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CHAPTER NINE: A
A
mast or antenna-mounted amplifier (preamp)
is used primarily to eliminate "snow" on the TV
screen. "Snow" (Figure 9-1) is actually electrical
noise that is generated by the TV receiver and other
electrical devices. The object of any installation is to
deliver to the TV input terminals, signals that are
strong enough to override the noise (snow).
Traditionally a "weak" signal is defined as one that is
not strong enough to override the level of the noise in
the receiver.
Fig. 9-1. Weak signals cause "snow" on the TV screen.
When to Use a Preamp
There is no simple or universal rule that tells
you when to install an antenna-mounted preamp
(Figure 9-2). Generally if the level of the received
signal is less than 1,000 uV at the receiver input
terminals, preamplification is needed.
Customer preference is another factor that must be
weighed when considering the use of a preamp. Some
customers may not agree to the added expense of a
preamp and will be content with a picture that is less
than perfect. Others may insist on a preamp just
because they want only the best.
Aside from customer preference, there are many
other variables that will influence your decision. For
example, a preamp generates a small amount of noise.
In areas with extremely weak signals, even the low
level of noise in the preamp will be too near the level
of the received signal. Because both the received
signal and the noise are amplified an equal amount,
the difference in their relative levels will never be
great enough to permit the received signal to override
the noise and eliminate the snow on the screen.
Consequently, you might want to consider another
method of boosting a very weak signal.
Vertically stacking two antennas and pointing them in
the same direction is probably the best alternative to
a preamp. This will provide additional gain without
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introducing more noise. However, this will not always
solve the problem especially if the received signal is
exceptionally weak.
In extremely deep fringe or otherwise difficult
reception areas, you might have to use both antenna
stacking and a preamp. However, this is very rare.
Selecting a Preamp
A major consideration when selecting a preamp is the
possible sources of interference in the area. It is vital
that as an installer, you become familiar with the
commercial broadcast and other radio frequency
signals that are being used in your area. Local FM
stations, police and fire frequencies, military commu-
nications, and other sources of radio frequency
signals can cause interference that should be trapped
(filtered out) at the preamp stage.
For this reason, preamps are available with almost
every conceivable arrangement of traps (filters).
Tunable traps can be adjusted to eliminate a specific
frequency, while switchable traps attenuate (reduce
the level of) an entire band of radio frequency signals.
If an installation is experiencing severe interference
from one local station (located at 101.5 MHz for
example), a tunable trap can be adjusted so that any
signals at this frequency are eliminated without
attenuating the rest of the FM band. If interference is
caused by several FM stations, you will have to use a
switchable trap that attenuates the entire FM band.
Many customers want to receive both TV and FM
signals from one antenna. Adding a trap to a preamp
will not necessarily make this impossible. In general,
any FM signal strong enough to require trapping will
also be strong enough to be picked up by an FM
receiver even though it is trapped out of the TV
bands. (An FM receiver requires a much lower level of
signal to operate well than does a TV.) When a
switchable trap is used however, some of the weaker,
more distant FM stations will be lost.
Antenna Terminals
Fig. 9-2. A preamp mounted on an antenna.
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MPLIFIERS
Antenna Crossarm
Standout
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MPLIFIERS
Mast
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