Practical_Antenna_Handbook_0071639586

C h a p t e r 2 8 : S u p p o r t s f o r W i r e s a n d V e r t i c a l s 641
Halyards and Other Support Lines
There are many choices for materials to use as a halyard or support line, but not all of
them are good choices. Of paramount concern is long-Âterm exposure to sun, wind, and
moisture. In general, natural cordage (hemp, for instance) is a very poor choice except,
perhaps, for a very short period (e.g., a few weeks). Nylon rope—despite having a
much longer life—is far too stretchy. Instead, various “poly” compounds, including
polyester and polypropylene, are worth considering, as is braided Dacron. Possible sources
include large discount stores, building supply stores, boat dealers, outdoor (mountaineering)
stores, your local hardware store, and a number of stores specializing in Internet
sales, as listed in App. B. If total cost is an issue, careful shopping is a must; prices for
comparable cordage may vary by as much as 4:1, depending on the market the seller is
focused on.
Keep in mind that the safe working range for rope is typically less than 15 percent of
its rated breaking strength. Thus, for an antenna that experiences up to 100 lb of tension
during windy weather, a 750-Âlb rated rope should be on your shopping list.
Choosing the best diameter for your cordage is a tradeoff among required strength,
size and cost of any pulleys (or blocks, in nautical terminology), and what is comfortable
for pulling by hand (preferably with reinforced gloves). For supporting a typical dipole,
somewhere around 5 ⁄16-Âin is a good starting point.
Virtually all cordage will eventually unravel at a cut end unless the end is treated.
Laid or twisted three-Âstrand rope can be eye-Âspliced or back-Âspliced, but braided ropes will
need to have their ends fused (melted), taped, or whipped with twine. Fusing can be done
with a soldering gun that has a special tip, a propane torch, or even (on a windless day)
a simple match. Always fuse cordage outdoors; the fumes from heated synthetics
should not be inhaled, and burning poly rope ends often melt and drip on the floor.
The antenna support line is connected to a spring on one end (see Fig. 28.6) and/or
a counterweight on the other end. As mentioned earlier, the spring should be stout
enough so that it is partly extended on a calm day with the antenna support lines normally
tensioned. (Ideally, with no wind the spring should be extended less than 50
percent of its total possible expansion.) For an 80-Âm dipole and about 100 ft of open-Â
wire line, both constructed from #10 enameled copper, a stiff garage door spring seems
to work best. The author’s 80-Âm dipole has stayed up for nearly 20 years using this approach!
The counterweight should be just enough to balance the weight of the antenna for
a reasonable amount of sag along the wire—probably between 15 and 40 lb for the 80-Âm
dipole and OWL described in the previous paragraph. As wind causes the supporting
tree limbs to move up and down or back and forth, the counterweight rises and falls,
thereby reducing the chances of snapping a support line or the antenna wire itself. The
choice of counterweight is limited only by your imagination: drapery cord weights, a
small bucket of rocks, a gaggle of fishing weights, a cinder block, and (in one case) a
burned-Âout automobile starter motor. (The mounting hole on the front boss of the motor
was ideal for accepting the rope!) Just be sure to locate the counterweight where it cannot
harm anyone or anything should the rope break. In that respect, it’s helpful to keep
the counterweight in balance at a point no more than a few feet above the ground, if
possible.
Don’t underestimate the value of making the dipole out of ordinary soft-Âdrawn
copper (house wiring, in other words). Although you may periodically have to retension
the support lines, such wire is more able to withstand the shock of wind gusts