Practical_Antenna_Handbook_0071639586

C h a p t e r 1 7 : E m e r g e n c y a n d P o r t a b l e A n t e n n a s 391
Improving Radiator Efficiency
If at all possible, the smartest solution to the first component of loss is to replace the
mobile antenna with a longer, more efficient, stowable antenna that can be brought
out and erected when needed. In real-world antenna systems, efficiency of radiation
increases dramatically as the length of the radiator approaches l/4. One candidate is
a surplus military HF whip antenna. Intended for jeeps and communications trucks,
these antenna/tuner combinations are collapsible and are as efficient as any on the
market. Alternatively, a quarter-wave length of wire (or, with a suitable antenna
tuner, anything longer than the mobile whip) can be attached where the mobile whip
was located previously. Carrying a ball of twine attached to a baseball with a smalldiameter
hole drilled through its center might not be a bad idea. It should be possible
to get one end of the antenna wire up in the air perhaps 20 or 25 ft, draped over
a tree branch, with such a rig. If necessary, arrange to support the far end of the wire
a little distance from the rear of the vehicle so that the lack of height is made up for
by the added horizontal distance the wire is covering. (Move the vehicle, if necessary.)
This creates a sloping vertical, which is not all that bad an antenna, believe it
or not.
Reducing Ground Losses
Even for frequencies as high as 6 m the typical vehicle body is not large enough to provide
an adequate ground plane. A better solution is to provide a counterpoise or elevated
ground plane. As we saw in the chapter on verticals (Chap. 9), the basis for this
recommendation is that a vertical monopole, such as a mobile whip, is really only half
an antenna. The other half is distributed throughout the ground return for the antenna,
and a failure to provide an adequate ground system is equivalent to further compromising
the antenna itself.
Figure 17.1A is the schematic representation of a proposed way to enhance the
ground return efficiency for a mobile whip used in a portable or emergency (i.e., vehicle
not in motion) situation. At its minimum it consists of the addition of three or four radials,
each l/4 in length on the band(s) of interest and spread as equally as possible
around the full 360-degree compass rose. Ideally, such a ground-plane antenna should
be mounted l/8 or higher with some kind of additional support, but the degradation in
performance is gradual as the height becomes progressively lower, so you do the best
you can with what you have.
When selecting radial wires, the primary requirement is that they be strong enough
to withstand the mechanical rigors of being repeatedly installed, taken down, coiled up,
and stored. Current-handling capability in radials is seldom an issue, regardless of wire
diameter. A number of suppliers of antennas and accessories sell spools of bare #18 wire
for radials; a 500-ft spool takes up little space in a car trunk. For many portable and
emergency setups, however, insulated wire may be best because there may be a need to
drape one or more radials across a metallic object of some sort; clearly, direct metal-tometal
contact with other objects would be undesirable and could cause extra noise in
the receiver. For 40 m, four l/4 radials total 132 ft. Allowing some extra wire for connections
to the base of the whip and for attaching to rope or twine at the other end, 150
ft of #14 wire is all that is needed.