Development of cross-polarized log-periodic dipole antenna for low frequency radio spectral observations

Abstract

The solar coronal mass ejections (CMEs) play a direct and vital role in deciding the space weather, so it becomes inevitable to study their dynamical properties in the lower corona (i.e., over the radial distance range: 1.05 – 3.0 RΩ; where RΩ = Radius of the solar photosphere) both prior to and after their time of eruption in the context of reliable spaceweather forecasting. Since the CMEs are found to be magnetically driven and often associated with different kinds of non-thermal energy releases, one can use the latter to study the properties of CMEs and to estimate the strength of the associated magnetic field (B). The above objectives can be met by obtaining radio spectral records (both total and circularly polarized intensities) over a broad bandwidth (i.e. in the 50 – 500 MHz range in order to cover the aforesaid radial distance range) with the help of an array consisting of crosspolarized, broad band, low frequency radio antennas. Hence we aim to design a crosspolarized log-periodic dipole antenna (CLPDA) which can operate over the above mentioned frequency range and to commission an array using CLPDAs with suitable backend receiver systems to record the radio spectra of the solar corona. In order to accomplish the above objective, we have designed a log periodic dipole antenna (LPDA) that can operate over the 50-500 MHz frequency range and determined its VSWR, HPBW, gain, effective collecting area, etc. through various tests in the field. Having measured the VSWR values to be less than 2.0 throughout the band, we ensured its impedence to be nearly equal to that of the feeding system which is 50 Ω. Since CLPDA is nothing but a combination of two LPDAs fixed to a common axis whose orientations are orthogonal to each other, we replicated the LPDA mentioned above and clamped two of them together orthogonally with specially designed insulators. At this stage, one has to ensure a minimum level of cross-talk between the two LPDAs of the CLPDA since they are fixed very close to each other. As the test reports of CLPDA at microwave frequencies indicate, we minimized the cross-talk level by reducing the spacing between the transmission lines of the LPDA; this led to the replacement of square-tube-transmission-line of a LPDA with a thinsolid- flat for our CLPDA design. We measured a cross-talk of -30 dB for our antenna as against a -20 dB CLPDA fabricated with a square tube transmission line; the test setup for measuring the cross-talk was readied using a linearly-polarized-transmitter and the above CLPDA-receiver. Later on an observational setup to detect the circularly polarized wave was developed using the CLPDAs, analog frontend receivers, quadrature combiner, etc. In order to characterize the system, we had developed both right and left circularly polarized feeds. Having done that, we went ahead and measured the degree of circular polarization (dcp) by correcting for different kinds of offsets due to various components of the receiver chain; after a careful study, we could detect the dcp of the transmitted signal within an error limit of ± 2 %. Finally observations of the Galactic center and the Sun were carried out successfully and the spectra were obtained with the setup.