Antennas and Feeds

The GMRT antennas are 45m alt-azimuth mounted parabolic prime-focus dishes. While the dishes can go down to an elevation of 16$^\circ$, at present, the elevation limit has been set 17.5$^\circ$, giving a declination coverage from $-$53$^\circ$ to $+$90$^\circ$. The slew speed of the antennas is 20$^\circ$/min on both axes and they are not operated when winds are higher than 45 km/h. There is a rotating turret at the focus on which the different feeds are mounted.

The GMRT has frequency range 30$-$1500 MHz, of the following five main frequencies are in operations viz. 150 MHz, 233 MHz, 325 MHz, 610 MHz & 1000$-$1450 MHz. The 50 MHz feed is under construction and testing. Herewith we present the description of type of each feed.

The table 2.1 gives a description of the feeds of the GMRT.

Table 2.1: Feed Description.

Frequency	Type of feed
(MHz)
50	Half-wave dipole with linear reflector (yet to be installed)
153	Two Orthogonal pairs of dipoles over a plane reflector
233	Dual concentric coaxial cavity
610	Dual concentric coaxial cavity
327	Half-wave dipole over a ground plane with a beam-forming ring
	(Kildal feed)
1420	Corrugated horn

The 610/233 MHz feed is a dual concentric coaxial cavity in which both the feeds are mounted on same face and one can use any one of them with both the polarization or both of them, but having only one polarization for each frequency. The L-band (1420 MHz) feed is a corrugated horn having four sub-bands each of 120 MHz bandwidth centered at 1060 MHz, 1170 MHz, 1280 MHz & 1390 MHz. The user can also choose the full band (from 880 MHz to 1500 MHz); this is called bypass mode of the 1420 band.