Limited-scan Array Research Articles

Application of Coherently Radiating Periodic Structures for Feeding Subarrays in Limited-Scan Arrays

Application of Coherently Radiating Periodic Structures for Feeding Subarrays in Limited-Scan Arrays

Random Feeding Networks for Reducing the Number of Phase Shifters in Limited-Scan Arrays

A new phased-array feeding network that results in low sidelobe levels (SLLs) over a limited scan region is demonstrated. The proposed concept groups the individual phased-array elements into random sequences of nonuniform subarrays and employs a single phase shifter (PS) for each subarray. When these random sequences are optimized, the resulting phased array can scan over a wide angle with low SLLs. A methodology for analyzing the random arrays is provided, multiple cases with different numbers of PSs and subarray groups are optimized, and design guidelines are presented. The performance of the random arrays is compared with conventional uniform subarrays and it is shown that random arrays reduce the number of PSs by up to 40% while preserving the same performance. A random array is demonstrated at 7.9 GHz for a 30-element linear design (scan in one direction) with 12 PSs and results in a half-power beamwidth of 4.1° with a scan angle up to ±14° and SLLs less than −15 dB. It is also shown that random arrays can be used in 2-D phased arrays with up to 60% reduction in the number of PSs. Application areas are in low-cost phased arrays with limited scan angles such as automotive radars or landing systems.

A new class of dielectric-loaded hybrid-mode horn antennas with selective gain: design and analysis by single mode model and method of moments

A mechanically simple dielectric loaded hybrid mode horn is presented. It can be designed to satisfy either hard or soft boundary conditions, or any boundary between hard and soft under balanced hybrid condition (low cross-polarization) for any aperture size. Design curves are developed based on a plane wave model. Radiation performance is computed based on a single mode circular cylindrical wave model (CCWM) as well as by method of moments (MoM). Based on CCWM aperture efficiency of about 94% has been computed at the design frequency for a 3.38/spl lambda/ aperture with hard boundary condition and a dielectric constant of 4.0. The same horn with a dielectric constant of 2.5 can provide higher than 89% aperture efficiency and under -30 dB cross-polarization over a 15% frequency range. Predicted peak sidelobes ranging from -19 to -26.5 dB and corresponding aperture efficiency between 92% and 78% at the design frequency under balanced hybrid condition have been obtained for various horn designs. Computations based on MoM show that cross-polarization patterns agree well with what was predicted based on an ideal single hybrid-mode model, while computed aperture efficiency is even higher than predicted by CCWM. Results also indicate that implementing dielectric-to-air matching at the horn aperture is crucial to achieve good cross-polarization performance and input match for hard horns in general. The horn can also be designed to radiate a flat-top pattern. The horn could be useful as an element in cluster feeds or limited-scan arrays, as a feed for quasioptical amplifier arrays or in millimeter wave applications.

Shaping of flat-topped element patterns in a planar array of circular waveguides using a multilayered disk structure-part II: experimental study and comparison

For pt.I see ibid., vol.51, no.5, p.1040-47 (2003). A symmetrical 19-element K/sub a/ band breadboard corresponding to the array considered in part I, as well as a similar X band breadboard with dipole exciters arranged in waveguides, are studied experimentally. In both breadboards the central ports are connected to microwave sources, while all the other ports are terminated with matching loads. The multilayered periodic structures placed above the waveguide apertures are composed of thin polymer films with conducting disks deposited by ion-beam. It is shown that the calculated and measured element patterns of the K/sub a/ band breadboard are in good agreement within the flat-topped main lobe. The edge effects and design elements explain some of the disagreement in the side lobe area. The measurement of the X band breadboard gives similar results, and confirms the desirable properties of the disk structures for application in limited-scan arrays.

Full and partial crosswalls between unit cells of endfire slotline arrays

Endfire slotline antenna arrays, also known as Vivaldi notch arrays, may be used to build broad-band phased arrays with wide scanning capabilities. However, scan blindnesses may render the phased array unusable over portions of its intended operating frequency range and scan volume. One such scan blindness occurs in the E-plane of single polarized arrays and can be predicted by a simple formula. By inserting full and partial crosswalls between unit cells of the array, this blindness can be moved to higher frequencies, moved toward endfire, or totally eliminated. Arrays with upper crosswalls exhibit another type of scan blindness. This scan blindness can he utilized to reduce far-out sidelobes of a limited-scan array.

