Installation guide

SELECTING & INSTALLING TRANSMISSION LINE
15
Selecting and Installing Transmission Line
Transmission line or downlead, is the wire that
carries the signal from the antenna output terminals
to the receiver input terminals. Even the best antenna
and the most expensive receiver will not produce an
acceptable picture if the transmission line has not
been carefully selected and correctly installed. The
transmission line is more important than most people
realize. Color television reception is sensitive and
highly susceptible to interference from many different
sources. Transmission line that is carefully chosen
and neatly run by an installer who knows what he is
doing will reward the customer with clear, distortion-
free color TV reception.
The Two Basic Types
There are two basic types of transmission line:
300 ohm twinlead and 75 ohm coaxial cable.
Most antennas are 300 ohm balanced output.
Therefore, an outdoor balun is required at the antenna
in order to use 75 ohm coaxial cable. If a preamplifier
with 300 ohm in put is used, a short length of 300
twinlead will be required between the antenna and
preamplifier. Fig. 5-3 shows examples of baluns.
300 ohm twinlead (Figure 5-1) comes in various colors
and thicknesses, is the least expensive, but requires
more careful installation, and picks up interference
signals if they are present. In modern installations, it
is only used to connect between an antenna and a
preamplifier.
Although more expensive, 75 ohm coaxial cable
(Figure 5-2) is easier to correctly install, has a longer
life, and does not pick up interference. Coaxial cable
is round with a central conductor wire surrounded
by plastic insulating material, a braided wire or
aluminum foil sheathing, and a water-resistant outer
covering jacket.
Most modern TV sets have a single 75 ohm VHF/UHF
input. Older TVs frequently have a 75 ohm VHF input
and a 300 ohm UHF output. In this case, a band
separator will be required. Coaxial cable has many
advantages over twinlead and is preferred by most
installers.
Installing Twinlead
Start the twinlead installation at the antenna and
work toward the receiver. First, assemble the antenna
according to the manufacturer’s instructions.
Carefully “snap-out” the elements and then mount the
antenna on the mast.
Attach the twinlead wire conductors to the antenna
terminals with lugs. Never twist bare wire around the
antenna terminals. It is a poor connection that will
deteriorate even more as rust and corrosion set in.
CHAPTER FIVE: SELECTING & INSTALLING TRANSMISSION LINE
Fig. 5-1. Standard 300 ohm TV transmission line (twinlead).
Fig. 5-2. A preferred type of 75 ohm coaxial TV cable and
its construction.
Fig. 5-3. Baluns are used to match 75 ohm coaxial cable
to the output of a 300 ohm antenna and to the
TV receiver 300 ohm input terminals.