150 MHz Feed

This feed employs four dipoles in a ``boxing ring'' configuration, placed above a plane reflector. The unique feature of the dipole is that it is wide-band i.e. has an octave bandwidth. It is a folded dipole with each arm being a ``thick'' dipole. A dipole is called 'thin' when its diameter, $d>0.05{\lambda}$. For such dipoles a sinusoidal current distribution can be assumed for the computation of input impedance and related radiation parameters.

Thin dipoles have narrowband radiation characteristics. One method by which its acceptable operational bandwidth can be increased is to decrease the $l/d$ ratio. For example, an antenna with a $l/d\:{\approx}\:5000$ has an acceptable bandwidth of about 3%, while an antenna of the same length but with a $l/d\:{\approx}\:260$ has a bandwidth of about 30%. By folding the dipole, one gets a four-fold increase in input impedance compared to a simple dipole. The 150 MHz feed also has a transmission line impedance transformer coupled to the excitation point [9].

Traditionally crossed-dipoles are used to give sensitivity to both polarizations. However since a crossed-dipole configuration in this design would be extremely cumbersome, a ``boxing ring'' design was instead chosen. Here one pair of dipoles at ${\lambda}/2$ spacing provides sensitivity to one linear polarization. Another pair orthogonally oriented with respect to the first pair gives sensitivity to the orthogonal polarization. The overall dimensions of the feed are:

• Folded dipole length : $0.39\:{\lambda}$
  
• Dipole height above reflector : $0.29\:{\lambda}$
  
• Reflector (diagonal of octagon) : $1.2\:{\lambda}$
  

The dipoles have an $l/d$ ratio of 6.48, and the phase center was determined to be at a height of 100 mm above the reflector. The feed's impedance bandwidth can be seen on the VSWR plot of Figure 19.8

Figure 19.8: VSWR for the 150 MHz feed.

The usable bandwidth for a feed is given approximately by the range for which SWR $\:{\leq}\:2.0$. By this criteria, the frequency coverage of the 150 MHz feed is from 117 MHz to 247 MHz, i.e. a bandwidth of 130 MHz, or 86% bandwidth. The radiation pattern gives an edge taper, ${T_{E}} = -9$ dB.

One undesirable feature of this feed is the high value of cross-polarization, as compared to that at other frequencies (see Figure 19.9)^19.4. The cross-polar peak for 150 MHz. is -17 dB and the on-axis cross polarization is also at about the same level.

Figure 19.9: The cross polarization of different GMRT feeds. The 150 MHz feed has relatively larger cross-polarization.

One-pair of outputs from the dipoles which are parallel to each other are connected to a power-combiner, whose output goes to one port of the quadrature hybrid (which adds two linear polarized signals to yield one circular polarized signal). Similarly the orthogonal pair of dipoles are connected to the other port of the hybrid. Both the power combiners and the quadrature hybrid are mounted inside one of the front-end chassis, placed behind the feed.

Footnotes

...fig:150XPolar)^19.4

Note that the cross polar pattern was measured using the standard technique outlined in [4 ; pp.177-79]. The cross-polar levels are measured with respect to a co-polar maximum of 0 dB.