Phased-array radiator reflection coefficient extraction from computer waveguide simulator data when grating lobes are present

Commercially available finite-element software that solves Maxwell's equations for arbitrary three-dimensional bounded structures has enabled phase-array radiator designers to perform waveguide simulator modeling of phased-array radiating elements on the computer very efficiently. Published work on waveguide simulator design has concentrated on array performance in the absence of grating lobes, a requirement for many radar applications. For such simulators, the reflection coefficient of each propagating mode at the waveguide simulator port gives the radiator reflection coefficient at a discrete scan angle. However, the design of limited scan arrays can lead to selection of an array element spacing that allows grating lobes in real space. When a waveguide simulator is modeled on the computer, and a grating lobe is present, the two waveguide modes representing the main lobe and the grating lobe will propagate in the waveguide simulator and they will be coupled together. The simulator port-reflection coefficient of either mode is not the true reflection coefficient seen by the radiating element. We describe a method for extracting the reflection coefficient of the radiating element from the waveguide simulator data when one or more grating lobes are present.

Blindness removal in arrays of rectangular waveguides using dielectrically loaded hard walls

We analyze an infinite planar array of open-ended rectangular waveguides in which the E-plane walls are loaded with dielectric slabs in order to create a hard boundary condition. The analysis is done by using the mode-matching method with Floquet modes in the exterior region. The results show that the hard walls remove the blind spots otherwise present in the element patterns of wide-angle scanning arrays. This occurs both for rectangular and for triangular lattices. In addition, the array becomes better matched to free-space in a rectangular lattice. In limited-scan arrays of large square waveguides, the use of the hard walls increases the array gain by 0.9 dB, it reduces the grating lobe level in the H plane by 11 dB and considerably decreases the difference between the array characteristics in the E and H planes.

Hard waveguide feeds with circular symmetry for aperture efficiency enhancement

Hard horns or waveguide feeds with circular symmetry have been studied with respect to gain (aperture efficiency) and crosspolarisation. The corrugated or strip-loaded representations of these feeds give almost identical radiation properties when surface waves are neglected. The contribution from the wall region increases the gain and crosspolar bandwidth of the feed. The gain increases and the crosspolar bandwidth decreases with increasing waveguide diameter and wall permittivity. The hard horns may be useful in cluster feeds and limited scan arrays due to reduced size and weight.

Grating lobe control in limited scan arrays

The use of multiple higher order modes for grating lobe control in arrays of large aperture elements scanned over restricted spatial sectors is described. Arrays of this type are useful in a number of applications including precision approach radar, weapons locators, and satellite communications systems. An extensive experimental and theoretical study that includes the design and use of dielectric lenses for control of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</tex> -plane null filling for an eight-element array, the investigation of null width effects on sidelobe levels, the experimental study of an element and control circuitry for two-dimensional scan, bandwidth effects, and the use of dielectric spatial filters and row displacement for residual grating lobe suppression is summarized.

Constrained feeds for limited scan arrays

Lossless modular hybrid coupled networks are described for application to phased arrays requiring a few beamwidths of scan. The simplest example of these networks for symmetrical limited scan is easily synthesized from analytical expressions which involve one parameter-the allowed grating lobe level. Formulas and design curves are presented for engineering applications. A practical example is calculated and compared with the conventional discrete subarray approach. Worst case radiation patterns are shown to provide the desired performance. The results show that the present technique in its simplest form requires only about half the number of phase shifters, drivers, and beam steering active devices as a conventional discrete subarray system which provides the same grating lobe level.

Aperture efficiency enhancement in dielectrically loaded horns

The effect of symmetrically loading a horn aperture with E -plane dielectric slabs is examined both theoretically and experimentally. It is shown that aperture efficiencies of the order of 92-96 percent may be obtained easily and inexpensively. The technique, which was demonstrated experimentally for a small horn aperture, might find application in limited scan arrays.