Transmission & Distribution: Monitoring RF systems
Monitoring RF Systems
By Don Markley
Transmitters keep getting more and more reliable. At least, that is what the manufacturers would all have us believe. In reality, they are becoming more complex, which is not necessarily the same thing as reliable.
One system currently in operation uses the new Bird Electronics power meters, which are installed in the six-inch transmission lines. The outputs of the Bird meters are routed to the computer that uses a proprietary program to display the forward and reverse power on all lines.
The issue of good preventative maintenance has been discussed in this column in lengthy and boring detail. But such maintenance has become even more important as transmitters have become more complex, and more difficult as the functions accomplished by the transmitter have become more numerous.
If you take care of good equipment, it will serve you dependably. That is the case now as it has been for many years.
With transmitter plants becoming almost universally unmanned, good monitoring systems have become even more necessary. Transmitter remote control systems have developed to the point where they monitor everything from primary line voltage to individual tower lights. But the transmission line and antenna are still sometimes overlooked.
It simply isn't sufficient to meter VSWR or reflected power using a very basic metering system. Equipment is available to measure forward and reverse power accurately and provide those readings to an on-site computer that can be accessed either by the Internet or a direct dial-in modem. As an example, one system currently in operation uses a computer to monitor three antennas fed by a total of eight transmission lines. The basic power metering devices are the new Bird Electronics power meters installed in the six-inch transmission lines.
The outputs of the Bird meters are routed to a computer that uses a proprietary program to display the forward and reverse power on all lines. If a fault is sensed on any of the lines, or if the power is inadvertently increased above the rating of the lines, the system shuts down the associated transmitters before further damage occurs. This also prevents the horrible habit practiced by some transmitter operators of pushing the “Power On” button over and over in an attempt to get the station back on the air. Operators should know that if the transmitter doesn't come up with the first attempt, any further attempts are probably just doing more damage — to very big and expensive components.
On some antenna systems the repeated button-pushing repair method has destroyed the entire vertical run of six-inch coaxial line. In one case, the repeated efforts of the station staff to get the transmitter back on line burned up about 75 feet of line, destroying most of the center conductor and burning huge holes in the outer conductor. The lesson here is very simple. When the transmitter goes off, check first to see if there is an overload condition. If there has been a VSWR trip, common sense would say to try bringing the transmitter back up once, although it probably would be a good idea to reduce the power before making that attempt. If the transmitter fails to come back to normal operation, it's time to go to the standby antenna if one exists. In any case, it is definitely time to find out what is wrong before hitting the high power button again.
Many transmission lines out there still use the familiar “watch-band” connections. As the years pass, the combination of continued wear and heat cause that spring-type assembly to lose its tension become loose and lie down in their groove. When the connection starts to go bad, heating increases and further loosens the spring. At the very least, those watchband springs should be replaced every 10 to 12 years. That repair job is not highly expensive and is certainly much, much cheaper than the repair job that will be necessary if a burnout occurs — probably at midday — during sweeps.
The next area of concern is simple mechanical maintenance. Those pieces of equipment that normally work properly for many years tend to be totally overlooked when maintenance is done. Such is the case for transmission line or waveguide hangers. Spring hangers are meant to allow the transmission line to expand at a different rate than the tower during temperature changes.
The hardware associated with those hangers — the necessary bolts, springs, hose clamps around the line, and other bits and pieces — do occasionally fail. The result can vary from mechanical damage to the line to a change in the tuning of a waveguide system as the waveguide is distorted by improper support. Again, the solution is simple. When the tower receives its annual inspection, be sure that the riggers check each hanger carefully for proper hardware and for correct spring tension.
The hangers come from the factory with a spring tension chart. That chart tells how long the spring should be for a given temperature. It is a simple matter to check the temperature and the chart when doing tower maintenance and then to measure the springs to ensure that they are at the proper tension. One final check should be performed during any tower modifications. The riggers need to check any tower additions such as new antennas or transmission lines to be sure that they do not interfere in any way with the normal expansion of the transmission line. If the line is not allowed to move freely, severe mechanical damage can result. The line is going to expand as it heats up — if it can't expand freely, something is going to be crushed in the process.
Finally, it is possible with today's equipment to find problems with the transmission line and antenna before they cause significant deterioration of the antenna performance or a burnout. A device such as the Dielectric VSWRvision system can help to identify changes taking place in the system, whether abrupt or gradual.
In addition, it is a very good idea to schedule a complete antenna and transmission line sweep at least every three years. This action is simple and can be done with no more than ten minutes of down time if no other actions are needed. The system performance can be compared with the measurements that were taken when the initial tuning was performed or at the time of the last sweep. The combination of frequency domain and time domain measurements can often spot developing troubles that, in turn, can be repaired at your convenience rather than leading to a system failure.
To that end, it is advisable to purchase an adapter, tuned to your channel(s) for performing system measurements. That way, the necessary hardware will be on hand to permit accurate measurements without waiting for the factory to tune and ship a connector. To answer the question before you ask, NO, a simple type N to whatever is not acceptable. Adapters should be tuned or optimized on your channels so that the measurement process becomes one of looking at the line and antenna rather than trying to determine how much the connector is screwing up the readings. If that seems implausible, it is recommended that a visit be made to the Agilent Web pages. The connectors to a piece of transmission line are absolutely critical if any kind of reasonable accuracy is desired. Just buy one and put it on the shelf. It will be a wise move in the long run, especially in the case of a system failure.
Don Markley is president of D.L. Markley and Associates, Peoria, IL.
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