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 629 Figure 28.3 Typical center-Âfed wire dipole. Center Insulators and Feedline Connection The l/2 dipole’s nominal feedpoint impedance at resonance is 73 W in free space and at multiples of a quarter-Âwavelength (l/4) above ground, so it is a reasonably good match to standard 50-Â or 75-ÂW coaxial cable. When fed with coax, a 1:1 balun transformer or common-Âmode RF choke is often inserted at the feedpoint. Far more important to the long-Âterm success of a horizontal wire than a balun are the quality, strength, and weather resistance of the electromechanical connection between the antenna and the feedline in the face of prolonged exposure to the environment. In virtually all installations (except perhaps for the attic dipole), this junction is subject to temperature and humidity changes, precipitation of all kinds (including ice and snow in many climates), ultraviolet rays from direct sunshine, continual vibration, and abrupt changes in tension resulting from gross motion in the wind. A common (but poor) practice is to strip the insulation back a few inches at one end of a coaxial cable, part the braid and center conductor, and connect them with ordinary electrical solder to each side of the dipole. We then wonder why the darn thing breaks apart in the next windstorm or why our VSWR measured back at the radio room seems to change over time. Unprotected solder joints exposed to the elements eventually turn gray, brittle, and powdery, and eventually crack under environmental stresses that conventional solder was never designed to resist. Solder for electrical and electronic circuits is not intended to provide a mechanical connection—it has little strength—nor is it meant to be exposed to the elements. Equally disastrous, coaxial cable that is open at one end is very hard to protect from moisture ingress; over time, it can become saturated with water. Over the years, there have been many reported cases of water dripping from the indoor end of cables onto the radio desk! Even if you elect not to use a balun transformer, a ready-Âmade center insulator that accepts a PL-Â259 or similar connector helps minimize or eliminate many of these problems. Figure 28.4A shows a common form of center insulator for use with dipoles and other wire antennas. At the bottom is an SO-Â239 coaxial cable receptable, shielded from direct rainfall by the assembly above it.Two eyebolts on the sidewalls provide mechanical strain relief for attaching the two halves of the dipole to the insulator. Although this particular style of center insulator is a compromise because it includes solder connections, if enough slack is provided in the pigtails that attach to the solder lugs the actual solder joint is under little or no mechanical strain and is far less likely to fail from stress-Â related mechanisms than the old-Âfashioned junction described above. Figure 28.4B shows a similar center insulator with a self-Âcontained 1:1 balun.
630 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 Figure 28.4A Center insulator. A recommended way to connect a wire dipole to the device is shown in Fig. 28.5. For simplicity, only one side is shown, but the other side is identical. The l/4 antenna segments (length B in Fig. 28.3) typically are made of #12 or #14 antenna wire, but the authors have used wire as large as #10 and as small as #18 for transmitting applications; the exact gauge is not important except that larger-Âdiameter wire is heavier and sags more, and very thin wire ultimately is not strong enough to support the weight of the center insulator and feedline. In years past, stranded wire was often used, but oxidation of the strands inevitably occurs and may result in some additional loss of transmit signal and the application of large RF voltages to oxidized junctions, so it is no longer favored. Conventional house wiring (THHN) found at electrical supply stores and discount home suppliers can be used (with or without its insulation), but be aware that it is soft-Âdrawn copper and can stretch in response to excessive tension such as that encountered during windstorms by dipoles strung from trees without provision for the movement of branches. (Most antenna tuners can compensate for the changing impedance caused by long-Âterm stretching; occasional retensioning of the end support lines may also be required.) To avoid stretch, some recommend copper-Âclad steel core wire (Copperweld is one well-Âknown trade name), but others have encountered problems with voids in the copper, pitting and rusting, and ultimately failure of the wire from poorer quality “off brands”. Copper-Âclad steel is also quite “springy” and very difficult to work with unless it is always kept under tension once unreeled. Given a choice, the author would rather have a stretched dipole than a snapped dipole! Pass one end of one side of the dipole through an eyebolt and then double it back and twist it onto itself four to six times to ensure that no slippage of the wire through the eyebolt will occur when the antenna is erected and put under tension. Bare an inch or so of the wire at the end of the pigtail with steel wool, sandpaper, or a utility knife
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630 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<br />
Figure 28.4A Center insulator.<br />
A recommended way to connect a wire dipole to the device is shown in Fig. 28.5.<br />
For simplicity, only one side is shown, but the other side is identical. The l/4 antenna<br />
segments (length B in Fig. 28.3) typically are made of #12 or #14 antenna wire, but the<br />
authors have used wire as large as #10 and as small as #18 for transmitting applications;<br />
the exact gauge is not important except that larger-Âdiameter wire is heavier and sags<br />
more, and very thin wire ultimately is not strong enough to support the weight of the<br />
center insulator and feedline. In years past, stranded wire was often used, but oxidation<br />
of the strands inevitably occurs and may result in some additional loss of transmit signal<br />
and the application of large RF voltages to oxidized junctions, so it is no longer favored.<br />
Conventional house wiring (THHN) found at electrical supply stores and<br />
discount home suppliers can be used (with or without its insulation), but be aware that<br />
it is soft-Âdrawn copper and can stretch in response to excessive tension such as that encountered<br />
during windstorms by dipoles strung from trees without provision for the<br />
movement of branches. (Most antenna tuners can compensate for the changing impedance<br />
caused by long-Âterm stretching; occasional retensioning of the end support lines<br />
may also be required.) To avoid stretch, some recommend copper-Âclad steel core wire<br />
(Copperweld is one well-Âknown trade name), but others have encountered problems<br />
with voids in the copper, pitting and rusting, and ultimately failure of the wire from<br />
poorer quality “off brands”. Copper-Âclad steel is also quite “springy” and very difficult<br />
to work with unless it is always kept under tension once unreeled. Given a choice, the<br />
author would rather have a stretched dipole than a snapped dipole!<br />
Pass one end of one side of the dipole through an eyebolt and then double it back<br />
and twist it onto itself four to six times to ensure that no slippage of the wire through<br />
the eyebolt will occur when the antenna is erected and put under tension. Bare an inch<br />
or so of the wire at the end of the pigtail with steel wool, sandpaper, or a utility knife