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.