GMRT Antenna Efficiencies

The efficiency relations shown in Section 19.5, have not considered the effect of aperture blockage by feeds and feed-support frames (``quadripod legs'' in GMRT-parlance). Simple geometrical optics based models for such computation exist,[16] which are used along with GMRT-specific 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)

$${{T}_{Sp}} = {{T}_g}{\cdot}{\frac{\int_{{\theta}_0}^{{\pi}/... ...{\pi}{\vert E({\theta})\vert^2{\sin( {\theta})}d{\theta}}}}$$ (19.8.24)

where ${{T}_g}$ is the ground temperature. Considering the reflectance of soil at microwave frequencies, it is presumed as $259^{\circ}\:$ K.

Similarly, the mesh-leakage ${{T}_{ml}}$, scattered radiation by the feed- support frames ${{T}_{sc}}$, can also be expressed in terms of ${{T}_g}$. The overall system temperature (see Chapter 3) is the sum of all these and the receiver noise temperature, ${{T}_r}$ and the sky temperature,${{T}_{sky}}$, which is assumed to be,

$${{T}_{sky}} = 3.0 + 20{\cdot}{(408/f)^{2.75}}, \\$$ (19.8.25)

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

$${{T}_{sys}} = {{T}_r}+{{T}_{sky}}+{{T}_{Sp}}+{{T}_{ml}}+{{T}_{sc}}. \\$$ (19.8.26)

Finally the figure-of-merit of any radio antenna, is the gain-by-system temperature ratio, ${G/{T_{sys}}}$, expressed as :

$$G = {\frac{S{A_{p}}{{\eta}_a}}{2k}}, \\$$ (19.8.27)

where $S$ is flux density in units of Jansky, ${A_{p}}$, is the physical area of the parabolic dish and ${{\eta}_a}$ is the overall aperture efficiency. For a 1 Jy. source at the beam of the antenna and value of Boltzmann's constant $k$ included in the above relation,

$$G = {\frac{{A_{p}}{{\eta}_a}}{2760}}. \\$$ (19.8.28)

Hence, the ratio ${G/{T_{sys}}}$ is expressed in units of ${{Jy}^{-1}}$.

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.

1. $T_r = 100^{\circ}\:$K for 150,233 and 327 MHz bands; $50^{\circ}\:$K for 610 MHz and $35^{\circ}\:$K for the 1000 to 1400 MHz bands.
2. The surface rms, $\sigma_1,\ \sigma_2,\ \sigma_3$ 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($^{\circ}\:$K)	428	229	152	92	65	77	62
$\frac{G}{{T_{sys}}}\:{\times}\:10^{-3}$	0.877	1.64	2.53	3.81	4.04	3.02	3.17
$HPBW$	$\:2^{\circ}\:$52'39''	$\:1^{\circ}\:$51'06''	$\:1^{\circ}\:$21'15''	$\:0^{\circ}\:$42'48''		$\:0^{\circ}\:$19'26''