A radio telescope in its simplest form consists of three components (see also Chapter 3), (i) an antenna that selectively receives radiation from a small region of the sky, (ii) a receiver that amplifies a restricted frequency band from the output of the antenna and (iii) a recorder for registering the receiver output. In this chapter we focus on the antenna, and in particular the antennas used for the GMRT.
The GMRT antennas are parabolic reflector antennas. The first reflector antenna was invented by Heinrich Hertz in 1888 to demonstrate the existence of electromagnetic waves which had been theoretically predicted by J.C.Maxwell. Hertz's antenna was a cylindrical parabola of and operated at a wavelength of 66 cm.(450 MHz). The next known reflector antenna was that constructed in 1930 by Marconi for investigating microwave propagation. After that, in 1937, Grote Reber constructed the prototype of the modern dish antenna - a prime-focus parabolic reflector antenna of 9.1 m. diameter, which he used to make the first radio maps of the sky. During and after World War II, radar and satellite communication requirements caused great advances in antenna technology.