Design of a Circularly Polarized Antenna System for Low Frequency Radio Astronomical Observations

Abstract

The Indian Institute of Astrophysics operates three different radio telescopes at the Gauribidanur Radio Observatory. These instruments are: I) Radio Heliograph to obtain twodimensional images of the solar corona; ii) Radio Spectrograph to obtain the dynamic spectrum of the radio emission associated with the transient energy releases in the solar corona; iii) Radio Polarimeter to obtain information on the solar coronal magnetic field. All the above instruments operate in the frequency range 40-120 MHz. The basic receiving element used in all the above three instruments is a linearly polarized log-periodic dipole (LPDA) which is a frequency independent antenna. Spectral observations of the solar corona in the frequency range 30 kHz-14 MHz are carried out using space-borne instruments. The ionospheric cut-off frequency (for ground-based observations) at Gauribidanur goes down up to 10 MHz at times. Taking advantage of this we wanted to explore the possibility of carrying out radio observations of the solar corona in the frequency range 10-40 MHz which will bridge the existing gap between ground- and space-based observations. Note that intense radio emission from Jupiter also occurs predominantly at low frequencies (18-39.5 MHz). To achieve the above, one needs to construct an antenna system that can work in the above frequency range (10-40 MHz). It is difficult to extend the low frequency coverage of the existing LPDAs in Gauribidanur since the antenna length will be extremely large (~10m) and there will be mechanical constraints. Also the low frequency band is usually crowded with Radio Frequency Interference (RFI) due to AM and other transmissions. The AM signals are predominantly linearly polarized and hence use of a linearly polarized antenna will have disadvantages. In view of the above it was decided to design and fabricate a circularly polarized antenna system. The aim of this project is to design a circularly polarized antenna system to cover the frequency range below 50 MHz. Since the ionosphere is opaque to radio signals below 10 MHz at Gauribidanur, the low frequency limit should be 10 MHz.