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327 MHz Feed

Generally a dipole has a broader H pattern than its E pattern (the E pattern being in the plane containing the dipole). Recall from the discussion in section 19.4.1 that for good cross-polarization properties it was essential to have matched $E$ and $H$ plane patterns. An elegant method for achieving this pattern matching was given by P.S.Kildal [10], and involves placing a beam forming ring (BFR) above the dipole19.5. The conducting ring is placed above the dipole in a plane parallel to the reflector and is supported by dielectric rods. The beam forming ring compresses the H-plane pattern while it has no significant effect on the E-plane.

The optimum dimensions of the dipole, BFR and reflector were arrived at by careful measurements done on a scaled-up version (i.e. at 610 MHz) and a follow-up measurements on a prototype 327 MHz model. The values arrived at were :

The measured phase center is at 26 mm above the reflector for both E and H- planes. Crossed dipoles are employed for dual polarization. The 327 MHz feed actually deviates slightly from the original Kildal's design - there are sleeves over the dipoles. These sleeves increase the bandwidth of the feed [5]. The VSWR plot for the 327 MHz feed is given in Figure 19.10.

Figure 19.10: The VSWR as a function of frequency for the 327 MHz feed.
\begin{figure}\centerline{ \psfig{figure=swr327Cor.ps,width=140mm} }\end{figure}

For SWR $\:{\leq}\:2.0$, the bandwidth is 138 MHz.(286 to 424 MHz.) The measured antenna pattern is given in Fig 19.11. The edge taper, ${T_{E}}$ is $-12.2$ dB. Fig 19.9 shows the cross-polar pattern. It is seen that a cross-polar maximum of $-27.5$ dB (mean value) has been achieved.

Figure 19.11: The measured antenna pattern at 327 MHz
\begin{figure}\centerline{ \psfig{figure=kf327Cor.ps,width=140mm} }\end{figure}

The linear polarized outputs of the dipoles are mixed in a quadrature hybrid at one of the front-end chassis to produce two circular polarized (both left and right) signals, which go further into the amplifying, signal conditioning circuits of front-end Electronics.



Footnotes

... dipole19.5
This design has been christened 'Kildal Feed' in the local jargon.

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Next: Dual-Frequency Coaxial Waveguide Feed Up: GMRT Feeds Previous: 150 MHz Feed   Contents
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