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Strong Towers Make Strong Signals

Do you have an ice bridge? If not, consider budgeting for one. Before Cold Weather Sets In, Make Time for FM Tower Site Inspection

Last column, we reviewed some of the things that should be checked at transmitter sites. Most of the suggestions were outside the transmitter building or antenna-coupling units (ACUs) found at AM sites.

(click thumbnail)Figure 1: Check the security of grounds on your transmission lines.
If you take care of an FM, remember to check these outside parameters. At the base of the tower, check that transmission lines are grounded properly and that the grounds have not broken off or have hardware missing.

Ensure that ground kits are installed so the wire runs down toward the earth. Looping a ground lead up the tower defeats the purpose. As lightning energy travels down the transmission line, a properly installed ground kit leads the energy to ground – not back up the tower to a ground stud.

In Figure 1, in addition to having each cable grounded with a ground kit, the station has numbered each cable to trace or identify lines in the event of problems. This is helpful particularly at sites with multiple lines going up the tower.

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As you inspect the base of the tower, check that cable support clamps, ties or spring hangers are not damaged. Do you have an ice bridge? If not, consider budgeting for one.

Shards of ice falling off towers can damage transmission lines. If your manager needs a cost comparison, show him the price of the ice bridge vs. a new run of transmission line. Add in the off-air time, or at least, consider the reduced power you’ll be running because of the VSWR caused by the dented cable.

You know if the failure is going to happen, it will be the week before Christmas, so the station will lose all the Christmas spots. Murphy’s Law loves holidays.

An ice bridge also provides adequate cable support from the building to the tower. Leaving a cable to dangle as you see in

Figure 2 is inviting trouble. Not only can falling ice damage the coaxial line, it’s hanging so low that someone can either walk into it or accidentally strike it with a tool or ladder.

(click thumbnail)Figure 2: An ice bridge will also provide cable support.
Note in the background of Figure 2, the ice bridge and how the cables are routed beneath it.

How is your tower paint? With the end of year in sight, budgeting for tower painting may be in order. As you obtain painting quotes, get at least three references and call them. If you can visit the stations that have been painted, then that is even better.

There are a few bad apples out there, posing as qualified tower riggers and painters. Don’t get caught with a job you have to do twice.

. . .

(click thumbnail)Figure 3: Ensure that grounds are connected properly.
You may encounter the older hollow leg towers as you make your inspection. Check the weep holes at the base, small holes that permit water to flow out of the hollow leg.

If they are clogged at the base, they probably are clogged further up the tower. Have a qualified tower company inspect each section and clear the holes.

Also check any braided grounds, such as those used on the Austin Ring transformer in Figure 3. These can deteriorate in time.

The newer prefabricated communications shelters have made grounding a breeze. Typically, they are tied into the tower ground system when the shelters are installed. Check the ground leads running outside the building as well.

Figure 4 shows a ground lead exiting the building and tying into the station ground strap. Note that this building also has a hurricane mount. A screw-type anchor is set into the ground at each corner of the building.

A metal strap connects the building bracket to the anchor. It is hard to believe that any hurricane would develop enough energy to move one of these concrete communications shelters, but I guess it can happen. The anchor bracket reduces the chance, and in some areas, is required by the local building code.

(click thumbnail)Figure 4: Make sure that building grounds are intact.
Check tower light photocells and the tower lighting conduit. Conduit should be secure, so that rain does not enter.

Some conduit has a vented junction box, to provide airflow inside the conduit. This prevents moisture from building up inside, due to temperature variations. Check that the vent is not clogged or torn open, exposing the box to bees.

For the AM engineer, that aluminum enclosure or small building at the base of the tower contains a lot to look at. We’ll start reviewing these dilemmas next issue.

Submissions for this column are encouraged and qualify for SBE recertification credit. Fax your submission to (703) 323-8044, or send e-mail to [email protected]