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CHAPTER TEN: M
M
any homes have more than one TV set.
Others have at least one FM radio receiver
and VCR. Consequently, the installer often
must use an antenna system that feeds the received
signals to two or more receivers. This is called
multi-set reception.
Supplying the signal from one antenna to two or more
receivers requires the use of special devices that pre-
vent problem-causing interaction between the TV
receivers. Without the devices, the interaction
between receivers will reduce the quality of the
pictures displayed by the receivers. Problems caused
by such interaction include multiple images (ghosts)
and loss of picture detail (smearing).
Passive Devices
One method of multi-set reception uses what are
called passive devices. The term "passive" means
that they do not amplify (increase) the strength of the
signals. Passive devices prevent receiver interaction
by "isolating" the TV receivers from each other.
However, because passive devices cause reduction of
signal strength, multi-set reception systems equipped
with these devices require more signal strength out of
the main transmission line than do antenna systems
that feed only one TV set. Typical signal strengths
required for multi-set reception vary from a minimum
of 2000 microvolts (uV) to over 4000 microvolts
depending on the number of passive devices used in
the system.
There are two types of multi-set passive devices:
couplers and splitters. Couplers are used in multi-set
systems equipped with 300 ohm twinlead. Splitters
are used in systems that are equipped with 75 ohm
coaxial cable. Couplers and splitters perform the
same basic function.
Most couplers are designed to distribute signals
to either two or four sets. The coupler (Figure 10-1)
usually is mounted on or near one of the TV sets,
preferably one that is centrally located. Twinlead is
run from the coupler's outputs to the antenna input
terminals of the TV receivers.
Couplers are the most economical way of feeding two
or more TV sets. However, because twinlead is used,
couplers present all of the interference pickup and
other installation problems associated with twinlead.
Be careful where and how you mount couplers.
Most splitters (Figure 10-2) are also designed to feed
either two or four TV sets. However, because they are
designed for use with coaxial cable, splitters can be
mounted just about anywhere without picking up
interference or causing other problems that reduce
picture quality.
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Fig. 10-3. A four-set splitter with
one output terminal
"terminated".
When a four-set splitter is used to feed only three
receivers, the unused splitter output must be
terminated" to prevent signal problems. "Terminated"
means that a small 75 ohm device (Figure 10-3) is
connected to the unused output connection of the
splitter. This is done to prevent the unused signal
from "bouncing back" through the coax and inter-
fering with the signals fed to the TV receivers.
Terminating devices are available for use with most
standard splitters.
ECEPTION
Fig. 10-1. A four-set coupler.
Fig. 10-2. A two-set splitter.
M
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ECEPTION
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