Ahmed El-Makadema

Geometry Design Optimization of Large-Scale Broadband Antenna Array Systems

The next generation radio telescopes such as the Square Kilometer Array (SKA) are expected to contain thousands of antenna array elements operating over a broad frequency range where the signals from each antenna element are combined and processed simultaneously providing high sensitivity with multiple beams providing a wide field of view. One crucial design aspect influencing both the performance and the cost of such systems is the array geometry. Due to the large bandwidth and number of broadband antenna elements, the optimization of such array system is difficult to achieve with the current array geometry optimization techniques which rely mainly on genetic algorithms and pattern search techniques. This paper provides a study of the effects of array geometry on the performance broadband array system. In addition, it provides a method where the array geometry can be more easily optimized for different applications. This is demonstrated for optimizing a typical SKA station in the frequency band between (70-450 MHz).

On the geometry of the Square Kilometer Array Mid-Frequency Aperture Array

This paper looks at the performance limits of reducing the total number of elements in a tightly coupled dense aperture arrays for radio astronomical applications particularly the case of the SKA-MFAA. It shows that by using a combination of triangular grid configuration and random thinning, a reduction of up to 20% of the total number of elements can be achieved without any significant sacrifice on performance. This has a major impact on reducing the cost since the number of elements is a main cost driver in such large array systems. The results are demonstrated through full wave simulation of the ORA array which is a major candidate for the SKA-MFAA.

Differential front-end design for mid-frequency aperture array design

Differential front-end design presents a good prospect to be integrated with differential beamformer inputs without a transformer needed otherwise for a single-ended front-end design. A front-end solution with a complete differential configuration is presented. The potential advantages of this design are: planar antenna elements mean a lower cost; differential LNAs in close proximity to the antenna elements reducing feedline loss hence potentially a low noise figure and a less power consumption without an active balun needed in the RF signal chain.

Analysis of Phased Aperture Array Geometries for Low Frequency Radio Astronomy

The proposed SKA telescope will rely heavily on the use of aperture phased arrays in the sub 1 GHz frequency band. Currently two different arrays (from nominally 70 MHz to 450 MHz and from 300 MHz to 1 GHz) are being studied for inclusion within the overall SKA configuration. In this paper we focus on analysing the real sky contribution to system temperature for various AA-lo antenna configurations composed of 10,000 elements. We evaluate six geometries, with four minimum inter-element separations of 0.5λ, 0.8λ, 1λ, and 2λ. We assume uniform excitation of antennas as well as Taylor and Dolph-Chebyshev weighting with a side-lobe level of 35dB. Although the simulations are tuned to observations at 100 MHz, the results can be temperature scaled to the appropriate frequency band. Our analysis of array temperature is carried out by assuming observations of three cold regions above and below the Galactic plane. The results show comparisons between regular and random array geometries.

Dual polarised crossed ring antenna array with high polarimetric purity

Polarisation purity becomes increasingly important in modern radio sciences. Accurate measurement of the state of polarization of the sources is crucial for pulsar timing and cosmic magnetism in radio astronomy. A polarization purity of-40 dB is expected ideally across the entire scan range as wide as ±60o from the zenith. With aperture array technologies for such applications, it is a demanding requirement. A crossed ring antenna array of a planar structure is proposed and a stable cross polarization over the wide scan angle is expected. The proposed antenna array is low profile and can operate in wide bandwidth. A finite array with 10×10 dual polarized elements has been manufactured and the radiation patterns of the 4×4 beamformed subarray and the centre elements in the finite array have been measured. The co-pol and cross-pol patterns both have been measured against a standard reference antenna, the intrinsic cross-polarization ratio is reported as it relates to the Jones matrix from the measurement quantities even after full calibration.

System noise reduction through null steering in large wideband sparse arrays

Null steering is an important beam-forming technique for phased antenna arrays in general particularly in applications involving high interference. In radio astronomy, where the signal of interest may be much weaker than interference (particularly from manmade sources) a severe limitation may occur on the system noise temperature, dynamic range and processing budget at interfering frequencies. One way of minimizing this impact is via null steering. For large wideband sparse array system such as the SKA-AALO, traditional null steering techniques are inappropriate due to the sparse geometry. Some methods have been reported using numerical techniques, but due to the large number of antenna elements involved and the random element distribution these require long computations and search methods to obtain pattern control. In this paper, a deterministic and computationally simple null steering technique is introduced which can be directly applied to null multiple interference sources. The weights for several nulls can be directly computed from the array radiation pattern data regardless of element distribution making it particularly useful for sparse arrays operating over a large bandwidth.

Smart surface for aperture array by using nonuniform distribution

Different geometry arrangement for elements in an array yields various electrical interactions between them. In a wide band and wide scan angle array, these interactions are quite complex. A smart array structure is proposed based on the planar octagonal ring antenna. Thanks to the flexibility of this array element, element sizes and the bonds between the elements in the array can be non-uniform, these non-uniform-configured micro-arrays can add flexibility in the entire array design, it can be engineered in a way that the current distribution at different frequency points involves different size of patch area on the active aperture array surface, and in each effective patch area, the electrical property are tuned to frequency and scan angle. This allows for wider scan angles while reducing the overall number of elements hence reducing the cost. As a result, a smart and conformal aperture array surface for broad band use can be achieved.

“Sparseness” in broadband array antennas

Broadband phased arrays are being used in a wide number of imaging application in radio astronomy and radar. To improve the cost effectiveness, the system must meet the minimum performance requirements with the fewest number of elements. Since the array geometry affects many aspects of the overall performance such as directivity, sidelobes, and mutual coupling, the geometry design and optimization of the array can vary significantly depending on the required application. In this paper the main aspects broadband antenna array geometry design are outlined with different design and optimization techniques of broadband antenna array are divided into three main categories. The paper also outlines the main tradeoffs involved with each category with design examples.

Scanning performance of SKA-low sparse array configurations incorporating realistic element patterns and sky noise contributions

Broadband antenna arrays have become widely used for high sensitivity imaging application such as radio astronomy and are envisaged for the low frequency SKA telescope which will contain approximately 500,000 antennas in phase 1 alone operating over the 70 MHz to 450 MHz band. For wide angle beamforming a crucial design aspect is the antenna element radiation pattern which directly affects the performance of the instrument particularly at wide scan angles off zenith. These effects have not been studied in detail, particularly for sparse and irregular geometries and are therefore crucial for analyzing performance trade-offs as well as for understanding the underlying assumptions made for instrument calibration. This paper provides a brief study of the various tradeoffs regarding the antenna element radiation pattern and irregular array geometries which can be utilized to optimize the performance. For this purpose the far-field station patterns will be convolved with the sky brightness temperature distribution from the Haslam 408 MHz survey which is then scaled to observations in the SKA-low frequency band.