Transmission line: The do's and don'ts of proper installation

Will you be one of those whose DTV antenna and rigid line run must go in at the height of DTV crunch time? You may be forced to utilize a second-tier
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Will you be one of those whose DTV antenna and rigid line run must go in at the height of DTV crunch time? You may be forced to utilize a second-tier crew. Even if you've been fortunate in booking a top installation service, remember that they will be under heavy time pressure and possibly nearing exhaustion.

To protect yourself, you should understand the basics of proper rigid transmission line layout, installation and maintenance so that you can supervise and inspect your new RF system properly. This article will fill you in on what you need to know.

Proper layout for the run Rigid transmission lines expand significantly when power is applied. To prevent buckling, they must be suspended from spring-type hangers. When the long vertical run expands, a bending moment will be applied to the horizontal run that can fracture the miter welds on the bottom elbow unless the horizontal run is also able to move freely. To avoid this overstress, lay out a minimum length for the horizontal run relative to the vertical run (as shown in Table 1) and insist on reinforced elbows throughout.

Placement of the bottom elbow is also critical to avoid interfering with any tower members. Figure a differential expansion of 1/2-inch per 100 ft between the copper of the line and the steel of the tower, over a temperature range of -25???F (-32???C) to +125???F (+52???C), based on the ambient temperature. The minimum number of elbows needed is usually four: three at the top and one at the bottom. In today's world of crowded towers, six or even eight elbows seem to be the norm. But elbows add cost and installation time and do nothing to improve signal quality. A top-down installation may help you avoid large elbow complexes.

Standard rigid line comes in sections 20 feet long, and special broadband line comes in sections of variable length averaging just under 20 feet. In either case, pre-ordered variable lengths or field cut sectionswill probably be needed to make everything fit. Field cuts are preferable because they are measured and made on the spot. Variable lengths result in a delay in installation while the factory makes them up and delivers them.

Installation sequence Installation may begin at either the top or bottom of the tower. The uppermost vertical section is attached with rigid hangers and the vertical pieces below it with spring hangers. When starting installation at the bottom of the tower, temporary rigid hangers are used on the first vertical line section installed and removed after the permanent rigid hanger(s) has been installed on the top vertical line section to avoid damage to the line. Follow manufacturer's instructions as to orientation of the line sections during installation.

Hanger considerations Standard hanger spacing is every 10 feet. The minimum distance from the horizontal run to the lowest vertical spring hanger depends on the length of the horizontal run. Follow the spacing given in Table 2.

Make sure the crew aligns all hangers properly so that free travel is maintained and the line will be straight. Verify that the installers are using torque wrenches to torque the hardware. (You'll also need torque wrenches to tighten the flange bolts on the line itself.)

Vertical spring hangers require adjustment of their spring tensions to match the weight they are supporting (proportional to line length), compensated for ambient temperature at installation. This is done by stretching the spring(s) for each hanger to a preset length and setting by tightening one or more hose clamps around the line. Since there is considerable variation between different sizes and brands of line, make sure the crew consults the manufacturer's installation instructions for your exact material.

A lateral brace must be installed at the bottom of the vertical run to restrict lateral movement while permitting free travel vertically and horizontally. The sections of the horizontal run are hung from three-point suspension hangers every 10 feet. These hangers have springs that permit the horizontal run to dip when the vertical run expands. Finally, the rigid line should be double wrapped where it passes through the wall feed-through to form a watertight seal, and the feed-through halves should be caulked all around the edge.

Hoisting and connecting If any section of your line is dented, bent, has bad welds or is wet inside, don't install it. (A good policy is to order only transmission line components that are 100 percent pressure tested at the factory.) Discard any damaged O-rings that you see.

Make sure the crew removes all burrs from field-cut sections, on both inner and outer conductors. Watch that flanges are soldered on carefully.

Never hoist coupled sections of line. The flange welds aren't designed to withstand such stress. Watch to be sure the crew doesn't force sections together. If something doesn't fit, stop the installation to check for correct hanger alignment, etc. Make sure every O-ring is fully seated in its groove before tightening.

Tightening the flange bolts properly is troublesome but necessary. Don't let the crew get lazy here or try to cut corners to save time. Bolts should be snugged alternately, as near to 180 degrees apart as possible (exactly 180 degrees for an even number of bolts) and tightened to the manufacturer's specified torque using the same alternate sequence. Be sure not to exceed the manufacturer's torque specifications.

Most importantly, don't let the riggers leave until you pressurize the line, verify that it holds pressure, obtain acceptable electrical test results and complete antenna tuning.

Testing When installation is complete, the rigid line system must be purged to ensure dryness, using nitrogen bottles or other pressurization methods such as an automatic membrane dehydrator or an automatic membrane nitrogen generator. Purge the line with a minimum of three volumes of nitrogen or dry air. Pressurize it immediately, without exceeding the pressure rating of any component, and monitor carefully forleaks 24 hours later. Correct any leaks found.

Perform all electrical tests at once. Start by attaching a tuned test adapter to a predetermined break point and an additional adapter for a tuned termination load at the end of the run under test. With a vector network analyzer, do a sweep test across the channel using the frequency domain to show overall return loss/VSWR. Next, transform to the time domain to reveal impedance mismatches in the line causing high VSWR, such as a split or bent inner conductor. Use a broader frequency range (+/- 25MHz equal to four TV channels) in the time domain to magnify the flange connections and highlight any poor joints. Once each run has been optimized separately, the full run should be tweaked to obtain the most favorable performance. Finally, verify that the line is holding pressure and apply power.

Inner conductor issues If using rigid line with sliding contacts (including watchband spring line), record the need to replace the bullets every seven years. If using bellows-type line, no replacement is required since the bullets do not slide.

That's it. You can now look forward to years of successful broadcasting.

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