Aperture Amplitude Distribution Research Articles

ANALYSIS OF DIFFERENT TAPERING TECHNIQUES TO REDUCE SIDE LOBE LEVEL OF A LINEAR ANTENNA ARRAY FOR X BAND SAR APPLICATIONS

Through amplitude and phase control, antenna arrays provide a wide range of options for varying their directivity patterns. Using amplitude control or weighting throughout the array aperture, peak side lobe levels (SLL) can be decreased. Several authors have contributed significantly to the development of procedures for synthesizing these aperture amplitude distributions for SLL control. A compact series-fed center-fed open-stub (SFCFOS) 10-element linear antenna array is presented in this work for X-band applications. A symmetrical Chebyshev amplitude distribution (CAD) is used to obtain a low side lobe level by comparing with uniform amplitude distribution (UAD). By adjusting the micro-strip patch element width, the amplitude is controlled, and an open stub is positioned close to the final antenna element to make better use of the radiating signal's energy. This study illustrates the utilization of two amplitude tapering strategies at the same time to produce the required SLL. The linear antenna array is designed with the center frequency of X-band is 9.65 GHz. The proposed tapering techniques applied to the antenna structure are very effective in reducing SLL. The design shows an SLL -28.6 dB for UAD and -34.7 dB for CAD. The simulation was performed in CST Microwave Studio.

Generalized Sequential Rotation Arrays With Full Control of Dual-Circularly-Polarized Aperture-Field Distribution Based on Elliptically-Polarized Elements

In this article, the theory, design, and experimental validation of wideband generalized sequential rotation arrays (GSRAs) are reported. In particular, the GSRA has the ability to provide dual-circularly polarized (dual-CP) beams with different handedness using a single feeding network, whose aperture amplitude and phase distributions can both be independently controlled. Such functionality is achieved by jointly utilizing the dynamic phase and Berry phase, as well as exploiting elliptically polarized (EP) array elements with different axial ratio (AR) values. The proposed technique and the design methodology are verified by two proof-of-concept microstrip GSRAs consisting of an <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$8\times8$ </tex-math></inline-formula> array of EP stacked-patch elements operating in the K-band. The first prototype exhibits a 3 dB gain difference between the two CP beams, while the other offers a 6 dB gain difference and different sidelobe levels (SLLs) for the generated dual-CP beams. Both arrays are fabricated and measured, experimentally achieving a joint <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$S_{11} &lt; -10$ </tex-math></inline-formula> dB, AR < 3 dB, and 3 dB gain bandwidth of more than 28.4% and 20.1% with a beam squinting of less than 3°. In addition, the two GSRAs enable a bandwidth of about 19% and 3% within which the gain difference between the two CP beams of different handedness deviates from the targeted value by less than 1 dB. With the large degree of freedom in aperture field control, the proposed GSRAs could be potential candidates for use in satellite communications, point-to-multipoint communications, and so on.

A dielectric free near field phase transforming structure for wideband gain enhancement of antennas

The gain of some aperture antennas can be significantly increased by making the antenna near-field phase distribution more uniform, using a phase-transformation structure. A novel dielectric-free phase transforming structure (DF-PTS) is presented in this paper for this purpose, and its ability to correct the aperture phase distribution of a resonant cavity antenna (RCA) over a much wider bandwidth is demonstrated. As opposed to printed multilayered metasurfaces, all the cells in crucial locations of the DF-PTS have a phase response that tracks the phase error of the RCA over a large bandwidth, and in addition have wideband transmission characteristics, resulting in a wideband antenna system. The new DF-PTS, made of three thin metal sheets each containing modified-eight-arm-asterisk-shaped slots, is significantly stronger than the previous DF-PTS, which requires thin and long metal interconnects between metal patches. The third advantage of the new DF-PTS is, all phase transformation cells in it are highly transparent, each with a transmission magnitude greater than − 1 dB at the design frequency, ensuring excellent phase correction with minimal effect on aperture amplitude distribution. With the DF-PTS, RCA gain increases to 20.1 dBi, which is significantly greater than its 10.7 dBi gain without the DF-PTS. The measured 10-dB return loss bandwidth and the 3-dB gain bandwidth of the RCA with DF-PTS are 46% and 12%, respectively.

