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 633
that connection is made, wrap high-Âquality black vinyl electrical tape around the joint,
starting on the vinyl cable jacket below the plug and continuing completely over and
past the PL-Â259 until reaching the molded case that supports the SO-Â239 receptacle.
Half-Âlapping the tape like siding or roof shingles should ensure minimal water ingress at
the end of the coaxial cable. To minimize water vapor ingress caused by changes in humidity,
the vinyl tape should also be sprayed with acrylic lacquer or painted with nail
polish. Alternatively, use heat-Âshrink tubing and a heat gun to totally encase the connection
from the threads of the SO-Â239 above the PL-Â259 to the outer jacket of the cable
a half-Âinch or so below the bottom edge of the PL-Â259.
In most installations, the weight of the feedline will keep the entire center assembly
oriented as shown in Fig. 28.5, thus eliminating the need for a drip loop on the coaxial
cable. If, however, a specific installation results in the SO-Â239 connector being anywhere
other than beneath the body of the device, the coaxial line near the connector should be
routed with a half-Âloop that is arranged so as to send any precipitation running along
the cable away from the connector.
At the top of the center insulator a small eyebolt is provided for strain-Ârelief of the
system or for suspension from a center support (such as when the antenna is an inverted-Â
vee). One disadvantage of coaxial cable and commercial center insulator assemblies (as
compared to open-Âwire line directly connected to the two sides of the dipole) is that both
add quite a bit of deadweight to the center of the antenna. Unless some kind of auxiliary
support (nylon twine, polypropylene rope, or small-Âdiameter aircraft cable pulling up to
a high point on a tower, mast, or tree) is available, the sag in a dipole for 80 or 40 m fed
with RG-Â8 or RG-Â213 cable may be unacceptable to the user. If the VSWR on the line is
kept close to 1.0:1 or power to the line is kept substantially below 1000 W, some weight
can be removed by using the smaller-Âdiameter RG-Â58 or RG-Â59 cables instead of RG-Â8—
especially at 10 MHz and below, where line loss is lowest.
The longer, lower barrel of the center insulator shown in Fig. 28.4B indicates the
presence of either a 4:1 or 1:1 impedance ratio balun transformer. The 1:1 size is recommended
for ordinary dipoles, whereas a 4:1 unit is a good match for folded dipoles and
certain other wire antennas with feedpoint impedances substantially larger than the Z 0
of the feedline.
Some users form the coaxial cable into a 6-Â to 12-Âin multiturn loop just below the
coaxial connector, as shown in Fig. 28.6. This forms a common-Âmode RF choke intended
1:1 BALUN OR
CENTER INSULATOR
WIRE SPLICES
END INSULATOR
HEAVY DUTY
DOOR SPRING
ROPE TO SUPPORT
STOUT STRING
COAX LOOP
(15-25 CM DIAM.)
COAX CABLE TO RECEIVER
OR TRANSMITTER
Figure 28.6 Dipole and wire antenna installation.