The development of new sensing devices has made temperature-testing tools extremely useful for broadcasters. The devices permit testing under power — a technique that simply isn't possible with conventional instrumentation.
Temperature testing equipment ranges from little handheld sensors to complex camera systems. It is the latter of those two systems that offers the greatest amount of information. This method of failure prediction should be performed on a regularly scheduled basis.
For antenna systems, the use of a network analyzer has essentially become the norm. Whether for routine system evaluation or to locate failures, the vector network analyzer, with its inverse Fourier transform capabilities, permits operators to spot many system irregularities, up to complete burnouts. It also allows users to identify the location of those problems to within a small distance on the tower. Problems can usually be located to within a couple of feet or less.
This ability to locate and identify minor mismatches in the system impedance allows broadcasters to find a component going bad so it can be replaced when that station wants to do the work. The alternative is to proceed with normal operation until the transmission line fails, taking the station off the air. As a rule, waiting to fix the problem until the burnout occurs is much more expensive, causes the station to spend several hours off the air and greatly upsets the suits in the front office.
The network analyzer cannot be used for testing at full power, which is where thermography comes into use. Figure 1 shows three coaxial lines, all 3in rigid, operating at three different power levels. The elbows are operating at a higher temperature than the straight-line segments, which is normal. The horizontal run is also warmer than the vertical run, as the heat can dissipate away from the elbows up the line. The components shown in Figure 1 are operating normally. A hot spot would show up as brighter than the rest of the lines. The temperatures can be scaled so that a hotter area will stand out from the remainder of the components.
The idea here is to scan the transmission line looking for components that are running unusually hot. As a rule, that signifies either damage or wear that may lead to a total failure.
An example of this occurred at WALA-TV in Mobile, AL, some years ago. Two 3in lines were feeding a batwing antenna. Ghosts were appearing in some directions and not in others. The station was taken off the air and immediately tested for voltage standing wave ratio (VSWR). One line showed a 1.04:1, while the other showed 1.4:1. After some discussion about what might be causing the problem, an engineer noticed that both lines read 1.04:1.
The high VSWR condition only occurred after the station ran at power for some time to warm up the lines. A climber felt all the elbows under the antenna and noticed that one was noticeably warmer than the others. The watchbands in the bullets at the elbow were replaced, and the problem was solved. When cold, the connections were good, but they went bad under full-power operation.
That particular project occurred before the FCC's Office of Science and Technology (OST) determined that climbers shouldn't be that close to the antenna. Under today's rules, that testing would be much more difficult to do. With modern thermography equipment, a technician can climb up the tower close to the antenna and then check all the components with the camera. The hot spots can be located, the picture and data can be recorded on the device, and evaluation can occur on the ground.
Figure 2 shows a 600-amp disconnect switch with problems occurring on the center phase. The switch is in significant trouble. Finding this problem before the switch totally burns out allows the station to install a replacement component before a major burnout occurs with accompanying arc flash damage.
Broadcasters are recommended to visit the transmitting site on an annual basis with both thermographic equipment and with a vector network analyzer. The analyzer should be used to sweep the antenna system from the output of the transmitter both through and after all combiners, filters and traps. Then the entire plant should be checked with the thermographic equipment from the incoming power connection to the antenna.
If either the thermographic or the network analyzer detects an improper condition on the tower, further testing can be done. In this case, an engineer will climb the tower with thermographic equipment. It isn't necessary to open the line to find hot spots. The interior components show up clearly, even in a closed transmission line system.
Besides just transmission line problems, a thermographic camera can pick up power line irregularities, disconnect and circuit breaker problems, transfer switch problems, and other irregularities that generate heat.
Regular annual testing can help a station avoid failures and burnouts that might take it off the air. Those failures cause a lot of heat from the front office.
Don Markley is president of D.L. Markley and Associates.
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