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GMRT Antenna Efficiencies
The efficiency relations shown in Section 19.5,
have not considered the effect of aperture blockage by feeds and feedsupport
frames (``quadripod legs'' in GMRTparlance). Simple geometrical
optics based models for such computation exist,[16] which are used along
with GMRTspecific efficiency relations, to produce the following table.
Limitations of this model are highlighted in [17].
Some of the loss terms can be expressed as equivalent noise
temperatures (see Chapter 3). The spillover temperature
is given by (see also Eqn 19.4.14)

(19.8.24) 
where is the ground temperature. Considering the reflectance
of soil at microwave frequencies, it is presumed as K.
Similarly, the meshleakage , scattered radiation by
the feed support frames , can also be expressed in terms of
. The overall system temperature (see Chapter 3) is the sum
of all these and the receiver noise temperature, and the sky
temperature,, which is assumed to be,

(19.8.25) 
where is the frequency of the received signal (in MHz). Hence

(19.8.26) 
Finally the figureofmerit of any radio antenna, is the
gainbysystem temperature ratio, , expressed as :

(19.8.27) 
where is flux density in units of Jansky, , is the physical
area of the parabolic dish and is the overall aperture
efficiency.
For a 1 Jy. source at the beam of the antenna and value of Boltzmann's
constant included in the above relation,

(19.8.28) 
Hence, the ratio is expressed in units of .
A summary of the relevant parameters for the GMRT antennas
is given in Table 19.4. These have been computed based
on the following assumptions.

K for 150,233 and 327 MHz bands;
K for 610 MHz and K for the 1000 to
1400 MHz bands.
 The surface rms,
values are
8.0, 9.0, and 14.0 mm respectively.
The agreement between the observed HPBW, gain and system temperature
and the computed values is in general quite good.
Table 19.4:
Calculated aperture efficiencies and system temperatures for the GMRT
antennas.

Frequency (MHz) 
Eff. 
150 
233 
327 
610 
1000 
1200 
1400 
Tap Eff. 
0.689 
0.823 
0.715 
0.775 
0.566 
0.533 
0.592 
Spill. Eff. 
0.952 
0.799 
0.944 
0.835 
0.967 
0.971 
0.971 
Mesh Eff. 
0.999 
0.999 
0.998 
0.991 
0.943 
0.941 
0.94 
RMS Eff. 
0.997 
0.992 
0.986 
0.948 
0.88 
0.835 
0.78 
Aper. Eff. 
0.652 
0.651 
0.664 
0.608 
0.452 
0.405 
0.422 
Tsys(K) 
428 
229 
152 
92 
65 
77 
62 

0.877 
1.64 
2.53 
3.81 
4.04 
3.02 
3.17 

52'39'' 
51'06'' 
21'15'' 
42'48'' 

19'26'' 


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