US3530475A - Active zone plate lens antenna - Google Patents

Description

p 2, 1970 w. E. DANIELSQN 3,530,475

ACTIVE ZONE PLATE LENS ANTENNA Filed Aug. 26, 1966 4 Sheets-Sheet 1 FIG.

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Sept. 22, 1970 Filed Aug. 26. 1966 United States Patent 3,530,475 ACTIVE ZONE PLATE LENS ANTENNA Warren E. Dauielson, Fair Haven, N.J., assiguor to Bell Telephone Laboratories, Incorporated, Murray Hill, N.J., a corporation of New York Filed Aug. 26, 1966, Ser. No. 575,429 Int. Cl. H01q 3/26, 15/02 US. Cl. 343-754 9 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a system for focusing electromagnetic energy; more particularly this invention relates to an electronically tunable active zone plate lens antenna.

A zone plate is a structure having concentric zones which are alternately transparent and opaque to incident electromagnetic energy. Points in the transparent zones act as secondary sources of electromagnetic energy and, according to Huygens principle, scatter the incident energy in spherical waves. It is possible to select the transparent zones such that there exists a focal point where energy from all scattering zones adds constructively; (see IRE Transactions on Antennas and Propagation, May 1961, volume AP-9, No. 3 pages 319-320, The Zone Plate as a Radio Frequency Focusing Element). By providing gain at the focal point, the Zone plate functions as a lens antenna for received signals. By placing an isotropic source of electromagnetic energy at the focal point, or focus, the zone plate can also be used as a directional transmitting antenna.

Typical zone plate applications have been described in US. Pat. 3,189,907 issued on June 15, 1965 to L. F. Van Buskirk. These and other well-known applications employ zone plates having fixed geometrical configurations and therefore fixed frequency response. That is, once the zones are constructed, a physical replacement of components is necessary to operate at a different frequency. Also, because of the rigid construction of previously-used zone plates, any tilting or distortion from the fixed geometry will result in degraded performance. Existing zone plates rely on the mechanical arrangement of zones and the precise orientation of the planar ensemble to achieve the desired constructive interefence.

Briefly stated, one embodiment of the present invention is an active zone plate lens antenna comprising means for actively selecting from a plurality of scattering elements those elements which in response to incident electromagnetic signals give rise to in-phase energy at a feed located at the focal point. The selection process is accomplished electronically by using auxiliary local sources of electromagnetic radiation. The auxiliary sources are spatially arranged and relatively phased so as to activate threshold devices located at the chosen scattering elements by constructive interference. Those scattering elements whose threshold devices are not activated do not provide any substantial scattered energy.

It is therefore an object of the present invention to provide for electronic control of the propagation of electromagnetic waves through a medium.

It is another object to provide electronically controllable focusing of electromagnetic waves.

It is another object to provide a zone plate antenna which, under electronic control, can operate over a wide range of frequencies.

It is another object to provide a large, economical directional antenna for deployment in space.

It is another object to provide a nominally planar antenna which is insensitive to inclination of the plane or distortions from planarity.

One feature of the present invention is that it comprises a large number of identical scattering elements with associated threshold devices which lend themselves to easy manufacture, typically by printed wiring and microelectronic techniques.

Another feature of one embodiment of the present invention is that it comprises a zone plate constructed such that the spatial arrangement of components need not be critically maintained.

Another feature of the present invention is that it provides for the activation, through the agency of electromagnetic fields, of scattering elements which are possessed of no internal source of power.

Other objects, features and related advantages of this invention will be readily appreciated by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 shows the intersection of a family of confocal paraboloids with posible scattering surfaces;

FIG. 2 shows a schematic representation of an active zone plate antenna having two auxiliary illuminating sources;

FIG. 3 shows one quadrant of an illuminated surface with activated scattering elements;

FIG. 4 shows one embodiment of an active scattering element;

FIG. 5 shows a configuration associated with a numerical example; and

FIGS. 6a, b, c and d compare ideal zone plate boundaries with those generated by one embodiment of the present invention under various conditions of nonideal alignment.

