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 and plane patterns. An elegant method for achieving this pattern matching was given by P.S.Kildal , 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 . The VSWR plot for the 327 MHz feed is given in Figure 19.10.
For SWR , the bandwidth is 138 MHz.(286 to 424 MHz.) The measured antenna pattern is given in Fig 19.11. The edge taper, is dB. Fig 19.9 shows the cross-polar pattern. It is seen that a cross-polar maximum of dB (mean value) has been achieved.
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.