Plasma controlled antenna

Details Plasma controlled antenna Plasma controlled antenna

2002-01-30

2003-09-16

Wimer, Michael C. (Department: 2821)

Communications: radio wave antennas

Antennas

With spaced or external radio wave refractor

C343S755000, C343S910000

Reexamination Certificate

active

06621459

ABSTRACT:

TECHNICAL FIELD
The present invention relates generally to a scanning antenna. More particularly, the present invention relates to a plasma controlled scanning antenna operable in the microwave (&mgr;W) or millimeter wave (MMW) bands, for example.
BACKGROUND OF THE INVENTION
Scanning antennas are necessary to form and scan an electromagnetic beam. Historically, there have been generally two types of scanning antennas, either mechanically scanned or electronically scanned. Mechanically scanned antennas perform scanning by forming a fixed beam with the antenna and physically moving the antenna. Electronically scanned antennas have been based on phased arrays which often employ hundreds to thousands of phase shifters to individual elements or groups of elements.
Mechanically scanned antennas are generally slower than desired and require precision hardware which is often expensive. Because mechanically scanned antennas rely on moving parts, reliability is an issue. Electronically scanned phased array antennas offer many advantages, but the large numbers of phase shifters make such systems costly.
Accordingly, alternative scanning methods have been of recent interest. Generally, these alternative methods are motivated by a desire for higher performance at lower cost. For example, a non-mechanical scanning antenna, without phase shifters, has been developed and is based on a type of Fresnel zone plate. The antenna forms and steers a beam of millimeter wave or microwave radiation using a light-modulated photoconducting wafer. See, e.g., U.S. Pat. No. 5,159,486 to Webb, entitled “Instrumentation apparatus and methods utilizing photoconductors as light-modulated dielectrics”; U.S. Pat. No. 5,360,973 to Webb, entitled “Millimeter Wave Beam Deflector”; Webb et al., “Light-Controlled MMW Beam Scanner”, Proc. 1993 SBMO International Microwave Conference, Vol. II, Sao Paolo, Brazil, IEEE Cat. No. 93TH0555-3, p. 417; and Webb et al., “MMW Beam Scanner Controlled by Light”, Proc. Workshop on Millimeter-Wave Power Generation and Beam Control, Huntsville, Ala., Special Report RD-AS-944, U.S. Army Missile Command, 1993, p. 333, the entire disclosures of which are incorporated herein by reference.
As another alternative, antennas have been developed which use at least two thin semiconductor reflecting plates (e.g., silicon) which are supported (e.g., on glass) and separated by a synthetic foam spacer of dielectric constant near one. There are, however, disadvantages associated with such technique. The use of two or more plates presents complications which require the spacing of the plates to be controlled. A synthetic foam spacer is fragile and easily damaged either mechanically or by temperature. The use of thin plates, especially in the case of silicon of about 50-200 &mgr;m in thickness, makes it difficult to achieve the required plasma density under photo-injection because of the effect of surface mediated recombination in the thin plates. See, e.g., U.S. Pat. Nos. 5,084,707, 5,585,812 and 5,736,966, each to Reits.
Recently, antennas have been disclosed which use a single photoconducting plate, e.g. silicon, and a transparent millimeter wave reflector. See, e.g., Webb et al., “Photonically Controlled 2-D Scanning Antenna,” PSAA-8 Proceedings of the Eighth Annual DARPA Symposium on Photonic Systems for Antenna Applications, The Naval Postgraduate School, Monterey, Calif., Jan. 13-15, 1998 (available from DTIC No. AD-B233444); Webb et al., “Experiments on an Optically Controlled 2-D Scanning Antenna,” 1998 Antenna Applications Symposium, Allerton Park, Monticello, Ill., Sep. 16-18, 1998, p. 99; Webb et al., “Optically Controlled Millimeter Wave Antenna,” Proceedings International Topical Meeting on Microwave Photonics, Melbourne, Australia, Nov. 17-19,1999, p.275; and Webb et al., “Novel Photonically Controlled Antenna for MMW Communications,” Proceedings International Topical Meeting on Microwave Photonics MWP 2000, Oxford UK, Sep. 11-13, 2000, p. 97. However, there is no indication of optimum thickness of the photoconducting plate, the nature of the transparent millimeter wave reflector, or the MMW phase relations of the wafer which are desirable for best performance.
In view of the aforementioned shortcomings associated with existing scanning antennas, there remains a strong need in the art for a further improved scanning antenna.
SUMMARY OF THE INVENTION
An improved plasma controlled millimeter wave (MMW) or microwave (&mgr;W) antenna is provided in accordance with the present invention. A plasma of electrons and holes is photo-injected into a photoconducting wafer. A special distribution of plasma and a MMW/&mgr;W reflecting surface behind the wafer allows the antenna to be generated at low light intensities and a 180° phase shift (modulo 360°) to be applied to selected MMWs/&mgr;Ws. The selected phase change produces superior performance over similar antennas without the phase change.
As is known, Fresnel zone plates (FZP) are of two general types, blocking and phase correcting. The simplest form of FZP works by blocking radiation. Rays going through different parts of an aperture add in-phase or out-of-phase at a detection point. If those rays which add out of phase are blocked, then there is a large gain in received intensity. Generally the phase conditions which produce a large increase in power are present in a given direction and thus the FZP produces a beam of radiation in that direction.
In previous transmissive-type antennas, a technique was used which involved a transient blocking FZP in which a spatially varying density of plasma of charge carriers, electrons and/or holes, was created by optical injection into a semiconductor or photoconductor wafer. The un-illuminated parts of the photoconductor with no plasma allow incident MMW from a feed behind the wafer to be transmitted through the wafer. In the illuminated regions, however, the photo-injected charge carriers alter the index of refraction of the wafer locally. At sufficient light intensity the plasma density was large enough to substantially block MMW in those local lighted regions; at large enough plasma density the plasma caused the transmitted MMW to asymptotically approach zero in magnitude. The wafer, modified by light in this way, is made to diffract incident radiation into a beam and thus comprised a transient FZP. Because the wafer responds rapidly to changes in optical injection, it is possible to change rapidly transient Fresnel diffractive conditions and thus rapidly change the beam direction.
In accordance with an exemplary embodiment of the present invention, a MMW feed is positioned in front of the wafer and an optically transparent MMW reflecting surface (reflector) is positioned in close proximity to the back surface of the wafer. The reflector is designed to be highly reflecting to MMW but transmit visible or infrared light of a wavelength below the band gap of the wafer in order to photo-inject plasma. A controllable light source behind the reflector can be positioned close to the reflector to minimize the need for focusing optics for the light patterns. The wafer thickness is chosen to be nominally an odd integer multiple of the wavelength of the MMW in the wafer material. With this choice of parameters MMW incident on a lighted region of the wafer containing plasma will be phase shifted by nominally 180° from MMW incident on a dark region.
These features of the present invention enable two advantageous modes of operation. One mode is an improved blocking FZP antenna, and the second mode is as a phase correcting FZP which uses all the incident MMW radiation. As a blocking FZP antenna, a low plasma density can be chosen which provides for the principle of destructive interference to be used to completely block the undesired out-of-phase MMW. With proper control of phase in the MMW this blockage can be made to be complete, not just asymptotically approaching zero, and at much lower plasma density than in previous designs. The fact that a lower plasma density is suitable for operation allows for much less

Affiliated with

Also associated with

Rating

Plasma controlled antenna does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Plasma controlled antenna, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Plasma controlled antenna will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3080923