It will be assumed for present discussion purposes that it is desired to receive plane waves from a distant source. FIG. 1 shows the traces in the plane of the page of two possible surfaces 10 and containing elements capable of scattering incident energy. A feed is located at the point at which it is desired to collect the incident energy. A simple geometric exercise will show that the intersec tion of a family of confocal paraboloids with surfaces 10 or 20 defines the points from which scattered energy will arrive in-phase at the paraboloidal focus, F. These paraboloids, indicated by their partial traces 25, have the feed 30 as the focus and are spaced a distance /2 apart along the axis of symmetry which for purposes of explanation has been chosen parallel to the direction of propagation of the incident energy. Here, A, is the wavelength of the incident energy.

If surface 10 is chosen to be a plane perpendicular to the direction of propagation of incident energy, the intersection of the paraboloids with the plane will mark a series of concentric circles on the plane. These circles define the centers of the transparent zones of a zone plate. Of course, scatterers in a band about these circles will give rise to energy which arrives nearly in-phase at the focus. These bands then constitute the complete transparent zones of the zone plate.

FIG. 2 shows a schematic representation of a typical embodiment of the present invention. Here the scattering surface is taken as a plane oriented perpendicular to the direction of propagation of incident energy. Also shown are auxiliary illuminating sources and located on a line 51 passing through the above-mentioned paraboloidal focus and perpendicular to the plane, i.e.,

the axis of symmetry for the paraboloids. This axis 51 will subsequently be referred to as the focal axis. The illuminating sources 40 and 50 are taken for present purposes to be coherent isotropic sources independent of the incident electromagnetic energy. The isotropic field pattern of these coherent sources is such as to give rise to constructive addition at points along hyperboloids having the two auxiliary sources 40 and 50 as respective foci F and F The partial trace 52 of one of these hyperboloids in the plane of the page is shown in FIG. 2. Also shown in FIG. 2 is the partial trace 54 of one of the paraboloids whose intersection with the scattering plane marks the location of scatterers giving rise to in-phase scattered energy at the focus 30. The embodiment of the invention presently under discussion relies on the fact that a high degree of congruence between the two traces 52 and 54 is possible in the vicinity of the plane 35.

Each scattering element in the plane has associated with it a threshold device responsive to radiation from the illuminating sources. The threshold is chosen so that only those scatterers illuminated by in-phase or nearly inphase signals from the auxiliary sources will be activated. By adjusting the threshold appropriately, the width of a transparent zone can be made to closely approximate that of a corresponding passive zone plate.

FIG. 3 shows one quadrant of a planar array of scattering elements as illuminated by the two auxiliary sources 40 and 50 shown in FIG. 2. Each of the crosses represents an activated scattering element. The totality of activated scattering elements in a given band comprises a transparent zone of a zone plate. The areas between successive activated areas then represent the regions of the zone plate which do not serve as strong scatterers. They play a role analogous to that of the opaque regions of a conventional zone plate lens.

An enlarged schematic view of one possible type of scattering element is shown in FIG. 4. The relatively larger dipole 75 is cut to be responsive at the signal Wavelength A and the relatively smaller dipole 60 is cut to be responsive at the illuminating frequency M. Signals arriving in-phase at a particular dipole 60 from the pair of illuminating sources 40 and 50 are selected by a tuner detector 65. This tuner detector typically might take the form of a tuned circuit followed by a rectifier. A capacitor 70 is shown as one means for smoothing the detected output. This smoothed output is then applied as a bias to a threshold element 80 which might illustratively be a diode. The threshold element is then effective in controlling the scattering characteristics of the dipole 75. This control can be exercised by having the bias exceed the threshold and thereby short circuit the dipole elements.

It is possible to adjust the various parameters shown in FIG. 2 to achieve a high degree of congruence between the hyperboloidal surfaces 52 and the paraboloidal surfaces 54 in the vicinity of the plane. Typically, it is possible to cause one hyperboloidal surface to be substantially congruent with one paraboloidal surface in the vicinity of the plane. Other paraboloidal surfaces can be approximated with slightly less precision. Accordingly, for a zone plate having many zones, it is desirable to match exactly a paraboloidal surface defining some intermediate transparent zone, rather than attempting exact matching at the innermost or outermost transparent zone.

