Selecting and Installing TV Antenna Transmission Line

Transmission line or downlead, is the wire that carries the signal from the TV antenna output terminals to the receiver input terminals. Even the best antenna and the most expensive receiver will not produce an acceptable picture if the transmission line has not been carefully selected and correctly installed. The transmission line is more important than most people realize. Color television reception is sensitive and highly susceptible to interference from many different sources. Transmission line that is carefully chosen and neatly run by an installer who knows what he is doing will reward the customer with clear, distortionfree color TV reception.

The Two Basic Types

There are two basic types of transmission line: 300 ohm twinlead and 75 ohm coaxial cable. Most TV antennas are 300 ohm balanced output. Therefore, an outdoor balun is required at the TV antenna in order to use 75 ohm coaxial cable. If a preamplifier with 300 ohm in put is used, a short length of 300 twinlead will be required between the TV antenna and preamplifier. 300 ohm twinlead comes in various colors and thicknesses, is the least expensive, but requires more careful installation, and picks up interference signals if they are present. In modern installations, it is only used to connect between an TV antenna and a preamplifier. Although more expensive, 75 ohm coaxial cable is easier to correctly install, has a longer life, and does not pick up interference. Coaxial cable is round with a central conductor wire surrounded by plastic insulating material, a braided wire or aluminum foil sheathing, and a water-resistant outer covering jacket. Most modern TV sets have a single 75 ohm VHF/UHF input. Older TVs frequently have a 75 ohm VHF input and a 300 ohm UHF output. In this case, a band separator will be required. Coaxial cable has many advantages over twinlead and is preferred by most installers.

Installing Twinlead

Start the twinlead installation at the TV antenna and work toward the receiver. First, assemble the TV antenna according to the manufacturer’s instructions. Carefully "snap-out" the elements and then mount the TV antenna on the mast. Attach the twinlead wire conductors to the TV antenna terminals with lugs. Never twist bare wire around the TV antenna terminals. It is a poor connection that will deteriorate even more as rust and corrosion set in.

CHAPTER FIVE: SELECTING & INSTALLING TV Antenna TRANSMISSION LINE

SELECTING & INSTALLING TRANSMISSION LINE

Next, pass the twinlead through a standout or standoff.  Install the standout on either the mast or the TV antenna boom as recommended by the TV antenna manufacturer. Leave just enough slack to relieve tension on the TV antenna connections. Crimp the standout firmly enough to securely hold the twinlead but not so firmly that it deforms the twinlead. Thoroughly spray the antenna connections with an acrylic insulator such as Krylon. This will retard corrosion and rust. Also, seal the end of the twinlead if foam is used as part of the twinlead insulating material. This will prevent moisture absorption. Twinlead must not be run close to metal. Metal interacts with the twinlead conductors. This causes signal mismatch, resulting in inefficient signal transmission through the line. Horizontal runs of twinlead also act as an  TV antenna. This causes two or more sets of identical signals to reach the receiver at different times producing ghosts on the TV screen. It may also cause "suck out," or loss of signal. Some types of twinlead also develop high attenuation (increased impedance) in wet or humid weather. This causes severe signal loss. If an TV  antenna rotor is used, never run the twinlead and rotor wire through the same standout. The wires and signals will interact, and the quality of the TV picture will be decreased. Use two evenly spaced mast standouts for the top 5 feet of mast or in-line double standouts designed to carry both rotor and transmission line. Use additional standouts as necessary to keep the twinlead away from the mast. Standouts should be at least three inches long. Twistthe twinlead once every three feet to prevent wind lashing. Use additional standouts at ends and turns to keep the twinlead away from eaves, gutters, drainpipes and any other metal surfaces. When running twinlead indoors, drive staples or tacks only in the center portion of the insulation between the conductors. Do not use any staples or tacks large enough to "bridge" the conductors. This will short the conductors. Run twinlead directly to the back of the set from the wall, floor, or baseboard. Don’t leave more twinlead than absolutely needed. Extra twinlead will coil up and act as additional TV antennas. This causes ghosting and signal loss.

SELECTING & INSTALLING TV ANTENNA TRANSMISSION LINE

Install Coaxial Cable

Because 75 ohm coaxial cable is shielded it is completely unaffected by contact with metal structures, and it will not pick up unwanted signals as twinlead does. Also, its performance is not affected by moisture, and it generally has a much longer life span than 300 ohm twinlead. Aluminum/mylar-equipped coaxial TV cable provides superior low-loss performance. (Examples are Channel Master coaxial cable model numbers 9533, 9539, 9540, and 9544.) Begin installing coaxial cable by first connecting the TV antenna end" to a balun. (The balun is not necessary if the TV antenna is one of the few with a 75 ohm output.) Attach the input lugs of the balun to the TV antenna terminals. Apply acrylic insulator or silicone grease to the connections. A weather boot (Figure 5-7) should also be installed over the connections. Next, run the coax through a standout mounted on the boom or the mast. This will prevent the weight of the transmission line from creating stress on the TV antenna connections. From this point on however, the coax may be taped to the mast at appropriate intervals. Use plenty of good-quality vinyl tape to secure it. (If you also are installing a rotor, sufficient slack must be left in the transmission line between the mast and the antenna to permit rotation of the TV antenna. Also, coax and rotor wire can be run together without interaction or signal distortion.

These are precautions that must be observed when you install coax cable

1. Do not bend coax too sharply. This may collapse

the dielectric and ruin the necessary spacing

between the center conductor and shield.

2. Do not crush or deform the coax. Ghosting and

smears may result.

3. When attaching connectors to coax, do not nick

the center conductor. This will cause a stress point

that will probably break the next time the wire is

flexed.

4. Remember that in almost all cases baluns are

required to match the 75 ohm coax impedance to

the 300 ohm impedances of the antenna output

and the TV input.

GENERAL GUIDELINES FOR INSTALLING

TRANSMISSION LINE

Both coax and twinlead should be run as directly as possible to the receiver. Avoid excessively sharp bends or turns. You should also try to use one continuous transmission line without splices. (However, if necessary, two lengths of coax can be joined with a splicing connector. Also, keep the line away from anything with sharp or jagged edges. Run the transmission line into the house through an attic or basement section if possible. Never run the line through a window or door. This invites damage to the line and is the mark of a careless installer. If your entry is through shingles or shakes, remove one of the shingles and drill a hole for the line through the wall. Use a brace with an extension bit to drill through the insulation, plaster, and/or drywall. After you run the line through the hole, replace the shingle, and you’ll have a neat, waterproof entry.

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