Phase correcting reflection zone plate for focusing microwave

Details Phase correcting reflection zone plate for focusing microwave Phase correcting reflection zone plate for focusing microwave

1993-08-09

1995-02-14

Hajec, Donald

Communications: radio wave antennas

Antennas

Antenna components

343914, H01Q 1502

Patent

active

053899440

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

This invention relates to a zone plate for focusing microwave energy and in particular to a phase correcting reflection zone plate for focusing microwaves. This invention also relates to apparatus and a method for manufacturing such a zone plate.
The use of zone plates for focusing microwaves is well known. One particular type of zone plate disclosed in "Millimeter-Wave Characteristics of Phase-Correcting Fresnal Zone Plates" by D. N. Black and J. Wiltse, IEEE Transactions on Microwave Theory and Technique Volume 35 No. 12 (1987) Page 1122-1128, is the phase-correcting Fresnel zone plate. Such a zone plate is shown schematically in FIG. 1 for quarter wave correction, although a phase correcting zone plate can be made for any wavelength fraction. The radius of each zone r.sub.n can be given by ##EQU1## where n is the zone number, f is the focal length of the zone plate, .lambda. is the wavelength of the radiation and P is an integer greater than 2. For quarter wave correction P=4. For such a zone plate both in and out of phase zones contribute to the energy at the focus thus increasing the efficiency compared to a conventional zone plate. The correction of the phase of the zones is achieved by changing the path length of the energy reflected from that zone. Thus the energy reflected from the zone 2a of the quarter wave zone plate of FIG. 1 would be out of phase with respect to the energy from the zone 3 by .lambda./4 at the focus, unless the pathlength was decreased or increased by .lambda./4. An increase in pathlength of .lambda./4 is achieved by providing steps .lambda./8 in depth. Thus zone 2a is .lambda./8 higher than zone 3 and zone 2b and 2c are .lambda./4 and 3.lambda./8 higher than zone 3 respectively. More generally, the different phases of the zones of the zone plate are stepped by d where
The construction of such a zone plate may be achieved by a number of manufacturing processes such as machining out of solid metal, stamping out of a thin metal sheet, moulding and subsequently metallising a plastic material or by vacuum forming plastics.


SUMMARY OF THE INVENTION

According to the present invention a reflection zone plate for focusing microwave energy comprises a plurality of reflective portions corresponding to zones of said zone plate; said reflective portions being positioned in P parallel planes, so that each said reflective portion reflects energy .lambda./P out of phase with respect to adjacent reflective portions, where .lambda. is the wavelength of the energy, such that energy reflected from said reflective portions constructively interfere at a focus of said zone plate; wherein the reflective portions in each said plane are formed on respective low dielectric loss sheets.
In a preferred embodiment each reflective portions reflects energy .lambda./4 out of phase with respect to adjacent reflective portions and said reflective portions are positioned in 4 parallel planes and separated by an electrical thickness of .lambda./8. In such an arrangement the sheets may be constructed from a plastics material.
The present invention also provides apparatus for the manufacture of such a zone plate, comprising means to apply reflective portions corresponding to zones of said zone plate to the surface of a plurality of sheets of low dielectric loss material; and means to stack said sheets of low dielectic loss material to form said reflection zone plate.
The present invention further provides a method of manufacture of such a zone plate comprising the steps of applying reflective portions corresponding to zones of said zone plate to the surface of a plurality P of sheets of low dielectric loss material, and stacking said plurality of sheets of low dielectric loss material to form said reflective zone plate.


BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the invention will now be described with reference to the drawings, in which:
FIG. 1 illustrates a cross-section of a prior art quarter wave zone plate;
FIG. 2 illustrates a cross-section of a quarter wave zone plate c

REFERENCES:
patent: 1906546 (1933-05-01), Darbord
patent: 2856602 (1958-10-01), Carlin
patent: 2921312 (1960-01-01), Wickersham, Jr.
patent: 3189907 (1965-06-01), Buskirk
patent: 3530475 (1970-09-01), Danielson
patent: 4905014 (1990-02-01), Gonzalez et al.
"International Journal of Infrared and Millimeter Waves", vol. 2, No. 3, Mar. 1991, New York, US, pp. 195-219; J. E. Garrett et al., Fresnel Zone Plate Antennas At Millimeter Wavelengths, see pp. 209-210.

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