Manipulation and control of 3-D caustic beams over an arbitrary trajectory.

We present an algorithm for manipulating and controlling 3-D field patterns, with energy confined to the narrow vicinity of predefined 3-D trajectories in free-space, which are of arbitrary curvature and torsion. This is done by setting the aperture field's phase to form smooth caustic surfaces that include the desired trajectory. The aperture amplitude distribution is constructed to manipulate both the on-axis intensity profile and the off-axis beam-width, and is updated iteratively. Once the aperture distribution is calculated, the radiation from a finite sampled aperture is computed numerically using a Fast Fourier Transform-based scheme. This allows for both verification of the design and examination of its sensitivity to parameters of realistic discrete implementation. The algorithm is demonstrated for the cases of an Airy beam of a planar trajectory, as well as for helical and conical-helical trajectory beams.

Multiobjective Optimal Antenna Synthesis for Microwave Wireless Power Transmission

In microwave wireless power transmission, the end-to-end transmission efficiency is of major concern and should be improved to reduce costs. Toward this end, high beam collection efficiency (BCE) as well as a flat beam pattern is desired. Previous studies have mainly focused on methods aimed at maximizing the BCE alone or only to obtain a flat beam. However, single-objective optimization fails to provide decision makers with enough choices in the tradeoff between BCEs and flat beam patterns. To overcome such a problem, the aperture amplitude distribution is approximated with the Taylor series expansion, and then a multiobjective optimization model is established. Here, the objectives are twofold. The first is to maximize the attainable BCE; and the second is to achieve a flat beam. Afterward, a novel algorithm named multiobjective multiverse optimizer is employed. To investigate the validity of the proposed method, numerical experiments on continuous apertures and array antennas are carried out.

Aperture Illumination Designs for Microwave Wireless Power Transmission With Constraints on Edge Tapers Using Bezier Curves

A new method is proposed for the antenna aperture illumination design on microwave wireless power transmission. Bezier curves are used to describe the aperture amplitude distributions. By optimizing the coordinates of the associated Bezier points, an aperture distribution yielding the maximum beam collection efficiency (BCE) can be achieved. Moreover, edge tapers can be easily controlled by the Bezier curves. The proposed method shows its effectiveness from carrying out extensive numerical experiments on different Fresnel parameters as well as edge taper constraints. The results show that larger aperture power coefficients obtain with a little sacrifice of BCEs when larger edge taper constraints are considered. Besides, the peak lobe irradiated outside the rectenna can be suppressed as well.

Improved design of a SIW long slot leaky wave antenna with low SLL

This study presents a new design for the substrate integrated waveguide (SIW) long slot leaky wave antenna (LWA) with narrow beam, low side lobe level (SLL) and low cross-polarisation. To meet these specifications, the changes of phase constant, β, along the structure are compensated, the required leakage constant, α r , is determined more accurately by considering the loss and a straight slot along the antenna is used. In order to calculate the leakage constant of the leaky-mode more accurately, thru-reflect-line method along with HFSS simulations is used. By properly determining the shape of SIW walls, the amount of leakage along the slot and the phase constant can be controlled. This results in a desired aperture amplitude and phase distribution. The presented LWA realises a Taylor distribution with a predefined SLL of -25 dB. Such a design also leads to a low cross-polarisation level and narrow beam at centre frequency of 17 GHz. The measurement results are consistent with the simulation results.

Impact on focal parameters for near‐field‐focused aperture antennas

AbstractNear‐field‐focused (NFF) antennas have been studied to focus the electromagnetic power in the near‐field (NF) region of the antenna aperture over a long time. Some focal parameters have been adopted to evaluate the focused performance of NFF antennas. Research work on smaller NFF aperture antennas with electrically large size is limited for the similarities between array antennas and aperture ones increasing with their size increasing. It is meaningful to study smaller aperture (several wavelengths) antennas because the physical size of antennas is limited for low‐frequency applications. This paper investigates the impacts of four factors on six focal parameters for NFF aperture antennas when the electrical size is not large enough. The results of six focal parameters are given as functions of the focal distance for different aperture shapes, aperture sizes, and aperture amplitude distributions, respectively. Finally, the differences of two focal parameters between the focal plane and the focal sphere are discussed as well. Unlike the previous work on the scalar diffraction field of NFF aperture antennas using the Fresnel region field approximation, the integral expression of scalar diffraction field is calculated directly and exactly in this paper to produce the focused field at an arbitrarily selected point.

