The invention relates to a small (0.5 wavelength or less) adaptable antenna system. In particular it relates to the use of loaded parasitic components in the antenna aperture for the purpose of controlling the RF properties of the antenna. Such an antenna system is here referred to as a controlled parasitic antenna (CPA). Parasitic elements within the radiating aperture are terminated by active (controllable) impedance devices. A feedback and control subsystem periodically adjusts the impedance characteristics of these devices based on some observed metric of the received waveform. Such antenna systems can provide multifunctionality within a single aperture and/or mitigate problems associated with the reception of an interfering signal (or signals) or multi-path effects. Such antenna systems are particularly suitable to a situation where an aperture size is desired that is too small for the use of an adaptive phased array.

Small controlled parasitic antenna system and method for controlling same to optimally improve signal quality

Reduction of radar cross sections (RCS) is one of the major problems in stealth technology. Basic techniques used for RCS reduction are well known. They involve appropriate shaping, use of radar absorbing materials (RAMs), and passive and active cancellation of the scattered jield. Detailed description of these techniques and related bibliography have been presented in RCS handbooks. In this paper we discuss these techniques briefly from the physical point of view and summarize their limitations. Attention is concentrated on discussions of the physical structure of radar waves scattered from large objects (compared with the radar wavelength). These waves consist of ordinary and diffracted rays, thier beams, and shadow radiation. The latter is caused by the transverse diffusion of the radar beam in the vicinity of the shadow boundaries behind the scattering object, i.e., in the penumbra region. Shadow radiation creates strong forward scattering and is responsible for RCS enhancement at large bistatic angles. Possible passive and active techniques to control and reduce reflected beams, rays, and shadow radiation and potential limitations of these techniques are considered in the paper. In particular, it is emphasized that grazing rejlected rays and shadow radiation cannot be eliminated by absorbing materials.

Comments on diffraction principles and limitations of RCS reduction techniques

Electromagnetic scattering from a conducting object can be controlled by loading portions of its surface with distributed or lumped impedances. The scattered field from a loaded object is expressed as a superposition of fields scattered by the unloaded object and fields reradiated by the loading. The interference of these fields can be used to produce the required scatter control. Techniques for evaluating the reradiated field are discussed. A general form for the scattered field as a function of load impedance is shown, and techniques for studying this form are discussed. Theoretical and experimental results for thin, impedance loaded dipoles are compared. For dipole lengths not much greater than λ/2, broadside backscatter can be reduced 30 to 40 dB with a passive center load. Greater lengths and other aspects require multiple passive loads. Backscatter reduction of up to 20 dB has been reported for thick dipoles at broadside incidence, planar surfaces at normal incidence, and long thin objects near end-on incidence.

The control of electromagnetic scattering by impedance loading

Reconfiguration of parasitically controlled elements in a phased array is used to expand the range of operational functions. Embedded array elements can be frequency tuned, and bandwidth can be improved by using reconfiguration to broaden the bandwidth of the embedded elements. For high gain arrays, beam squint can be a limiting factor on instantaneous bandwidth. Reconfiguration can alleviate this problem by providing control of the element phase centers. Scan coverage can be improved and scan blindness alleviated by controlling the embedded antenna patterns of the elements as well as by providing control of the active impedance as the beam is scanned. Applying limited phase control to the elements themselves can alleviate some of the complexity of the feed manifold. A presently preferred method of designing reconfigurable antennas is to selectively place controlled parasitic elements in the aperture of each of the antenna elements in the phased array. The parasitic elements can be controlled to change the operational characteristics of the antenna element. The parasitic elements are controlled by either switching load values in and out that are connected to the parasitic elements or are controlled by applying control voltages to variable reactance circuits containing devices such as varactors. The parasitic elements can be controlled by the use of a feedback control subsystem that is part of the antenna system which adjusts the RF properties of the parasitic components based on some observed metric. The controllable characteristics include directivity control, tuning, instantaneous bandwidth, and RCS.

Reconfigurable parasitic control for antenna arrays and subarrays

Corrections to "Comments on Diffraction Principles and Limitations of RCS Reduction Techniques"

The scattering of a plane electromagnetic wave by a metallic sphere loaded with a circumferential slot in the plane normal to the direction of incidence is investigated. The slot is assumed to be of small but nonzero width with a constant electric field across it, and under these assumptions the analysis is exact. The field scattered in any direction is then obtained by the superposition of the field diffracted by an unloaded sphere, and that radiated by an excited slot at the position of the load, with the radiation strength of the slot related to the loading characteristics in the combined problem. Numerical results are presented, primarily for the case of back scattering, and these are compared with experimental measurements made using a sphere with an equatorial slot backed by a radial cavity of adjustable depth. The agreement is excellent.

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V.V. Liepa

Papers

Modification to the scattering behavior of a sphere by reactive loading