CONNECTORS
AND CABLE
Radio frequency cables are referred to as “coax” as a
generic term. It is short for coaxial. A coaxial cable consists of
an inner conductor inside an insulating core. This is surrounded on
the outside by a metal braid or foil, called the shield. This
shield is in turn covered by an insulating jacket of plastic
material. Coaxial cables are specified in terms of impedance which
for most micropower broadcasting purposes is 50 ohms except for
dipole phasing harnesses.In the 50 ohm category, there are a number
of choices when selecting coaxial cable. The most important
characteristic of coax is its level of signal attenuation. This
depends on the length of the cable and its particular frequency
response. RG58 coaxial cable has a high degree of attenuation and
should only be used for short connections. RG8X or mini 8 works
well for lengths under 50 feet and is suited for portable and
mobile set ups since it is rather flexible. RG8 and its higher
performance cousins such as 213 and Belden 9913 are the best for
fixed installations. Belden 9913 has the lowest loss for any given
length as compared to other variations of RG8. In fact, it has a
loss figure at 100 MHz that compares well with commercial broadcast
hard-line coax. It is rather stiff cable and must be installed
correctly.Coaxial cables do not take rough treatment very well,
especially 9913. They must be carefully rolled up by hand, not
wrapped between palm of hand and elbow like a rope. Kinks are to be
avoided at all costs. When routing a cable keep the bends from
being sharp and keep it away from circumstances where it can be
pinched or slammed.Three types of connectors are in general use -
BNC, PL259 and N. Most micropower broadcasting equipment uses PL259
and its mating socket known as the SO239. Any connector will
introduce some small degree of signal loss. N connectors are used
where high performance and reliability are of most
importance.