Adrian Sutinjo

Understanding instrumental Stokes leakage in Murchison Widefield Array polarimetry

This paper offers an electromagnetic, more specifically array theory, perspective on understanding strong instrumental polarization effects for planar low-frequency “aperture arrays” with the Murchison Widefield Array (MWA) as an example. A long-standing issue that has been seen here is significant instrumental Stokes leakage after calibration, particularly in Stokes Q at high frequencies. A simple model that accounts for interelement mutual coupling is presented which explains the prominence of Q leakage seen when the array is scanned away from zenith in the principal planes. On these planes, the model predicts current imbalance in the X (E-W) and Y (N-S) dipoles and hence the Q leakage. Although helpful in concept, we find that this model is inadequate to explain the full details of the observation data. This finding motivates further experimentation with more rigorous models that account for both mutual coupling and embedded element patterns. Two more rigorous models are discussed: the “full” and “average” embedded element patterns. The viability of the full model is demonstrated by simulating current MWA practice of using a Hertzian dipole model as a Jones matrix estimate. We find that these results replicate the observed Q leakage to approximately 2 to 5%. Finally, we offer more direct indication for the level of improvement expected from upgrading the Jones matrix estimate with more rigorous models. Using the average embedded pattern as an estimate for the full model, we find that Q leakage of a few percent is achievable.

BIGHORNS - Broadband Instrument for Global HydrOgen ReioNisation Signal

The redshifted 21cm line of neutral hydrogen (HI), potentially observable at low radio frequencies (~50-200 MHz), should be a powerful probe of the physical conditions of the inter-galactic medium during Cosmic Dawn and the Epoch of Reionisation (EoR). The sky-averaged HI signal is expected to be extremely weak (~100 mK) in comparison to the foreground of up to 10000 K at the lowest frequencies of interest. The detection of such a weak signal requires an extremely stable, well characterised system and a good understanding of the foregrounds. Development of a nearly perfectly (~mK accuracy) calibrated total power radiometer system is essential for this type of experiment. We present the BIGHORNS (Broadband Instrument for Global HydrOgen ReioNisation Signal) experiment which was designed and built to detect the sky-averaged HI signal from the EoR at low radio frequencies. The BIGHORNS system is a mobile total power radiometer, which can be deployed in any remote location in order to collect radio-interference (RFI) free data. The system was deployed in remote, radio quiet locations in Western Australia and low RFI sky data have been collected. We present a description of the system, its characteristics, details of data analysis and calibration. We have identified multiple challenges to achieving the required measurement precision, which triggered two major improvements for the future system.

Characterization of a Low-Frequency Radio Astronomy Prototype Array in Western Australia

We report characterization results for an engineering prototype of a next-generation low-frequency radio astronomy array. This prototype, which we refer to as the Aperture Array Verification System 0.5 (AAVS0.5), is a sparse pseudorandom array of 16 log-periodic antennas designed for 70-450 MHz. It is colocated with the Murchison widefield array (MWA) at the Murchison radioastronomy observatory (MRO) near the Australian square kilometre array (SKA) core site. We characterize the AAVS0.5 using two methods: in situ radio interferometry with astronomical sources and an engineering approach based on detailed full-wave simulation. In situ measurement of the small prototype array is challenging due to the dominance of the Galactic noise and the relatively weaker calibration sources easily accessible in the southern sky. The MWA, with its 128 “tiles” and up to 3 km baselines, enabled in situ measurement via radio interferometry. We present array sensitivity and beam pattern characterization results and compare to detailed full-wave simulation. We discuss areas where differences between the two methods exist and offer possibilities for improvement. Our work demonstrates the value of the dual astronomy-simulation approach in upcoming SKA design work.

Intrinsic Cross-Polarization Ratio of Dual-Linearly Polarized Antennas for Low-Frequency Radio Astronomy

This note discusses the intrinsic cross-polarization ratio (IXR) from an antenna engineering perspective in that we seek to identify an a priori (coordinate) system where IXR is well approximated by the raw cross-polarization numbers. We begin by establishing a special case where IXR is identical to the raw cross-polarization ratios for in-phase dual-linearly-polarized antennas when the Jones matrix is expressed using circular polarization bases. This insight allows physical interpretation of IXR which may be useful in antenna design and system calculations. In addition, we discuss comparisons between direct IXR calculations and circular polarization approximations for more realistic cases involving dual-polarized Murchison widefield array (MWA) bow-tie antennas.

