Instrumental Faraday-Like Rotation and Temporal Autocorrelation for Radio Astronomical Antennas

We investigated oscillations in the frequency spectra of dual-polarized, radio astronomy antennas by applying the Fourier transform to obtain their temporal autocorrelation. The temporal autocorrelation is equivalent to the so-called delay transform of zero length baselines. Of particular interest are wideband antennas at low frequencies, which are planned for use in next generation Faraday rotation observations. We found that antennas such as the one proposed for the SKA-Low telescope, the SKALA4.1, exhibit spectral oscillations at 150 MHz and at 20 MHz that may appear as Faraday rotation. The strength of this effect tends to be stronger at lower elevations, as anticipated, but still exists even for zenith elevation, which is bore-sight. This effect is seen mainly in wideband antennas, and suggests that the additonal complexity of wideband antennas needs more attention in full polarimetric calibration than narrowband antennas more commonly used for Faraday measurement.