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 dipole^19.5. 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 :

• Reflector diameter : $2.2\lambda$.
• Height of dipole above reflector : $0.26\lambda$.
• BFR diameter : $1.22\lambda$.
• BFR height above reflector : $0.51\lambda$.

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.

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

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

... dipole^19.5

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