Antenna Rotation Error Tolerance for a Low-Frequency Aperture Array Polarimeter

We present antenna rotation error tolerance analysis for a polarimeter consisting of dual-linearly polarized dipole-like elements. Treating the elements as a phased array and expressing the measurement basis as circularly polarized (CP) results in a concise expression for the Jones matrix for the array. For the type of elements being considered, the matrix shows that the intrinsic cross-polarization ratio (IXR) of the array at the intended beam scanning direction is unaffected by small rotation errors. For random rotation error and very large number of elements, we further find that the relative Jones matrix estimation error converges to that of the error-free case at the intended beam scanning direction; however, the effect of element rotation error on array directivity and radiation pattern remains. Recasting the analysis with the array observing an unpolarized source, a relation between rotation error and cross-polarization “leakage” is obtained, wherein similar trends with very large number of elements hold true. Practical examples involving “large” number of elements such as the low frequency Square Kilometre Array are discussed.

Antenna array characterization via radio interferometry observation of astronomical sources

We present an in-situ antenna characterization method and results for a “low-frequency” radio astronomy engineering prototype array, characterized over the 75-300 MHz frequency range. The presence of multiple cosmic radio sources, particularly the dominant Galactic noise, makes in-situ characterization at these frequencies challenging; however, it will be shown that high quality measurement is possible via radio interferometry techniques. This method is well-known in the radio astronomy community but seems less so in antenna measurement and wireless communications communities, although the measurement challenges involving multiple undesired sources in the antenna field-of-view bear some similarities. We discuss this approach and our results with the expectation that this principle may find greater application in related fields.

First results from AAVS 0.5: A prototype array for next-generation radio astronomy

This paper provides an overview of the Aperture Array Verification System 0.5 (AAVS 0.5), co-located and operated in conjunction with the Murchison Widefield Array (MWA) near the Australian SKA core site. AAVS 0.5 is based on log-periodic antennas of a type potentially useful in next-generation low-frequency arrays such as SKA-low. We report on our progress by discussing results obtained thus far as well as test plans for the near future. A number of lessons learned will be presented, demonstrating that hands-on experience constitutes an essential knowledge-base in the pre-construction phase of a radio-telescope such as the SKA.

Efficiency Measurement of Connected Arrays Using the Improved Wheeler Cap Method

We present a method for measuring antenna efficiency of connected arrays. The method is a modified version of the improved Wheeler cap (IWC) method where the antenna under test (AUT) is electrically connected to two walls of the Wheeler Cap which simulates array connection. Theoretical analyses based on plane wave and cylindrical wave problems and numerical simulations using HFSS and FEKO are presented to establish accuracy estimates for the method. Close attention will be made in comparing two data collection techniques: frequency sweeping versus wall shifting. Finally, we discuss an apparatus for efficiency measurement of a 1-D connected array and the measurement results.

Calibration and Stokes Imaging with Full Embedded Element Primary Beam Model for the Murchison Widefield Array

The Murchison Widefield Array (MWA), located in Western Australia, is one of the low-frequency precursors of the international Square Kilometre Array (SKA) project. In addition to pursuing its own ambitious science program, it is also a testbed for wide range of future SKA activities ranging from hardware, software to data analysis. The key science programs for the MWA and SKA require very high dynamic ranges, which challenges calibration and imaging systems. Correct calibration of the instrument and accurate measurements of source flux densities and polarisations require precise characterisation of the telescopes primary beam. Recent results from the MWA GaLactic Extragalactic All-sky MWA (GLEAM) survey show that the previously implemented Average Embedded Element (AEE) model still leaves residual polarisations errors of up to 10-20 % in Stokes Q. We present a new simulation-based Full Embedded Element (FEE) model which is the most rigorous realisation yet of the MWAs primary beam model. It enables efficient calculation of the MWA beam response in arbitrary directions without necessity of spatial interpolation. In the new model, every dipole in the MWA tile (4 x 4 bow-tie dipoles) is simulated separately, taking into account all mutual coupling, ground screen and soil effects, and therefore accounts for the different properties of the individual dipoles within a tile. We have applied the FEE beam model to GLEAM observations at 200 - 231 MHz and used false Stokes parameter leakage as a metric to compare the models. We have determined that the FEE model reduced the magnitude and declination-dependent behaviour of false polarisation in Stokes Q and V while retaining low levels of false polarisation in Stokes U.

Element rotation tolerance in a low-frequency aperture array polarimeter

We present a rotation error tolerance analysis for dual-polarized dipole-like antennas commonly found in low-frequency radio astronomy. A concise Jones matrix expression for the phased array is derived which facilitates calculations of rotation error effects in polarimetry. As expected, for random rotation error and number of elements approaching infinity, the estimation error converges to that of the error-free case. However, as in practice large but finite number of antennas are involved, we present a simple analysis to estimate rotation error effects. An example calculation based on a “baseline” design for a low-frequency Square Kilometre Array (SKA) “station” is discussed.