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INTERFERENCE
Adjacent Channel Interference
Though it is not uncommon, often the signals of one
channel will show up as interference on the channel
next to it (Figure 11-4). This usually happens only
when the receiving antenna is midway between the
transmitter towers of two adjacent channels. Because
the frequencies of the two channels are relatively
close, the TV tuner amplifies both the desired channel
signal and that of the adjacent channel. Some older
TV tuners which have poorer selectivity, are especially
prone to adjacent channel interference. The effect on
the screen is the picture of one channel superim-
posed on the other.
Fig. 11-4. The most common symptom of adjacent channel interference.
Adjacent channel interference may also appear as a
"herringbone pattern" on the screen (Figure 11-5).
This is caused by the sound carrier of one channel
"beating" against the picture carrier of the other. This
produces a signal whose frequency is the difference
between the two carrier frequencies. The different
frequency is passed by the TV receiver and appears
as a herringbone pattern on the screen.
Fig. 11-5. A herringbone pattern caused by adjacent channel interference.
Solution: Traps and filters are available that will
minimize this type of interference. Using a highly
directive antenna and a rotor will also help. A
combination of these methods may be necessary in
extreme situations.
Fig. 11-6. Co-channel interference.
Co-Channel Interference
Co-Channel interference (Figure 11-6) occurs when a
TV set picks up two stations that are broadcasting on
the same frequency. The FCC carefully assigns
frequencies so stations on the same frequency are in
different reception areas. Nevertheless, certain
atmospheric and geographical conditions can still
cause co-channel interference. The picture of one
station will be considerably weaker than that of the
other, but the effect is still annoying. Co-channel
interference caused by atmospheric conditions
usually is sporadic and lasts only a few seconds or
minutes.
Solution: Co-channel interference is difficult to
completely eliminate. However, a highly directive
antenna (one with a high front-to-back ratio) usually
minimizes the problem.
Airplane Flutter
Picture flutter and/or roll caused by airplanes is
a problem that usually affects only receivers near
airports or along other low-level flight paths. It is
caused by signals that bounce off low-flying airplanes.
These signals add to or cancel out the primary
(direct) signals. The TV picture may roll (loss of
vertical synchronization) and/or fade in and our
(variation of signal strength).
Solution: A highly directive antenna usually will
eliminate airplane flutter. However, if the problem is
severe, it may be necessary to stack two identical
antennas.
T
ROUBLESHOOTING
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