Practical_Antenna_Handbook_0071639586

380 P a r t V : H i g h - F r e q u e n c y A n t e n n a s f o r S p e c i a l i z e d U s e s
A modified version of the coil-loaded HF mobile antenna is shown in Fig. 16.2A. In
this configuration the loading inductance is divided between two individual coils, L 1
and L 2 . Coil L 1 is adjustable with respect to the antenna, while L 2 is fixed; coil L 2 is
tapped, however, in order to match the impedance of the antenna to the characteristic
impedance of the coaxial-cable transmission line. When tuning this antenna, two instruments
are useful: a field strength meter and a VSWR meter. Placed at least a few wavelengths
from the mobile installation, the field strength meter gives a relative indication
of the signal radiated from the antenna, while the VSWR meter helps determine the
state of the impedance match.
Variations on this theme are found in Figs. 16.2B and 16.2C. In the first variation, a
helical whip, similar to the family of mobile whips available from Kenwood, is shown
in Fig. 16.2B. In this configuration, the inductor is distributed along the length of a fiberglass
antenna rod. Rather than being a cylindrical metal tube or rod, the conductor
is a wire helically wound on a fiberglass shaft and encased in shrink-wrap sleeving
to minimize abrasion damage. An adjustablelength
tip section tunes the antenna to resonance
if the mobile transmitter or transceiver
does not have a built-in ATU. The length of
this tip should be set with the help of a VSWR
meter. Fixed station versions of this type of an-
Adjustable
tip
L 1
COAX to
RIG or RCVR
L 2
Helically wound
wire embedded
in fiberglass
A
B
Figure 16.2A Practical mobile antenna for HF
bands below 10 m; L 1 resonates, L 2 matches
impedance.
Figure 16.2B Helically wound mobile antenna.