Overheating terminal in the electrical service panel. Note the temperature 20 degrees higher compared to other terminals. Throughout day-to-day operations in a broadcast facility lies a part of the broadcast chain often overlooked. Antenna and feed-line systems are the last link in the broadcast distribution network, but they are often the most difficult to troubleshoot and maintain.
Broadcast engineers do the best they can with the limited tools available. There are ways of tracking and maintaining various computer networks and transmission systems in a broadcast facility, and some maintenance can even be performed in a systematic manner thanks to modern technology.
However, antenna and feed-line systems still remain difficult to troubleshoot effectively. To make matters worse, when antenna problems do arise, there is often little that can be done to rectify the problem quickly without engaging an expensive tower crew to investigate. So the question becomes, “How do we troubleshoot problems that are out of reach and difficult to see?”
In February, this question was posed rather suddenly to McKenzie River Broadcasting in Eugene, Ore. McKenzie River Broadcasting combines two of their signals, KKNU(FM) and KMGE(FM), with a high-level combiner. KKNU and KMGE contribute a total of 50 kilowatts from a branch combiner into a 4-inch rigid feed line and antenna system. That month, I began to notice intermittent VSWR overloads from high-speed “Watt Watcher” detectors. Both transmitters were displaying VSWR overload and PA screen overload a few times per week — indications of a problem in the transmission system. Clearly something was arcing and causing a temporary short circuit. Where could it be?
Typical rigid-line flange connection. Temperature was consistently about 4 degrees higher on the low side of the flanges but this indicated normal operation. Early stages of arcing often are difficult to detect and even more difficult to pinpoint. There are only a few places to look, but a direct mechanical inspection is time-consuming, possibly requires a tower rigger and requires the stations to be off the air. Arcing became more regular in a short amount of time until the transmitters were tripping off the air several times a day.
The combiner and associated components were dismissed first by a physical inspection and looking for areas of excess heat. Options for detection and repair became limited once the problem was tracked to the tower transmission line and antenna system. A time-domain reflectometer at ground level is only helpful if there is a total burnout resulting in an open circuit and that wasn’t the case yet. Of course, a total burnout is also the worst-case scenario for the antenna system, and the one thing McKenzie River Broadcasting wanted to avoid. Both FM signals for McKenzie River Broadcasting function into a single broadband antenna at the top of a 600-foot tower, so a failure would take both stations off the air at the same time.
Conventionally, after a problem of this sort is tracked to the antenna system, the only solution is to remove and inspect the entire feed line, which for McKenzie River Broadcasting is 600 feet of 4-inch rigid line. If the line sections look adequate, then the antenna would be taken down for inspection. This would be an expensive and a time-consuming project. It would have required the other FM stations on the tower to operate at safe power levels for days.
Thermal image of an elbow at 540 feet. Woods Communications offered an alternative solution. Tom Woods has been a broadcast engineer for some 20 years. McKenzie River Broadcasting hired him to help identify the antenna system failure.
In 2011, Woods had acquired a professional thermal imaging camera. Woods knowledge of antenna systems, combined with his ability to climb towers, led him to believe that thermal imaging could be used to help identify the problem without having to resort to taking down the transmission line or antenna.
Woods believed the thermal imaging camera provided a way to locate an imminent burnout. The idea was to use a thermal camera and map the antenna and rigid line system temperatures in hopes of finding discrepancies with the system that would indicate a pending failure. This would require the other high-power FM and TV stations on the tower to only operate at the low and safe power level for only an hour.
THERMAL MEASUREMENT BASICS
Thermal technology works on the theory that all objects emit heat. An infrared camera is a non-contact device that detects infrared energy (heat) and converts it into an electronic signal, which is then processed to produce a thermal image on a video monitor and perform temperature calculations. Heat sensed by an infrared camera can be precisely quantified, or measured, allowing you to not only monitor thermal performance but also identify and evaluate the relative severity of heat-related problems.
This internal connection at the elbow is nearly gone — the entire end is burned off. Note the metal fragments piled up on the insulator — why the arcing gets worse after the first event. For this application, Woods Communications used a FLIR T-400. We started temperature measurements in the transmitter suite and immediately found a lurking problem, although it wasn’t the problem we were looking for. Fig. 1 (on page 1) shows an overheated contactor in KKNU’s three-phase disconnect panel. This excess heat eventually will cause the metal to fatigue, get even hotter and eventually fail. I can now schedule time to replace this panel before a failure occurs.
From there, measurements of the branch combiner and all rigid feed line paths to the tower were taken. Although no further weaknesses were discovered inside the building, the information gathered provides a frame of reference for future problems.
Next, Tom and I took our efforts outside. Fig. 2 depicts one of the sections near the bottom of the tower. Thanks to the T-400, it was easy to see temperature both above and below each flange. The ambient temperature on the ground that day was 46 degrees. The temperature on the exterior of the line measured at about 52 degrees at the bottom and incrementally decreased further up the tower as ambient temperature dropped. However, the temperature difference between the top and bottom of the bullet remained consistent at 2 to 4 degrees. This temperature difference is attributed to infrared reflection from the bottom of the flange down on to the line. I noticed that the bottom of each bullet was slightly warmer than the top. I suspect that this has to do with the ability of the inner conductor above the bullet to sink heat away from the bullet.
The line bullet is heavily discolored, indicating overheating. After scanning 48 sections of rigid line, the problem finally was discovered at the 540-foot level in an elbow. Fig. 3 depicts the largest temperature difference. Heat at the elbow measured around 90 degrees. The weakness in McKenzie River Broadcasting’s antenna system had been identified.
The elbow was later removed and replaced, and McKenzie River Broadcasting quickly returned to full power. Fig. 4 shows the damage inside the elbow. A close look reveals the arcing path from the inner to the outer conductor. Although there wasn’t a complete burnout, severe heating damage to the bullet is visible in Fig. 5.
The survey took about 90 minutes and repairs were completed in less than six hours. Repairs would have been more extensive and costly in terms of lost air time had the problem worsened to the point of a complete burnout.
Using infrared photography to survey your entire antenna system while it is in operation is a tremendous troubleshooting tool that can prevent future catastrophic failures. The use of thermal technology has been invaluable to Woods Communications in the work of identifying weaknesses that can’t otherwise be observed.
Chris Murray is the director of engineering for McKenzie River Broadcasting in Eugene, Ore. He can be reached at [email protected]. Tom Woods is the president of Woods Communications, a broadcast communication consulting firm in Eugene. He can be reached at [email protected].