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 645 Figure 28.16 Strain relieving the center insulator or balun. Installing Vertical Antennas At frequencies above 5 MHz, erecting and supporting HF verticals—both ground-Â mounted and elevated ground planes—is a relatively easy task for two people, provided the vertical is made of aluminum tubing strong enough to support itself during the brief period when it is being tilted from a horizontal to an upright position. At lower frequencies, verticals should be treated as towers (Chap. 29) or erected with the help of extensive rigging—often including some form of gin pole—and a larger crew. For years the Hy-ÂGain 14-ÂAVQ (now sold as the AV-Â14AVQ) has been a popular multiband vertical monopole. By using three parallel-Âresonant traps and a small capacitive top hat, the antenna provides a physically short vertical (18 ft overall) that is an electrical quarter-Âwavelength on 40, 20, 15, and 10 m. (The design is essentially unchanged from the original, pre-ÂWARC band, model, so there is no guarantee of low SWR on the 17-Â and 12-Âm bands.) It can be ground-Âmounted or elevated atop a mast as in Figure 28.17. In either case, the antenna requires radials, but the number and length are different for the two cases. When ground-Âmounted, at least 16 (nonresonant) radials that are roughly as long as the vertical is tall (18 ft) should be laid out in a circle around the base of the antenna. When elevated, however, three (resonant l/4) radials for each band, equally spaced around a 360-Âdegree circle, are sufficient. Ground-Âmount installation of the AV-Â14AVQ and similar verticals is simplicity in itself: Drive a 3-Âft length of 1½-Âin OD steel post into the ground, stand the assembled vertical upright, and clamp its base to the post with the U-Âbolts supplied. To complete the installation, attach radials to the base or add a ring of copper wire or soft copper plumbing tubing around the base as an attachment point. Install a lightning ground (see Chap. 30) of multiple 8-Âft ground rods in a circle with a radius of about 8 ft centered at the base of the vertical, and connect each ground rod directly and separately to the base
646 p a r t V I I I : M e c h a n i c a l C o n s t r u c t i o n a n d I n s t a l l a t i o n T e c h n i q u e s Base insulator TR3 TR2 TR1 Capacity "hat" Mounting pipe Figure 28.17 Mounting commercial trap vertical. with #6 or heavier gauge wire. Finally, connect 52-ÂW coaxial cable at the bottom of the base as shown in Fig. 28.18A or 28.18B and weather-Âseal the connection with heat-Âshrink tubing or a half-Âlap winding of electrical tape (as described earlier in this chapter) and a spray coating of clear acrylic lacquer. Although Hy-ÂGain does not require or suggest the use of guy wires for the vertical itself, a single set of lightweight, nonconducting guys such as nylon twine or ¼-Âin poly rope tied just above or just below the middle trap is not a bad idea unless your installation location is such that the antenna is at greater risk from people tripping over the guy ropes than it is from high winds. In years past the author preferred the performance of the 14-ÂAVQ as an elevated ground plane, even though that installation is a little more involved. Telescoping TV antenna mast material, such as that long sold by RadioShack and others, is suitable for getting the base of the antenna 15 ft (corresponding to about l/8 at the lowest operating frequency) or more up in the air. Because three l/4 radials are required for each operating band, the radials for the lowest band (40 m) can double as guy wires for the top of the mast, if you so choose. The tradeoff is that adjusting the length of the radials for minimum SWR may be a little more difficult if the radials are doing double duty as guy wires. On the other hand, keeping separate sets of guy wires and radials from getting tangled up during the installation can often be frustrating. One alternative to a telescoping metal mast is a pivoted wood mast, as shown in Fig. 28.19. At a minimum, one set of guy ropes just below the top of the mast is sufficient, although a set of lightweight guy ropes at the midpoint of the vertical eases wind-Âloading stresses on the antenna. Although a ground-Âplane vertical does not require any additional RF ground system other than its elevated radial system, it does require a lightning ground at the base of the mast. It is a very good idea to also ground the coaxial cable outer conductor there as it comes down from the connector above and heads off to the radio room. A simple way to do that is to use two separate pieces of coaxial cable—one just long enough to run from the SO-Â239 at the base of the vertical to ground level, and the other to run from the base of the mast to the radio equipment—and join them at the base with a double female (or barrel) adapter mounted on a small piece of aluminum mechanically connected to the center of the lightning ground system. Alternatively, commercial lightning suppressors that house the barrel, a discharge circuit, and a convenient lug for the ground connection are not very expensive. Because the normal stresses on guy lines for verticals erected in this fashion are much lower than the tension on halyards and other support lines for horizontal wires, smaller-Âdiameter rope is appropriate. Certainly, ¼-Âin poly is about the maximum that would ever be needed for verticals of this size.
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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 645<br />
Figure 28.16 Strain relieving the center insulator or balun.<br />
Installing Vertical <strong>Antenna</strong>s<br />
At frequencies above 5 MHz, erecting and supporting HF verticals—both ground-Â<br />
mounted and elevated ground planes—is a relatively easy task for two people, provided<br />
the vertical is made of aluminum tubing strong enough to support itself during<br />
the brief period when it is being tilted from a horizontal to an upright position. At lower<br />
frequencies, verticals should be treated as towers (Chap. 29) or erected with the help of<br />
extensive rigging—often including some form of gin pole—and a larger crew.<br />
For years the Hy-ÂGain 14-ÂAVQ (now sold as the AV-Â14AVQ) has been a popular<br />
multiband vertical monopole. By using three parallel-Âresonant traps and a small capacitive<br />
top hat, the antenna provides a physically short vertical (18 ft overall) that is an<br />
electrical quarter-Âwavelength on 40, 20, 15, and 10 m. (The design is essentially unchanged<br />
from the original, pre-ÂWARC band, model, so there is no guarantee of low<br />
SWR on the 17-Â and 12-Âm bands.) It can be ground-Âmounted or elevated atop a mast as<br />
in Figure 28.17. In either case, the antenna requires radials, but the number and length<br />
are different for the two cases. When ground-Âmounted, at least 16 (nonresonant) radials<br />
that are roughly as long as the vertical is tall (18 ft) should be laid out in a circle around<br />
the base of the antenna. When elevated, however, three (resonant l/4) radials for each<br />
band, equally spaced around a 360-Âdegree circle, are sufficient.<br />
Ground-Âmount installation of the AV-Â14AVQ and similar verticals is simplicity in<br />
itself: Drive a 3-Âft length of 1½-Âin OD steel post into the ground, stand the assembled<br />
vertical upright, and clamp its base to the post with the U-Âbolts supplied. To complete<br />
the installation, attach radials to the base or add a ring of copper wire or soft copper<br />
plumbing tubing around the base as an attachment point. Install a lightning ground (see<br />
Chap. 30) of multiple 8-Âft ground rods in a circle with a radius of about 8 ft centered at<br />
the base of the vertical, and connect each ground rod directly and separately to the base
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