The new generation of radio telescopes, such as the proposed Square Kilometer Array (SKA) and the Low-Frequency Array (LOFAR) rely heavily on the use of very large phased aperture arrays operating over wide bandwidths at frequency ranges up to approximately 1.4 GHz. The SKA in particular will include aperture arrays consisting of many thousands of elements per station providing un-paralleled survey speeds. Currently two different arrays (from nominally 70 MHz to 450 MHz and from 400 MHz to 1.4 GHz) are being studied for inclusion within the overall SKA configuration. In this paper we aim to analyze the array contribution to system temperature for a number of regular and irregular planar antenna array configurations which are possible geometries for the low-frequency SKA (sparse disconnected arrays). We focus on the sub-500 MHz band where the real sky contribution to system temperature (T sys ) is highly significant and dominants the overall system noise temperature. We compute the sky noise contribution to T sys by simulating the far field response of a number of SKA stations and then convolve that with the sky brightness temperature distribution from the Haslam 408 MHz survey which is then scaled to observations at 100 MHz. Our analysis of array temperature is carried out by assuming observations of three cold regions above and below the Galactic plane. The results show the advantages of N. Razavi-Ghods (B) · Exp Astron (2012) 33:141-155 regular arrays when sampled at the Nyquist rate as well as their disadvantages in the form of grating lobes when under-sampled in comparison to non-regular arrays.

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).

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In this paper, we consider the problem of reducing the radar cross section of a wind turbine blade through the application of radar absorbing material (RAM). One problem encountered by these techniques is the integration of the RAM solution with the existing lightning protection system, which is mandatory requirement to protect the blade when in operation. A common form of lightning protection is the use of conducting lightning receptors on the surface of the blade. To ensure the protection system is effective, a clearance area around the receptor may be required before any RAM treatment is applied. The size of the clearance area and the number of lightning receptors therefore potentially reduce the effectiveness of the RAM treatment. Design guidelines are given in this paper for a generic 40 m blade geometry. Some modelling results of the radar cross section and Doppler signature from a RAM treated blade are presented, and a comment is also made on the importance the blade edges have in reducing radar effects.

Complementary to the conventional dish radio telescopes, aperture arrays provide a technically attractive approach to achieve a large field of view and flexibility in observational parameters e.g. Sky area vs. bandwidth. Designs of both aperture array elements and overall geometry for the SKA Mid Frequency Aperture Array are presented here, together with resulting trade-offs. The paper reports the latest developments of global efforts on the front-end design of Mid-Frequency Aperture Array, not attempting to make technology selections, as the priority of sciences and the time for implementing Mid-Frequency Aperture Array is yet to be fully confirmed. Different ongoing front-end solutions are introduced, particularly crossed ring antenna array with a planar structure is explored in more detail as it is less known in the community. Key performances of the candidate front-end technologies are addressed by examining the prototypes. The objective of the collaborative study is to increase technology readiness for implementation of Mid-Frequency Aperture Array in the future.