Transmission lines

As almost all TV stations have a DTV facility in operation, it is time to plan what moves need to be made in the transmission line on the tower. Should the station plan for major modification, replacement or maintaining the status quo?

Transmission line facilities can be divided into four classes:

  • Analog lines that will be abandoned in 2009;
  • Digital lines that may have to be upgraded and must be maintained; and
  • New digital lines that must meet the final power and channel assignment.

Strictly analog lines

The first class of lines simply needs to be held together until the big changeover occurs. Many of those lines have been in operation for 30-plus years, often with little maintenance.

Analog lines that will become digital lines to the same antenna;

  • Replace wristwatch band connectors about every 12 to 14 years. Don't believe manufacturer claims that watch bands never fail. They all fail — sooner or later. Pulse the lines to ensure nothing is in the upcoming failure mode. Remember, the front office is still making most of its money from that old analog system. You'll feel a lot more heat from the front office if the analog line goes down than if the DTV line fails — although that is starting to change.The problem is keeping the old analog lines working. Duct tape doesn't help in failures, but dual-sided sticky tape might help with broken hangers. There is no need to budget for major projects.Analog-to-digital linesFor the second class of systems, the care and feeding of the transmission line is a bit more critical. In this case, major projects can be planned and budgeted. The lines are probably old, and maintenance may be questionable. First, sweep and pulse the line to look for developing problems. Plan to replace the watch band if the system is more than 12 years old.In addition, the entire center conductor run may need replacing. A complete center conductor changeout with connector replacement will essentially provide the station with a new transmission line run, leaving only the big copper pipe with its flanges. This is a good way to upgrade the system at a reasonable cost, especially given the cost of copper.Maintaining digital linesDigital lines only need to be maintained until the changeover occurs. This includes stations where the DTV channel will be moved significantly with a major power increase.Many stations using low- to medium-power antennas have stayed with semiflexible lines for good reason. Often the runs are shorter up to the lighter antennas, all of which will be scrapped. Here the technical staff is lucky. These systems are only about three or four years old. They often are still within factory warranty. Nothing should be required between now and the change date other than keeping them pressurized. Remember, even brand new lines will die if you let them reach atmospheric pressure.Some lines are capable of operating a DTV channel without maintenance. These lines are fairly new so they should be in good condition. Usually these lines have been swept and pulsed as part of the original installation. The station staff now only needs to maintain good engineering practice. Again, keep the pressure on the lines, or waveguide, as required to maintain a dry status. Watch for any change in the indicated VSWR. The meter on the transmitter can't indicate the line VSWR, as the true condition is only determined by sweep and pulse measurements. But, the transmitter meter is invaluable for spotting a change in reflected power, which in turn indicates that the VSWR is changing at some frequency or group of frequencies. At that point, figure out what is wrong and get it fixed before you are off the air.Establish a regular schedule for testing the line with combiners and filters. The recommended time between such tests varies. Not more than five years should go by without the careful application of a network analyzer to the complete antenna and transmission line system. Add to that a careful examination of the entire system, including the primary power connections, breakers and power distribution wiring, by a good operator with a thermal imaging camera. That test can help you avoid downtime and physical injury to station staff.The cost of such testing is small compared with the cost of repairing a burnout with a day or two of lost commercial time. Some failures cannot be predicted, even with testing. All you can do is to try to catch failures that can be anticipated by careful observation and testing.

New digital lines

The last class of new transmission line systems need to conform to the new power or channel assignment. In short, what worked well on analog works on digital. The DTV signal is robust, the transmitters compensate for limited performance, and the later generations of receivers work well.

All the major manufacturers have complete transmission line products. Depending on the channels involved, other manufacturers can modify their line lengths to avoid the bad reflection problems that would otherwise occur. Of course, on single-frequency systems, that isn't much of a consideration.

Most manufacturers have Web pages that show their range of products. The biggies have hard lines from 7/8in through 9 3/16in, although the upper sizes are not usually needed in the DTV world, with its much lower power levels. They won't reach the higher channel numbers without moding, so they are relatively useless in multiple channel systems. This has led to higher power designs of the smaller sizes. Most hard line sizes are available in 50Ω or 75Ω. Both work well in DTV service. Therefore, the selection goes back to the old criteria of loss, power rating and cost.

Waveguide systems can operate well in DTV service. That includes rectangular, circular and truncated systems. For long runs, the efficiency of such systems can still make them a good choice based on the cost of developing DTV kilowatts.

Use the same method

The major point of the DTV selection process is essentially the same as has been used for years in analog operation. There are still some areas where research is needed to determine how the bit error rate or signal to noise ratios may suffer from mistuning of coaxial lines or waveguide. To date, major problems don't exist. If you select lines the same way as for analog systems, don't anticipate any problems.

Don L. Markley is president of D.L. Markley and Associates.

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