Thermal Imaging Saves Time and Money on Transmission Line Failures
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.
|Overheating terminal in the electrical service panel. Note the temperature 20 degrees higher compared to other terminals.
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
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?
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.
|Typical rigid-line flange connection. Temperature was consistently about 4 degrees higher on the low side of the flanges but this indicated normal operation.
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.
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.
|Thermal image of an elbow at 540 feet.
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 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
|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.
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.
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
|The line bullet is heavily discolored, indicating overheating.
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
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 firstname.lastname@example.org.
Tom Woods is the president of Woods Communications, a broadcast communication
consulting firm in Eugene. He can be reached at email@example.com.