One step-by-step procedure for achieving the desired congruence between paraboloids and hyperboloids is given below. Before this is presented though, the general rationale underlying the procedure will be presented.

Each of the hyperboloidal surfaces 52 in FIG. 2 has a plane tangent to it at each point of intersection with the scattering plane 35. These tangent planes generate a tangent cone having an apex located on the focal axis 51 for each hyperboloid. Similarly, each of the paraboloidal surfaces at its intersection with the scattering plane also has associated with it a tangent cone with an apex located on the focal axis 51. Thus there are two families of nested cones: one associated with the hyperboloids and one associated with the paraboloids.

Referring to FIG. 2, let x be the distance between the scattering plane and the mid-point between the two auxiliary sources. Let b be the distance between the midpoint between the auxiliary sources and the intersection of a given hyperboloid with the focal axis. If x is chosen much larger than b for each hyperboloid all of the tangent cones for the hyperboloids tend toward a common apex located midway between the two auxiliary sources. In FIG. 2, this requires that b /x approach zero.

Let y be the distance from the scattering plane to the focus of the paraboloids and let p be the distance from the common paraboloidal focus to the intersection of a given paraboloid with the focal axis. For all those paraboloids for which y can be chosen much greater than p, the associated tangent cones will have apexes which are nearly coincident at a point located a distance y to the left of the paraboloid focus, or a distance approximately 2y from the scattering plane.

For the case where x is much greater than b and y is much greater than p, the problem of achieving congruence between the paraboloidal surfaces and the hyperboloidal surfaces in the vicinity of the scattering surface then reduces to one of achieving near coincidence of the two common apexes.

Let c be half the distance between the auxiliary sources. In the practical case where c and a are small compared to x, a value for c may be chosen which is given in terms of signal wavelength, A and illumination wavelength, M, y

1 11+Pm=z pref ref (4) Calculate x and b using 2 x2 ref= ret) and 2 2 Tref i pref+y) s (4:)

where Eq. 3 forces the reference circles (intersections with lens plane) to coincide, and Eq. 4 makes the reference zone widths equal.

To see how well the proposed illumination pattern approaches the ideal, it is helpful to take a specific example which uses parameters likely to be of practical interest.

Problem: Desired beamwidth of lens, 10 angular mils;

a angle subtended at F; approximately 70 to A chosen as 20x Solution: Measure all distances in terms of k Then giving r =50 (10 mil beam) 3 z r cot 2 c-2y/2O which would make equal 6.5. It happens that a silghtly larger value, 6.61, was chosen for initial calculations, and this value will be retained to save recalculation. As noted earlier, the exact choice isnt important. We thus use Next we take r =30 (about .6 to .7r and from Eq. 2 we find Using Eq. 1,

Eqs. 3 and 4 can now be evaluated to give =(13.16+n) (sang- 2) for the ideal zone boundaries, and

m [sal 1 0 1 for the hyperbola-generated boundaries.

These equations are to be evaluated for successive integer values of n starting with 12. The resulting hyperboloidal zones are compared with the corresponding paraboloidal zones in FIG. 6a. To investigate sensitivity to displacement of the zone plate surface, similar calculations have been made in planes a distance of 20' to each side of the nominal lens plane. These results are shown in FIGS. 6b and 6c for the cases where the lens plane is closer to F and farther from F respectively.

Referring again to the numerical example, we see that an axial translation of the lens plane by 20 wavelengths (with F, F and F all fixed) produces a mismatch of about /3 zone width at the 10th zone (i.e., at the edge of a lens 35 wavelengths in radius). The quadratic dependence of this mismatch on translation distance assures that we can tolerate displacement of at least 10 wavelengths without experiencing a shift of more than about A of one zoneclearly an easily tolerable shift.

The allowable tolerance to changes in the W distance is also comfortable if We use a change in the illumination frequency as a compensation means. For the above example, the effect of a 10 change is IT, is quite accurately counteracted by an adjustment of about 14 percent in the illuminator frequency (see FIG. d).

