BEITC 24 Report: RF Fault Monitoring Beyond VSWR

State-of-the-art VSWR measurement and monitoring of broadcast transmission infrastructure is limited to in-band reflected power and typically incapable of detecting matched arcs. Finding and isolating the source of intermittent arcing and other tricky RF issues has recently become significantly easier.

A BEITC paper called “An Innovative In-Service Antenna Monitoring System to Protect Your Antenna and Transmission Line” was presented by Heidi Stamm, Anton Lindner, Christoph Neumaier at SPINNER GmbH, Munich, Germany and Todd Loney with SPINNER ICT Inc., Duluth, GA, USA in a BEITC session. Graphics and information in this BEITC coverage are Courtesy of NAB.

Every broadcast maintenance engineer knows the most frustrating troubleshooting issues are those that are infrequent, intermittent, identical, and of short duration. Depending on the speed and accuracy of VSWR monitoring equipment, some arcs or small reflected power changes can go unnoticed.

Spinner introduced the Antenna Monitoring System (AMS) in 2015 to scan for arcing from the transmitter building all the way up to the end of the antenna. The AMS provides protection to the entire RF system by alerting and/or opening the transmitter interlock when an arc is sensed. While the AMS provided protection, it couldn’t tell you where the fault occurred.

This paper discusses the addition of Distance to Fault (DtF) analysis added with AMS 2.0, it's methodology and benefits, along with some real-world results where the AMS protection detected and closely located faults before significant damage occurred.

Outdoor Hardware Degrades

Broadcast transmission RF hardware is designed and maintained regularly to last a long time. However, most stations RF components are outdoors and thus are exposed to extra environmental stress. There can be high temperature variations and humidity. Storms can lead to increased tower vibrations or even lightning damage, and UV-radiation may degrade plastic components, just to name a few impacts. Over time, material wear is unavoidable. Thus, the potential of degraded or even damaged parts always rises and may also affect the RF operation caused by water ingress.

Immediate post-commissioning transmitter failures are possible due to improper installation leading to arcing inside an RF component. A TV transmitter typically measures reflected power from the antenna system consisting of the transmission line system and the antenna. If the VSWR is too high a transmitter typically shuts down or drops to minimum power. However, arcs do not necessarily lead to reflected power at all frequencies. A matched arc may be present while virtually no degradation in transmitter VSWR is detectable within its specific frequency range. Therefore, the transmitter would not detect a matched arc and would not shut down.

Figure 1 shows a real-world example of a standing matched arc. Because the transmitter did not detect high reflection the RF power remained active, keeping the arc alive. On the lower part of the picture flames are visible. The jacket of the feeder line is already melted, and the outer conductor has changed color. Without intervention, severe damage to the RF system will occur.

Figure 1: This burning feeder line with a standing arc is not degrading the transmitter VSWR.

Figure 1: This burning feeder line with a standing arc is not degrading the transmitter VSWR.

In another incident occurring in 2011, the feeder lines of the transmitting station Zendstation Smilde, in the Netherlands, caught fire and the upper part of the mast collapsed.

A common method for maintaining an antenna system or finding the cause of a faulty system is to perform a time-domain reflectometry (TDR) test with a vector network analyzer. A TDR test requires shutting down the transmitter, moving the broadcast signal to a backup transmitter and antenna and the test could potentially create interference for other users of that frequency spectrum. TDR testing works but is complicated and disruptive.

Also, antenna systems usually contain frequency selective components like splitters or the antenna itself which will reflect most of the out-of-band TDR signal, limiting the ability to see beyond those components into the antenna.

Combining Measurement Principles

To overcome the risk of matched arcs, monitoring the reflected power of an antenna system is not sufficient. The aim was to find a method which is passive and able to detect any type of even very short arcs.

In the 2015 Antenna Monitoring System (AMS) the approach was not to interpret the arc as a reflecting object but as a new signal source introduced in the system. An insulation measurement was implemented which can detect water ingress at open circuit antennas. The sensors for both measurement principles were integrated in a line section to replace a piece of normal feeder line. The calculations and analysis are performed in a separate control unit with an SNMP interface.

Continuously intercepting its characteristic RF emissions as well as analyzing and evaluating them is essential. AMS allows detection of very small and short arcs which do not harm the antenna system at all. The minimal arc detection time is 100 µs. Depending on the properties of the arc, a pattern recognition algorithm can decide whether it is tolerable or if a shutdown of the transmitter is necessary.

A Distance to Fault (DtF) analysis was required to complement the AMS. The goal was to implement the DtF without the disadvantages of the out-of-band TDR testing. Instead of a normal TDR, a passive radar-like approach was chosen. Passive radars do not transmit anything but use signals emitted by other services like broadcast stations as sources. The approach analyzes these signals and their reflections to sample the environment.

Figure 2: Valuable data history is instantly available to reveal when faults are detected and where.

Figure 2: Valuable data history is instantly available to reveal when faults are detected and where.

Figure 2 was created with a test transmitter and an arcing unit for demonstration. The red crosses in the upper diagram are arc events captured by the non-reflection-based detection method. The amber curve is calculated by the DtF algorithm.

The first reflection event in the diagram displays a strong and stable arc. Degraded reflection is quite high and stable, and the arc duration and power are high enough to cause an alarm that would have opened the interlock if it were connected. When the arc ceases, the degraded reflection goes back to a low level.

The second reflection event shows fluctuating arcs. The degraded reflection is changing within seconds and there many short arc events. The third event indicates a degrading component or water ingress. Here no arc events occur, and the degraded reflection curve is smooth and changing slowly.

Weather and Icing Detection

In general, the degraded reflection is sensitive enough to indicate weather conditions, particularly temperature. Its influence depends on the construction and the height of the transmission tower. If the feeder lines are laid outside and completely exposed to the weather the impact is higher. In this case, daily cycles caused by temperature changes are usually visible. At stations where the feeder lines are inside the building the temperature profile is smooth and less observable in the degraded reflection. The DtF threshold should be adjusted to the specific station to hide acceptable results caused by weather impact.

Figure 3: The effects of temperature and antenna icing are easy to spot when comparing DtF data with weather data.

Figure 3: The effects of temperature and antenna icing are easy to spot when comparing DtF data with weather data.

Figure 3 shows the degraded reflection and the corresponding distance for an iced antenna. Weather data is displayed below. On the left side of the diagram the antenna is not iced. The degraded reflection is on its normal level which has been stable for months. Its alternation is caused by normal daily cycles as mentioned before. In the night between 2nd and 3rd of February freezing rain and snow fall and the degraded reflection significantly increases.

As soon as it exceeds the DtF threshold a distance is indicated. In this example, it corresponds to the top of the antenna. The 480 m located at first is the distance to the most temperature sensitive component in the antenna system. It was detected during stronger temperature changes before (not displayed in this diagram). On the 5th of February the weather is quite warm and sunny so snow and ice are melting, and the degraded reflection comes back to its normal level. Even during icing a low temperature sensitivity is visible.

The presentation went on to explain how AMS detected arcing in a split bullet and damaged inner conductor in a transmitter feedline and isolated it at 595 meters. It concluded with demonstrating how the AMS detected and isolated infrequently recurring malfunctions caused by objects falling into the antenna and creating arcs.

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