the phase shift imparted to radiation of wavelength incident at a distance y from the longitudinal axis of the phase shifting element,

F is the focal length of the phase shifting element, and p is an integer.

16. Antenna apparatus comprising a phase shifting element according to claim 12 or 13«

17• Antenna apparatus comprising a phase shifting element according to claim 14 or 15-

l8. Antenna apparatus comprising two phase shifting elements according to claim 10 or 11, and an antenna, .the longitudinal axis of said two phase shifting elements being arranged in line with the axis of said antenna and the relative orientation of the two phase shifting elements being such that a relative rotation of the phase shifting elements about the axial direction alters the beam pointing direction of the' antenna. 19. Antenna apparatus according to claim 18 wherein the antenna is a phase shifting element according to claim 12 or 13•

20. A method of steering an antenna using two phase shifting elements according to claim 10 or 11, comprising the steps of: a) aligning the longitudinal axis of a first of said two phase shifting elements with the axis of the antenna; b) aligning the longitudinal axis of the second of the two phase shifting elements with the longitudinal axis of the first phase shifting element on the side of the first phase shifting element remote from the antenna, the second phase shifting element being oriented upside down relative to the first phase shifting element; and c) rotating the phase shifting elements relative to one another about the axial direction.