Sidelobe‐level reduction of a linear array using two amplitude tapering techniques

Peak sidelobe levels (SLL) may be reduced via amplitude control or weighting across the array aperture. Several authors have made significant contributions in detailing processes for synthesising these aperture amplitude distributions for the purpose of sidelobe-level control. This study, however, shows the use of two amplitude-tapering methods simultaneously with pattern multiplication to achieve desired SLL. With a target bandwidth of 1.83 to 1.85 GHz and centre frequency of 1.84 GHz, a linear array is designed, fabricated and measured. Good agreement between simulated and measured results achieved shows that the proposed amplitude weighting methods applied to the antenna structure are very efficient in achieving reduced SLL below −20 dB across the operational bandwidth with an impedance bandwidth of 7.5% below −15 dB.

Full-Space Electronic Beam-Steering Transmitarray With Integrated Leaky-Wave Feed

A new low-profile, full-space, electronic beam-steering antenna architecture which combines the full-space beam-steering properties of reconfigurable transmitarrays with the low-profile feeding characteristics of leaky-wave antennas is proposed. The design uses an integrated leaky-wave feed to spatially distribute power across a reconfigurable transmitarray aperture in a low-profile manner while individual element phase control using varactor diodes enables full-space pencil-beam-steering. A $6\times 6$ element array was fabricated and experimentally verified, and full-space (both azimuth and elevation) beam-steering was demonstrated at angles up to 45° off broadside with a total efficiency for all scan angles on the order of 25%–35%. The design demonstrates a tenfold reduction in the overall thickness over the original transmitarray, improved aperture amplitude distribution control, as well as improved spillover control. The design also acts as a lower cost, scalable alternative to phased arrays as it does not require a complicated beamforming network and the feed and aperture are easily scaled.

Adjusting aperture illumination on dual reflectarray antennas for maximum aperture efficiency

Presented is a novel method for controlling aperture amplitude distribution on dual reflectarray antennas with the aim of achieving maximum aperture efficiency. This has been performed by appropriately shaping the sub-reflectarray which gives optimum illumination on the main reflectarray aperture. Applying this technique resolves the inherent deficiency of reflectarrays; which is the impossibility of controlling elements amplitude on the reflectarray aperture. In this regard, it has been shown that using this feature, the aperture efficiency of a classic dual reflectarray antenna can be enhanced significantly by optimally shaping the sub-reflectarray radiation pattern. Theoretical analyses demonstrate aperture efficiency of more than 84% which is an improvement of about 12% in antenna aperture efficiency comparing with conventional dual reflectarray antennas.

W-Band Steerable Composite Right/Left-Handed Leaky Wave Antenna for Automotive Applications

A novel structure for a frequency-independent steerable composite right/left-handed (CRLH) leaky wave (LW) antenna in the millimeter-wave band is proposed. This has the advantages of wide beam scanning and low profile, and is a suitable structure for mass-production. The proposed antenna has features wherein a movable dielectric slab is placed above the CRLH LW antenna, and the radiation angle can be steered by changing the distance between the slab and the antenna using compact actuators. Moreover, slots are added to the antenna to control the aperture amplitude distribution of the array antenna in order to enhance aperture efficiency. A prototype CRLH LW antenna has been fabricated with these slots, and backward-to-forward beam scanning characteristics at 76 GHz have been demonstrated successfully by measurement. A wide scanning angle from 73 to 114 deg. has been achieved experimentally. The aperture efficiency is 25.3%.

Effect of amplitude tapering and frequency-dependent phase errors on radiation characteristics of radial waveguide fed non-resonant array antenna

A high-gain low sidelobe level array antenna that is fed by a radial waveguide and designed to operate over a frequency range is considered. Assuming azimuthally symmetric excitation, the radiation pattern of the circular array is approximated by the space factor for the equivalent circular aperture and a simple and efficient tool is introduced for analysis of the array radiation pattern. The effect of the tapering, for both paraboloidal and bell-shaped aperture amplitude distributions, and the consequences of the phase errors, caused by the variation with frequency of the electrical lengths in the radial waveguide feed structure, on the gain and sidelobe level are studied in detail. Results obtained based on the proposed analysis are validated with respect to experimental data available in the literature. They are applicable to designs of arrays as well as aperture antennas.

