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TROUBLE SHOOTING
Antenna Systems Interference
There are times when despite all the careful planning and attention you've given an installation, outside interference still shows up on the TV screen. Some of the most common interference problems and their solutions are discussed in this chapter.
GHOSTING
Ghosting is the appearance of faint duplicate images alongside the true picture. Ghosting is caused by two sets of the same signal arriving at the receiver at slightly different times. Leading ghosts (Figure 11-1) appear to the left of the main images. Trailing ghosts (Figure 11-2) appear to the right.
Leading Ghosts
Leading ghosts are caused by signals arriving at the receiver a split-second
ahead of the direct (primary) signal. They are usually caused by direct pick
up (pick up other than the antenna). Solution: Take great care with shielding
and downlead splices! etc. and if ghosting is still present, reorient the
antenna to minimize ghosts.
Trailing Ghosts
Trailing ghosts are usually caused by TV signals that bounce off hills or tall buildings. Because these signals take a longer path to the antenna, they show up on the screen a fraction of a second later than the primary signal. Trailing ghosts also can be caused by shorted-out or otherwise defective connections that produce signal mismatch. This type of ghosting also shows up in a distribution system when signals are reflected back through the system by an unterminated output terminal.
Solution: Use
terminators on all unused distribution terminals. Use a highly directive
antenna (like a Channel Master QUANTUM) that will not pick up reflected signals.
Reorient the existing antenna; slight reorientation often will eliminate
the ghosting. As a last resort try a special "ghost killer" antenna.
Smeared or Blurred Pictures
Blurred or
smeared pictures (Figure 11-3) are frequently caused by mismatched impedances
or improper connections in the transmission line or other equipment.
Solution: Check all connections to ensure that they are tight and not rusted or corroded. Check for and eliminate any crimps or kinks in the downlead. Be sure that the line has not been crushed by staples and is not bent too sharply. Use only the best quality coaxial cable, hardware! and other equipment to ensure good impedance matching.
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 superimposed on the other.
Adjacent channel
interference may also appear as a "hernngbone 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.
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.
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 onestation 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.
CB andlor Ham Radio Interference
Strong, local
signals transmitted by Citizens Band (CB) or Amateur Radio (11am) operators
can cause a picture to roll, fade, or disappear completely. Sometimes, horizontal
lines appear on the screen (figure 11-7) and the voices of the radio operators
are heard in the TV audio.
Solution: If you know or can identify the radio operator, call the person and tell him/her about the interference. If the radio operator is cooperative (most are), he/she usually can eliminate the interference by equipment adjustment and/or by installing filters. If the operator will not cooperate or is unknown, call the FCC. If these efforts do not solve the problem, purchase a high-pass filter that eliminates frequencies below 54 MHz. Attach the filter to the input of the receiver or prior to any amplifiers if used.
Electromagnetic Interference
Electromagnetic
Interference is the most common form of TV screen disturbance. It causes
bright static across the screen (Figure 11-8). Often, it also causes a grinding
or whining sound. This type of interference can come from many different
sources, including household appliances, power lines, motor vehicles, and
hospital equipment. The motors of household appliances such as furnaces,
mixers, hairdryers and humidifiers are principal causes of electromagnetic
interference. The interference from these devices is carried to the receiver
through the house wiring.
Solution: A highly directive antenna often solves this problem if the sources from outside the house and the interference is fed to the set by the antenna. There are other measures you can take, however, if the problem persists. If you suspect power line interference, contact the power company. In most cases their linemen can quickly find the loose connector or clamp that is causing the problem. Remember. The use of coaxial cable reduces the probability of electromagnetic interference.
FM interference
TV interference
from the FM band is also a common type of reception problem. FM interference
most frequently affects the TV high band (Channels 7-13), but sometimes a
weak Channel 6 will also be affected. FM interference typically causes a
"herringbone" patternon the screen (Figure 11-9).
Solution: Use coaxial cable instead of twinlead. Use an FM trap (filter) for severe problems. (See the chapter titled Antenna-Mounted Amplifiers for specific instructions about tuning and installing FM traps.)
Preamp Overload
Preamp overload occurs when the transistorized circuitry of an antenna mounted amplifier is fed an excessively strong signal. On the TV screen, preamp overload looks exactly like FM interference.
Solution: When an installation that includes a preamp seems to be experiencing FM interference, always suspect the preamp. Use a preamp with a high input capability (capacity). The input capabilities of preamps are specified as microvolts (uV). Measure the strength of the overload-causing signal. An FM trap or other filter also may be necessary in extremely troublesome causes. Always mount the FM trap prior to the preamp.
When servicing an existing system, be sure to check the strength and quality of the signals being received and the condition of the equipment being used. Measure signal levels with a field-strength meter. Check general picture quality with a TV set you know is operating correctly. You may find that an improper installation or a change in reception conditions is causing problems that can easily be solved.
Defective components within the customer's TV set can also cause problems. To determine whether the cause of a reception problem is the customer's TV or the antenna system, connect your test TV to the antenna system. Compare the picture on the customer's set with the picture on your test TV. If your test set produces an acceptable picture, the customer's TV set probably is the source of the problem, not the antenna system. Unless you are a qualified electronic technician, do not attempt to fix the customer's TV set. Instead, advise the set owner to call a qualified TV technician. (Some cities and states have laws that prohibit anyone other than a licensed TV technician from repairing TV receivers. Some cities and states also require that antenna installers be licensed.)
CAUTION: Before checking out an antenna system, always disconnect the transmission line from the TV input terminals. Some TV set malfunctions can feed potentially dangerous levels of voltage Into the transmission line. This voltage can be high enough to cause severe shock, Injury, or even death. If you have a volt-ohm-millimeter (VOM) use it to test for dangerous voltages across the antenna terminals of the TV set. You also can use the VOM to test for shorted and open (broken) conductors in the transmission line.
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