Polarization properties of reflector antennas used as radio telescopes

Abstract

The distribution of cross polarization across the main beam and near sidelobes of a reflector antenna is calculated. Results are expressed in terms relevant to imaging in radio astronomy, using Stokes parameters, as plots of instrumental polarization Q/I, U/I, and V/I, showing conversion of total intensity of a signal which is unpolarized into apparent linear and circular polarization. The calculations use GRASP8, software that is based on physical optics and the physical theory of diffraction. For purposes of calculation, the symmetrical paraboloidal reflector (diameter ∼40 wavelengths) is fed at the prime focus with a linearly polarized signal. Computed radiation patterns at a number of feed orientations are averaged to establish the antenna response to an unpolarized radio astronomy signal. The results of the computations are consistent with measurements of instrumental polarization of the Dominion Radio Astrophysical Observatory Synthesis Telescope at 1420 MHz made using unpolarized radio sources. For this telescope, the dominant source of instrumental polarization across the field is the cross polarization of the feed. The next most significant effect is scattering by the feed struts; both three‐strut and four‐strut configurations are examined. Struts affect performance in linear polarization but also introduce some instrumental circular polarization. The contribution to instrumental polarization from the reflector itself is comparatively small. Roughness of the reflector surface has relatively little effect in the main beam in Q and U but introduces V and also randomizes the polarization of the sidelobes. In all cases considered, the computations show that the first and subsequent sidelobes are highly polarized, with levels of instrumental polarization up to 50%.

A Survey of the polarized emission from the Galactic plane at 1420 MHz with arcminute angular resolution

“…The reflector-induced instrumental polarization is not purely a scalar quantity: it has phase as well as amplitude. Furthermore, it is quite strongly dependent on the details of antenna construction; the calculations of Ng et al (2005) show this quite clearly. The ultimate correction must take into account the complex nature of this response, and to enable us to do so we made a further set of observations of the effect.…”

“…The Sun is such a source: it traverses the antenna sidelobes whenever it is above the horizon, contributing ∼1 K to antenna temperature. Conversion of I to Q or U can be as high as 50% in the far sidelobes (Ng et al 2005) so the Sun can contribute a polarized signal up to 500 mK. However, because of the Sun's large extent its effects are usually confined to shorter baselines.…”

“…The effect has been studied by Ng et al (2005) using electromagnetic analysis software. Instrumental polarization depends on the polarization performance of the feeds, on reflector curvature, and on aperture blockage by the feed-support struts, among other more minor effects such as surface roughness.…”

An absolutely calibrated survey of polarized emission from the northern sky at 1.4 GHz

“…Numerical calculations show that the far sidelobes are polarized by up to 50% (Ng et al 2005). Therefore, ground radiation may cause offsets in the cross-products as well as in total power, with amplitudes of several hundreds of mK.…”

Correcting the polarization leakage phases and amplitudes throughout the primary beam of an interferometer

“… Ng et al [2005] calculated theoretical leakage voltage patterns for the ST's three and four metal strut antennas. Applying them to correcting polarization leakage in the CGPS, Taylor et al [2003] confirmed that heterogeneity in the ST was having a noticeable effect on the CGPS polarization images that was not being corrected by subtracting the Stokes I images multiplied by leakage maps.…”