Chapter 10 - TV ANTENNA TROUBLESHOOTING
TV Antenna Systems Interference
Update: All of the types of ineterference below still occurs with today's digital reception. However, the interference won't manifest itself in the same way. When interference is severe enough it will cause loss of digital signal but not ghosting etc. Ghosting, blurred picture etc... only occurs when viewing analog TV reception.
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 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 appear to the left of the main images. Trailing ghosts appear to the right.
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 TV antenna).
Solution: Take great care with shielding and downlead splices, etc. and if ghosting is still present, reorient the TV antenna to minimize 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
Smeared or Blurred Pictures
Blurred or smeared pictures 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.
TV ANTENNA 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 itThis 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 "herringbone pattern" on the screen . 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: 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 (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 TV antenna (one with a high front-to-back ratio) usually minimizes the problem.
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 TV antenna usually will eliminate airplane flutter. However, if the problem is severe, it may be necessary to stack two identical TV antennas.
TV ANTENNA TROUBLESHOOTING
CB and/or Ham Radio Interference
Strong, local signals transmitted by Citizens Band (CB) or Amateur Radio (Ham) operators can cause a picture to roll, fade, or disappear completely. Sometimes, horizontal lines appear on the screen 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 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 TV antenna often solves this problem if the sources from outside the house and the interference is fed to the set by the TV 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: Theuse of coaxial cable reduces the probability of electromagnetic 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" pattern on the screen.
Solution: Use coaxial cable instead of twinlead. Use an FM trap (filter) for severe problems. (See the chapter titled TV Antenna-Mounted Amplifiers for specific instructions about tuning and installing FM traps.)
Preamp overload occurs when the transistorized circuitry of an TV 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.
CAUTION: Before checking out an TV antenna system, always disconnect the transmission line from the TV input terminals and unplug the preamplifier power supply if one is in use. 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 TV 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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