Practical_Antenna_Handbook_0071639586

396 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
depend on the magnitude of the planned operation and the conveyance mechanisms
available. A list of possible components for a functioning radio station for backpackers
will be substantially shorter than one for stowing some gear in a vehicle prior to motoring
into the backcountry. Size and weight are major constraints in both cases, but the
scale is different.
Probably the single biggest antenna item for portable or emergency use is the antenna
support, if required. In many parts of the world, there are trees all around but in
other areas, sand or fields are all that is visible for miles around, and some form of temporary
mast(s) must be employed.
Where trees are available, use of a bow with suitably modified arrows may be the
best way to get support lines up high. For nearly 30 years one of the authors used a
“portable” bow that could be broken down into small sections to erect not only temporary
dipoles but also all his home station wires that were supported by trees. See Chap.
28 for more information on this and similar techniques.
If trees are not plentiful, various mast alternatives, ranging from extremely heavy
(military surplus or steel TV antenna sections) to reasonably light (plastic pipe and fiberglass
rods) are available. Your choice is determined primarily by whether you expect to
have help erecting the mast and by how much room you have for stowing and transporting
it. Also, be aware that some masts consist of telescoping sections that get progressively
smaller (and more limber) toward the top. Most masts collapse to 6 or 8 ft in length for
transport and stowage, but can be pushed up to heights of 18, 25, 30, 40, or 50 ft, depending
upon the type selected. Keep in mind, when shopping for these masts, that the larger
models are considerably heavier than shorter models, and they require two or more people
to install them. Erecting a 40- or 50-ft telescoping mast is not a one-person job!
Masts made of PVC plastic plumbing pipe are somewhat lighter. If mast sections
can be carried on top of the vehicle, then lengths up to 10 ft are available and can be
joined together at the site with couplings. If the sections are going to be transported
inside the vehicle, be sure to determine the maximum length that will fit before cutting
longer sections into smaller ones. Be very careful not to use PVC pipe that is too small,
however; PVC pipe is relatively thin walled and can become quite flexible as it is extended
in length. Sizes below 1.5-in diameter will not easily stand alone without guying.
Nor is PVC pipe particularly self-supporting. While a single 10-ft section might be,
two or more sections together will not support themselves plus the weight of any reasonable
antenna. Guying will be necessary—either with ropes or on a temporary basis
with synthetic (or other water-repellant) twine.
If you plan to erect temporary or portable antennas that represent a heftier downward
or sideward pull on the support mast(s)—an 80-m dipole, for instance—and you
have a motor vehicle for transportation, you may wish to employ the heavier but more
rigid steel TV antenna masts. Available in 5- and 10-ft lengths, these masts are flared on
one end and crimped on the other for joining together to form longer lengths. Make
sure you procure the mating guy wire rings, too; although not absolutely necessary,
they can make the job of putting the antenna up and keeping it up easier.
Over the years the authors have used military and steel TV mast sections many
times—especially during Field Day, the annual American Radio Relay League (ARRL)
emergency preparedness weekend exercise. The inverted-vee antenna is, of course, a
natural for this kind of event, but once one group the author was with mounted a Hustler
mobile antenna at the top of a 20-ft mast with four radials; it worked surprisingly
well given the low output power (30 W) being used.