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.