Pattern characteristics of harmonic and intermodulation products in broadband active transmit arrays

A three-term power series model for the nonlinearity in the power amplifiers of an active array is used to explore the radiation characteristics of the harmonics and intermodulation products. This model leads to simple equations for predicting the beam-pointing direction and the aperture amplitude taper of the spurious signals. Cases of a single transmit signal, dual transmit signals within a single beam and dual beam operations are analyzed. This model shows that the second and third harmonics are co-boresighted with the fundamental signal but have squared and cubed aperture amplitude distributions. The third-order intermodulation products in general have beam-pointing directions offset from the fundamental signal beam(s). Graphical methods of locating the intermodulation beams in (u, v) space are outlined and illustrated by example.

Thermal Fields and Thermal Doses in Ultrasonic Surgery: A Model of a Gaussian Focused Beam

The condition of equivalence of the space-time distributions of thermal sources in the focal region is introduced for the sources caused by the absorption of focused ultrasonic beams with different aperture amplitude distributions in biological tissue. The Gaussian beam model, which allows the simplest analytical solution of the biothermal equation, is used to obtain fairly accurate approximations in the form of explicit functions describing the spatial distribution and the time dynamics of thermal fields and thermal doses along the beam axis and along the radius in the focal plane. This provides an opportunity to conduct a rapid analysis (without multiple numerical integration) of the results of active surgical (thermodestructive) action of ultrasound on biological tissue and, first of all, to determine the characteristic dimensions of the zone of thermal destruction of biological tissue. The approximate analytical solutions are important for the determination of the effect of the beam parameters (the radius of the aperture and the frequency) on the characteristic dimensions of the zone of thermal destruction.

Fresnel region power density levels from Earth station antennas

A method for predicting power density levels near transmitting parabolic Earth station antennas is described. The Fresnel region analysis applies to circular aperture antennas with an appropriate aperture amplitude distribution, and can be used to predict power densities along the antenna boresight or at off-boresight angles.

Synthesis of array feeds for optimum illumination of reflector antennas and low spillover

A novel synthesis technique for reflector-array feeds is presented. It consists of a quadratic programming algorithm for global optimization with constraints that are used to control the spillover radiation of the antenna. The method is capable of compensating for feed-phase aberrations and surface distortions, and also provides a prescribed aperture amplitude distribution with constant phase, which is useful to control side-lobe levels. © 1997 John Wiley & Sons, Inc. Microwave Opt Technol Lett 15: 334–338, 1997.

An E‐plane sectoral horn fitted with side‐lobe suppression devices

AbstractA finite difference method is used to design a new type of sectoral horn antenna with corner channel inserts which produces a tapered aperture amplitude distribution in the E‐plane. The channel inserts, which present a simple modification to the horn aperture geometry, act as effective side‐lobe suppression devices. The efficiency of the suppression technique was successfully tested on an experimental horn whose performance is compared to that of a horn with similar geometrical dimensions but with no channel sections.

Smooth aperture distribution synthesis for shaped beam reflector antennas

A method of synthesising smooth aperture amplitude and phase distributions from a prescribed far-field radiation pattern is proposed. A geometric optics (GO) solution of the problem, which gives a starting point for the final physical optics (PO) optimisation, is obtained using the Monge-Ampère approach of Westcott. During PO optimisation a derivative condition is invoked to control the smoothness of the aperture distributions.

Application of optical ray technique to the design of short microwave horn antennas with low side lobe levels

Optical ray theory is used to design a phase-corrected microwave horn antenna of shortlength which produces a tapered aperture amplitude distribution in the E-plane. The basis of the design is the creation of a region where the effective refractive index varies with hyperbolic secant (HYSEC) form in a direction normal to the horn axis. In a practical realisation this effective refractive index distribution is achieved in both smooth and stepped form by varying the plate separation of the planar waveguide supporting a TE01 mode. Good agreement is found between predicted and experimentally determined aperture and far-field distributions. Finally, the study is extended to the design of a novel antenna incorporating dielectric loading which, in theory, allows for operation at even shorter horn lengths.