If a steeper gradient in illumination intensity is needed to better define the zone boundaries, one or two more equi-spaced elements may be added in line with P and F One effect of this modification would be a narrowing of the zones used with a related small decrease in the antenna gain achieved.

It should be noted that additional combinations of illuminators and signal feed (each combination looking in a different direction), can make simultaneous use of the same active lens. The use of these additional combinations will, however, cause some reduction in antenna gain and an increase in sidelobe level. The maximum angular separation for the numerical example would be about 2.0 or 30 degrees.

Many advantages of the above embodiment of the present invention are at once apparent. Because the near congruence of the hyperboloids and the desired paraboloids extends for a considerable distance from the nominally planar lens surface, it is clear that departures of the lens surface from planarity will have minimized effect. Likewise, tilting of the plane will not introduce serious defocusing in any but the most severe cases of misorientation.

These latter two properties of this invention suggest as a possible application that of a space antenna. Because the above embodiment is largely a planar structure it would lend itself especially well to deployment in space. Its tolerance to planar distortion and tilting is of great importance in such an application because it relieves some of the burden on the attitude control and station keeping systems.

It is to be understood that the embodiment depicted above is but one of many applications in which the present invention may be used to advantage. Variations of that embodiment can easily be designed while retaining the essential features of the present invention. For example, it may be desirable to use broad-banded elements such as log-periodic elements in place of the di poles shown in FIG. 4. Similarly, the illuminating signals might be received and detected in a different manner. Some applications might advantageousuly use other than colinear sources to achieve the desired interference patterns. Likewise, the present invention is not limited to scattering element selection by the interference of coherent sources; it might be advantageous to intentionally introduce phase delays between sources to produce desired interference patterns.

It might also be desirable to select scatterers by other than interference patterns. For example, a rapidly scanning laser could be used to pulse signals in the vicinity of scatterers to be activated. A photoelectric device could then be used to activate the scattering element.

A more generalized focusing of electromagnetic waves may be achieved using a variation. of the present invention. Scattering elements can illustratively be located in a uniform three-dimensional array. Electromagnetic waves passing through this portion of space are then defiected or focused by the selective activation of these scattering elements.

The binary conditions of scatter or no-scatter are not essential to the present invention. Some applications may use scatterers having gradations of transparency, i.e., they may impose selected losses on incident signals. This could give rise to interference patterns having a variety of properties.

The above embodiments are by no means meant to exhaust the possible variations implicit within the spirit of the present invention. Other variations and embodiments will occur to those skilled in the art.

What is claimed is:

1. An active lens antenna comprising a nominally planar array of scattering elements, a plurality of electromagnetic illuminating sources having equal frequency and fixed relative phase, activating elements individual to said scattering elements and responsive to the vector sum of incident energy from said illuminating sources, and an antenna feed located at the focal point of said lens.

2. An antenna as in claim 1 wherein each of said activating elements includes additionally a threshold dev1ce.

3. An antenna as in claim 2 wherein selected ones of said scattering elements are completely activated and the remainder of said scattering elements are completely inactivated, said activated and inactivated elements forming zones which are respectively analogous to the transparent and opaque zones of a zone-plate.

4. An antenna as in claim 3 wherein said activated elements form bands about the points where signals from said electromagnetic illuminating sources add in-phase.

5. An antenna as in claim 4 wherein said illuminating sources comprises two coherent sources located on the axis passing perpendicularly through the center of said zone-plate.

6. A medium for the controlled propagation of electromagnetic energy comprising a plurality of scattering elements arranged to form a quasi optical lens, at least one source of radiated electromagnetic control signals, and activation means individual to said scattering elements and responsive to said radiated electromagnetic control signals for selectively activating said elements.

7. A medium as in claim 6 wherein each of said activating means comprises a switch and electromagnetic energy receiving means for controlling said switch in response to said electromagnetic control signals.

References Cited UNITED STATES PATENTS 2,840,820 6/ 1958 Southworth 343754 3,276,023 9/1966 Dorne et al. 343-754 3,392,393 7/1968 Spitz 343-454 ELI LIEBERMAN, Primary Examiner US. Cl. X.R.

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