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