Solar Radio Spectro-polarimetry (50-500 MHz) : Design and Development of Cross-Polarized Log-Periodic Dipole antenna and configuration of receiver system

TL;DR

This paper details the design, development, and testing of a specialized radio spectro-polarimeter and antenna system for studying polarized solar radio emissions in the 50-500 MHz range, demonstrating its capabilities through magnetic field estimation.

Contribution

It introduces a novel Cross-polarized Log-Periodic Dipole Antenna and a comprehensive receiver setup, with detailed characterization and calibration procedures for solar radio observations.

Findings

The CLPDA achieves ~6.6 dBi gain and low cross-talk (~-27 dB).

The receiver system has a noise figure of ~3 dB and high dynamic range.

Spectral data shows a polarization degree of ~80-100%.

Abstract

A radio spectro-polarimeter was developed at the Gauribidanur radio observatory to study the characteristics of the polarized radio waves that are emitted by the impetuous solar corona in the 50 - 500 MHz frequency range. The instrument has three major components : a Cross-polarized Log-Periodic Dipole Antenna (CLPDA), an analog receiver, and a digital receiver (spectrum analyzer). This article elaborates the design and developmental aspects of the CLPDA, its characteristics and briefs about the configurations of the analog and digital receivers, setting up of the spectro-polarimeter, stage-wise tests performed to characterize it, etc. To demonstrate the instrumental capability, the estimation of the solar coronal magnetic field strength (B {\it Vs} heliocentric height), using the spectral data obtained with it, is exemplified. Throughout the above band, the CLPDA has a gain, return loss and polarization cross-talk of $\approx$ 6.6 dBi, $\lesssim$ -10 dB, and $\lesssim$ -27 dB, respectively. The design constraints, the procedure to tune its impedance and to minimize its dimension, etc. are elaborated. The analog receiver has a noise figure of $\approx 3$ dB and a receiver-noise-temperature ($T_{rcvr}$) of about 290 K. The receiver-flux-density ($S_{rcvr}$) is $\approx 5.3 \times 10^3 $, and $\approx 5.3 \times 10^5$ Jy at 50 and 500 MHz, respectively. The observed spectral data shows a Signal-to-Noise Ratio and Dynamic range of about 30 dB and 40 dB, respectively, at 50 MHz. The average polarization isolation / cross-talk of the CLPD varies from -30 dB to -24 dB over an azimuthal angle of $\pm 45^\circ$ with respect to the reference position angle ($0^\circ$). The average degree of circular polarization (DCP) is $\approx 100\%$ at the reference position and found to decrease gradually and reaches $\approx 80\%$ at an azimuthal angle of $\pm 45^\circ$.