Practical_Antenna_Handbook_0071639586
246 P a r t I I I : H i g h - F r e q u e n c y B u i l d i n g - B l o c k A n t e n n a s Shorting bar Gamma rod C 1 Coax to XMTR C 1 C 2 Figure 9.11B Gamma-fed grounded vertical. Figure 9.11C Omega-fed grounded vertical. As the radiating element of a vertical monopole increases beyond l/4, both the resistance and the reactance seen at the series feedpoint increase, as well. Thus, for any significant increase in the length of the vertical radiator, direct matching to commonly available transmission lines will result in increased SWR on the transmission line, so some form of matching network or ATU should be employed. Again, an alternative is to directly ground the base of the radiator and use some form of shunt-feed network to accomplish the proper match. The base impedance of a 5 ⁄8-wavelength antenna is about 1600 Ω—certainly not an acceptable match for anything but a custom-designed transmission line. So some form of impedance matching is needed. For a single-band 5 ⁄8l antenna, one option is to use a form of stub matching employing unbalanced coaxial cable, such as shown in Fig. 9.12. The lengths are given by
C h a p t e r 9 : V e r t i c a l l y P o l a r i z e d A n t e n n a s 247 Radiator element 5 8 L 1 L 2 4 4 8 Radials Coax to XMTR Figure 9.12 Q-section for match feedpoint impedance to line. L 1 122 (feet) = F(MHz) (9.2) or L 1 38 (meters) = F(MHz) (9.3) and L 2 30 (feet) = F(MHz) (9.4) or L 2 9 (meters) = F(MHz) (9.5)
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C h a p t e r 9 : V e r t i c a l l y P o l a r i z e d A n t e n n a s 247<br />
Radiator<br />
element<br />
5 8<br />
L 1<br />
L 2<br />
4<br />
4<br />
8 Radials<br />
Coax to<br />
XMTR<br />
Figure 9.12 Q-section for match feedpoint impedance to line.<br />
L<br />
1<br />
122<br />
(feet) =<br />
F(MHz)<br />
(9.2)<br />
or<br />
L<br />
1<br />
38<br />
(meters) =<br />
F(MHz)<br />
(9.3)<br />
and<br />
L<br />
2<br />
30<br />
(feet) =<br />
F(MHz)<br />
(9.4)<br />
or<br />
L<br />
2<br />
9<br />
(meters) =<br />
F(MHz)<br />
(9.5)