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.
