Two-element Phase-switched Interferometers for Radio Star Scintillation Studies at 110 and 74 MHz

Abstract

We describe here two-element phase-switched interferometers built for the study of ionospheric irregularities by recording the amplitude scintillations in the flux of the radio stars, Cas A and Cyg A. Two interferometers have been built. The 110 MHz interferometer has been operating at Ahmedabad and the 74 MHz one, which is an improved version of Ahmedabad unit, has been operating at Gulmarg. The 110 MHz interferometer has been described in some detail and improvements incorporated in the 74 MHz interferometer are briefly discussed. The 110 MHz interferometer consists of two Yagi antennas, spaced ten wavelengths apart, connected to a superhet receiver. A half wavelength transmission line is switched in and out, at the rate of 340 Hz, between one of the antennas and the input of the receiver, which modulates the signal received from a radio source of small angular size. The fringe pattern of such an interferometer is relatively insensitive to the cosmic noise background, because it is an extended radio source.

The interferometer equipment includes a low noise cascode amplifier, a converter, an IF amplifier, a square law detector, an audio amplifier tuned to the switching frequency, a phase-sensitive detector, a d.c. amplifier and a recording milliammeter.

A few specimen interferometer recordings of Cas A and Cyg A are shown and calibration procedure is briefly described.