Practical_Antenna_Handbook_0071639586

376 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
But suppose your vehicle is leased? Or has a ragtop—in other words, it’s a convertible?
Or there’s only a few inches of clearance between the top of your sport utility vehicle
(SUV) and your garage door? Then you will need to consider alternative locations
for your antenna.
For VHF or UHF whips, the next best location is usually the center of the trunk
lid—assuming, of course, that your vehicle has a trunk! Even though the roof of a car
will be well above the base of the VHF or UHF whip, this isn’t necessarily as bad for
your signal or your antenna pattern as you might think. For one thing, in the typical
sedan or convertible, anything mounted on the trunk lid has a clear horizontal shot
(through the car’s windows) in many front-facing directions. Secondly, collinear (see
Chaps. 9 and 20) whips for VHF and UHF are relatively inexpensive and can bring at
least one element of the collinear up above the roofline for a full 360-degree “view”. A
secondary advantage of locating the mobile antenna at the rear of the vehicle is that in
many installations noise pickup from the various ignition components and microprocessors
in the front will be less.
If a roof-mounted or trunk lid–mounted whip is not an option (because of garage
doors or other constraints), sometimes a VHF or UHF whip can be mounted on top of a
front fender (directly above a wheel well, in other words) or along the side or rear of the
hood, in the gap between the hood and other vehicle body sheet metal sections. In other
cases—especially SUVs—whips can be mounted to the oversize outside rearview mirror
brackets found on larger vehicles, just as over-the-road drivers of those “big rigs”
have their CB antennas mounted on their mirror brackets. Bear in mind, however, that
with these latter locations, the overall symmetry of the ground plane is compromised.
Finding an acceptable ground plane on a fiberglass vehicle such as the Corvette is a
more difficult task. However, one advantage to having a Corvette (actually, the author
can think of many!) is that low whips, such as bumper-mounted ones, will encounter
less loss of signal from massive pieces of metal near the whip. Further, it should be possible
to get at least a skeletal ground plane from the frame and drivetrain members,
provided care is taken to make sure all these parts are electrically bonded together and
to the transmission line ground at the base of the whip.
If all else fails, bumper mounts, side fender mounts, and trailer hitch mounts can be
used, even though the typical VHF or UHF whip (even the collinear) will be well below
the roofline of the vehicle. However, for HF antennas, these last-named mounting techniques
are actually the most common!
Mobile Antennas for HF and MF
Radiating a strong signal from a mobile installation is very much a frequency-dependent
problem. While many vehicles provide a satisfactory ground plane and sufficient
vertical clearance for a l/4 or taller whip on 2 m and above, having a strong HF mobile
signal is one of the most enduring challenges of CB and amateur radio (and some other
services, as well). Even in the presence of a perfect ground plane, most mobile vertical
monopoles below 30 MHz are such a small fraction of l/4 that radiation efficiency and
radiation resistance are extremely low by comparison to well-designed fixed station
antennas.
The magnitude of the standing wave of current on a l/4 monopole is generally approximated
as a cosine curve having maximum amplitude at the base feedpoint and