Practical_Antenna_Handbook_0071639586

C h a p t e r 1 6 : M o b i l e a n d M a r i n e A n t e n n a s 379
lower frequencies, the 8-ft whip is too short and it becomes capacitive, thus requiring
an equal inductive reactance to cancel the capacitive reactance of the antenna at the
chosen operating frequency.
The lower the operating frequency, the worse the problem becomes. In fact, HF and
MF mobile operation suffers from a “double whammy” as the frequency drops: Not
only is the radiating monopole getting far too short for the frequency, but the ground
plane beneath it is becoming less and less effective, as well!
When the effects of imperfect ground planes and short vertical radiating elements
are combined, it’s not unusual to find that a typical HF mobile installation may have
overall efficiencies of between 1 percent and 10 percent those of a conventional quarterwavelength
radiator married to an extensive ground plane!
A
B
Types of Short Mobile Antennas
Given the impossibility of having a l/4 whip on the HF and MF bands while mobile,
the most common compromise is to add a combination of discrete and/or distributed
components to the mobile whip in hopes of increasing the feedpoint resistance and
eliminating or at least minimizing the feedpoint reactance. But what kinds of components?
And where do we place them in the existing antenna?
Figure 16.1 shows three basic configurations of coil-loaded HF antennas for frequencies
lower than the natural resonant frequency of the antenna. In each case, the
antenna is series-fed with coaxial cable from the base; point A is connected to the coaxial
cable center conductor, and point B is connected to the shield
and the car body, which serves as a (usually quite imperfect) ground
plane. The system shown in Fig. 16.1A is base-loaded. Although convenient,
simple to build, and the easiest to support mechanically,
base-loaded verticals are among the least efficient radiators around
and should be avoided if at all possible. Their inefficiency is a direct
result of the high-current portion of the antenna element being replaced
by a lumped-component loading coil, an approach we have
eschewed in earlier chapters.
Figure 16.1B depicts a center-loaded whip, with improved current
distribution. This configuration is by far the most common among
commercially available HF mobile antennas, although the coil may be
high enough and/or large enough to require special support.
Finally, we see the top-loaded coil system in Fig.16.1C. When
accompanied by a capacitive top hat, this is by far the most efficient
configuration, as has been proved time and time again at numerous
mobile “shoot-outs” at hamfests around the world. It also is the
most top-heavy of the three configurations, requiring the largest
coil, the strongest mast, and sometimes even guy wires or guy rods
A
B
A
B
high on the mast from anchors near the top of the vehicle.
Regardless of the specific configuration, in all three cases the
purpose of the coil is to cancel most, if not all, of the capacitive reactance
of the electrically short antenna at the operating frequency.
A B C
Figure 16.1 Loaded mobile antennas.