Hal Communications CWR6850, Instruction Manual

Page: 44 / 64

CHAPTER 5 PAGE 44
Also, if possible, avoid bringing an end of a half-wave dipole in close proximity to the operating po-
sition; there is a high voltage field at the ends of the dipole that may be hard to shield.
(A)
(B)
Figure 9 – Power Line Filters
In general, a shielded, coaxial cable feedline with low SWR is much preferred over open wire, twin-
lead, or single wire feed systems. The self-shielding property and lower voltages present act to
make the coaxial feedline much less susceptible to radiation of RF energy in the shack rather than
at the antenna. RF energy may also be conducted back to the station by conduction down the out-
side of the coaxial cable shield. This may be a particular problem with half-wave dipoles on 40 and
80 meters that are center-fed with only coaxial cable. A balun at the antenna tends to reduce this
problem. Also, dress the coaxial cable from the balun so that it drops perpendicular to the dipole,
rather than parallel. In stubborn cases, you may find that dropping the coaxial cable clear to the
ground and burying it (5 or 6 inches) for the horizontal run to the shack my help reduce RF cou-
pling considerably. This technique has worked particularly well for second-story station installa-
tions. As an alternate to the balun, construction of an RF choke out of the Coax itself is sometimes
effective; wind six or more turns of the coaxial cable in a six inch diameter coil. Place the coil at
the antenna and wrap it with electrical tape to hold its shape. If there is a moderate to high SWR
on the line (2:1 or more), you may find that varying the length of the line helps, although this is a
poor substitute for a properly matched antenna.