A violent, straight-line wind came through Brainerd, Minn., in the summer of 2015. Suddenly, operating parameters changed on the KLIZ(AM) 5 kW ND day and 5 kW three-tower directional night antenna systems. Not serious, but enough to notice.
Fig. 1: A partially shattered guy-line insulator.
Apparent transmitter efficiency decreased because of a shift of a few ohms in day tower impedance. Night tower phases changed 2 degrees and ratios about 3 percent. Tweaking phasor controls to get licensed phase and ratio resulted in normal night monitor point readings. Still, something was amiss.
A tower crew was brought in to inspect for damage.
From a high vantage point they found that about a dozen of the original guy line insulators were damaged or outright broken on the 380-foot tower at the center of the array. The violent wind had whipped the lines mercilessly.
Fig. 2: Insulator half gone from a guy line.
Fig. 3: Insulator completely missing from a guy line.
At least one insulator broke away entirely, causing metal guy line sections to be electrically connected, rather than insulated from each other. These are sometimes referred to as “Johnny Ball” or “egg” insulators.
It is a good thing that insulators are designed and built so that failure will not result in guy line separation. The photos you see were taken when the damaged insulators were finally on the ground for close-up inspection near the end of the project. It is important to note that a cracked insulator can arc over, especially when the crack fills with debris and water.
The repair was a big job. It was determined ultimately that all guy lines and insulators on the tower had to be replaced. The insulators are spaced at one-tenth or less wavelength intervals. At 1380 kHz, that is 71 feet, so there are quite a few.
Not all tower contractors know the requirement to put insulators in guy lines at the correct intervals. You, as the engineer, must watch over the project to make sure it is done right.
Fig. 4: Workers on the tower near an FM antenna.
Fig. 5: Tower worker with a new guy line in tow.
Time went by because the insurance company had to agree to the repair, then supplies were ordered and a tower contractor was hired.
I measured AM monitor points just before the tower crew started the repair. It was a reference point in case something went wrong. I could pin any new problem on the work, not something that might have happened a month or more prior.
Since KLIZ is a multi-tower array, I was able to divert AM power to the north tower, which is 180 feet away. I did that at 1,000 watts instead of the 5,000 for which the station is licensed. That was to keep the tower crew safe. I also grounded the unipole feed on the troubled tower to keep AM RF voltages to a minimum while workers were on it. Grounding was by a heavy cable with large alligator clips (as described in my Radio World article in the July 6, 2016, issue).
KLIZ is non-directional day, as mentioned, so it worked out using a single shorter tower in the array during the day, even if it was not the one normally used. By sunset, the plan was for the station to go directional, as required by FCC rules.
The tower requiring work has two FM transmit antennas. The FM transmitters had to be reduced in power or completely shut down for much of the project. Station management and programming weren’t happy about it, but what could they do? Safety comes first.
Fig. 6: Guy lines properly tensioned.
Fig. 7: Sunset at the end of the successful project.
The procedure involved installing and tensioning a temporary guy line to a slightly higher location on the tower than the original guy. Then the original line was taken down and a new line hoisted in its place. That new line had to be tensioned while tension was being released from the temporary line. It was a slow job that took three men a week to complete.
The first day ended in the installation of a temporary guy line that did not have insulators. Directional operation that night was impossible because the tower became, electrically, an odd length that could not be compensated for in the phasor system. So that was a lesson learned the hard way. From then on, any temporary guy line was insulated from the tower so it would not significantly distort the AM directional night pattern during the project.
Guy lines droop slightly, but that is normal when they are tensioned properly. Good engineering practice dictated that I confirm all AM operating parameters. Fortunately, they had returned to normal before I released the tower crew to go home. That included measuring AM monitor points while the crew was still cleaning up on the ground. One essential element in this lesson is that thinking ahead will help avoid problems later.
All was well after the repair work was done. The towers stood tall at sunset for everyone to admire. They have been there for 48 years and should be around for a while longer, providing broadcasting to the area.
This article is not just about what I did, but about what you should do in a case like this. Also, if you have an AM tower that has changed impedance, it might be good to pull out binoculars to see if one or more of the guy insulators is broken, especially where a guy line meets the tower. It makes perfect sense to check everything.
Mark Persons, WØMH, is a Certified Professional Broadcast Engineer and has more than 40 years’ experience. His website iswww.mwpersons.com.