Specifications

page 32
[
StarServe Installers Guide
]
Interference and
Trouble Shooting
Cross Modulation Interference
Cross modulation interference occurs in Broadband
preamplifiers and distribution amplifiers when one or more
signals (TV Channels) exceed the amplifiers rated output
capability. This causes two or more signals (TV channels) to
beat together resulting in the picture information of one channel
appearing superimposed upon another. This interference
usually manifests itself as a windshield wiper effect or as a
negative image.
The windshield wiper effect is seen as the vertical or
horizontal framing bars of the interfering channel appearing
on the channel being viewed. The negative image appears as
a superimposed image in the background of the picture on
the channel being viewed.
Highly directional antennas and the use of filters and taps to
attenuate and control the offending signals can usually
eliminate this type of interference.
It is always the strongest signal received that causes the
interference and it does not normally show up on the interfering
channel. The interfering signal may also be an FM signal or
combination of FM and TV signals.
Adjacent Channel Interference
Adjacent channel interference is caused by a strong signal
from one channel overriding a weaker signal on an adjacent
channel, producing a “herringbone” effect. An adjacent channel
is one, which is next to another channel. For example,
Channels 1 and 2, 7 and 8, 10 and 11 are adjacent channels.
Channels 2 and 3 are not adjacent because there is a 15
MHz band between them. In addition, channels 5 and 6 are
not adjacent because there is channel 5A between them.
This interference can be eliminated by using a higher gain
antenna to increase the weaker signals and by using
attenuators to reduce the stronger signals.
Co-Channel Interference
Co-channel interference is the result of two stations in different
locations operating on the same channel. It appears on a TV
as two different pictures, as though one were placed on top
of the other. The effect can be minimised by using a highly
directional antenna, or, if necessary, by stacking antennas.
Power Line Interference
Power line interference is caused by radiation from a high
voltage power line close to the antenna. To minimise this
interference, the antenna should be located as far away from
the power line as possible. A balun should be used as close
as possible to the antenna terminals to prevent direct pickup
of radiation.
Worn or cracked insulators on power lines can also cause
this interference. If the interference shows up intermittently
(especially during wet weather), it may be due to cracked
insulators on the power lines.