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Selecting and Installing Transmission Line

Transmission line or downlead, is the wire that carries the signal from the 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 antennas are 300 ohm balanced output. Therefore, an outdoor balun is required at the 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 antenna and preamplifier. Fig. 5-3 shows examples of baluns.

300 ohm twinlead (Figure 5-1) 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 antenna and a preamplifier.

Although more expensive, 75 ohm coaxial cable (Figure 5-2) 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 lacket.

CHANNEL MASTER WIRE PAGE

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

Next, pass the twinlead through a standout or standoff (Figure 5-4). Install the standout on either the mast or the antenna boom as recommended by the antenna manufacturer. Leave just enough slack to relieve tension on the 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 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 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. Twist the twinlead once every three feet to prevent wind lashing (Figure 5-5).

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 antennas. This causes ghosting and signal loss.

Installing Coaxial Cable

Because 75 ohm coaxial cable is shielded (Figure 5-6), 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 "antenna end" to a balun. (The balun is not necessary if the antenna is one of the few with a 75 ohm output.) Attach the input lugs of the balun to the 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 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 antenna. Also, coax and rotor wire can be run together without interaction or signal distortion. See Chapter 22 for details about rotor installation.

These are precautions that inust be observed when installing coax:

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 (Figure 5-8). 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 todrill 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.

Note: Always make a drip loop (Figure 5-9) at the entry point so that water will run off the line and not into the house.

If you are running the line through wooden siding, drill a hole through the waIl, run the line through and then seal the entry point with caulking compound for weather protection.

For an entry through brick, concrete, or stucco, use a hammer and star bit or a carbide-Tip masonry drill bit. Again, caulking should be used to weatherproof the entry

CAUTION: Before making an entry hole, always carefully check the interior surface of the waIl for electrical fixtures or wiring that may be in the path of the drill bit. Do not enter near electrical wires or outlets especially when using twinlead.

Indoor runs of transmission line should be concealed as much as possible. With twinlead this is a practical necessity because it should be shielded from unwanted signals whenever possible. Coax should be concealed too, but mostly for neatness. lf possible, run the line in closets, behind walls, behind baseboards andmoldings, or up through the floor from the basement. Snaking the line down between wall studs however, is difficult and time consuming. Consequently, it shouldn't be done unless the customer has demanded it and has agreed to pay the extra cost. Before attempting to conceal the line behind baseboards and moldings, be sure that you can replace them exactly as they were without damage.

There are various couplers, amplifiers, and wall outlets that are useful in many installations. Because these devices usually come with detailed instructions, their installation will not be described in detail here. However, we suggest the following general procedures and guidelines:

1. Use only plastic outlet boxes or plaster straps for 300 ohm twinlead.

2. Locate TV outlets near electrical outlets. However avoid running twinlead parallel to the electrical wiring in the wall. (Coax is not affected by the electrical wiring because it is shielded.

3. When running transmission line from a coupler or amplifier, be sure it runs to the outlet(s) as directly as possible. The shorter the run the better.

4. Twinlead lines running in and out of any device should never be run close or parallel to each other.

5. If you must use twinlead outdoors, select the kind  with a black polyethylene jacket. The sun's ultra-violet rays will not penetrate it, and it will not crack or chip in extreme heat.

Choosing the correct type and best quality of transmission line and then installing it correctly, is one of the keys to a customer-pleasing TV antenna installation. Don't skimp on quality. "Cheap" transmission line can be very costly in the long run. Most so-called "cheap" lines not only produce excessive attenuation and poor impedance matching,but also are made of poor grade polyethylene which deteriorates rapidly. This makes it necessary to replace an installation within a relatively short time. Because the dollar difference between "cheap" and quality line is minimal, it always pays to invest in the best.

PAGE #10 F TYPE CONNECTORS, for RG6, RG59 and RG11, ground Block, DC Voltage block, right angle adapters, 75 ohm terminators, F Inline splice

PAGE #11 Compression Seal F TYPE CONNECTORS , Tools, SPLICES AND INTERSERIES ADAPTORS also  Compression installation Tool.

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