Mar 1, 2002 12:00 PM, By John Battison, P.E., technical editor, RF
Directional antenna maintenance is primarily an ongoing process based on experience and adequate written records. Admittedly today, with the FCC's greatly reduced logging requirements, the paperwork can be considerably less than in years past. However, there is still no excuse for a DA that does not have adequately written and available records in the form of maintenance logs. Not only does a well-maintained logbook provide a guide to the daily operation but it can, and usually does, give a great deal of diagnostic help when a crisis occurs (and even with the best of maintenance, crises do occur; usually after normal working hours). Often a quick glance at operating parameter records will show a situation that has developed and sometimes might have been averted if a record had been kept carefully and analyzed regularly.
A thorough inspection of the tower and any accessory items must be done regularly. Photo by Jerry Goforth, WCLV-AM/FM.
A properly recorded, correctly taken and regularly examined monitor point log is the best maintenance tool. The Commission no longer requires regularly taken and spaced monitor point readings, so it behooves the conscientious engineer to make a point of having these readings taken on an acceptable routine. A slow change will show up when compared to previous measurements, and should lead immediately to a check of antenna monitor phase and ratio readings as well as common point current.
A sudden large change in antenna monitor readings should be followed immediately by a check of the monitor points and log; it should never be followed by a frantic turning of phasor cabinet knobs in an effort to regain the normal monitor readings. In the absence of any drastic change in other operating parameters such a monitor change should be followed by the usual checks including common point current. If the system has a built-in Operating Bridge, the common point impedance should be checked whenever any of the above out of limit readings are observed and before assuming that the array has returned to operation in accordance with the license.
Even though regular inspections are a thing of the past there is a lot to be said for the old-timer's �handy feely� hand check of capacitor and inductor temperatures at sign-off, or even pattern change. A hot capacitor, or high temperature or discolored inductor is one of the easiest checks make to run down undesired and excessive RF current in the wrong place.
Look for warm spots in the transmission line. There should be none. It's not unusual for a line to be very slightly warm, especially indoors where there is no breeze or air movement to cool it. Any hot spots are an immediate indication of high standing waves. AM antenna systems are usually quite tolerant of standing waves, but VSWRs high enough to cause heating are usually an indication of an improperly adjusted antenna system. This means getting out the O-I-B and checking the phasor and ATU lines to find the mismatch. At this point it will probably be a good idea to check actual base operating currents as well.
It is very good practice to post the phasor dial readings by each control knob; and also the common point's upper and lower current limits by the common point ammeter. Similarly posting the antenna monitor's phase and current ratio limits close to the monitor makes for quick referral in a panic situation.
The fact that an antenna monitor is brand new or just rebuilt doesn't mean that it is working properly. I've had several cases where a new, or rebuilt, monitor has come back with the same poor relay contacts that caused the original problems.
If the monitor points are �in,� RF currents are normal and the antenna monitor is �out,� check the sampling lines. They should all be buried and any excess treated equally and also buried. If the system was properly installed originally, there will be a record of the original sampling line impedances and DC resistances. Checking the immediate operation against the original values will give a good idea of their condition. It is not unknown for trucks to drive over soft places around the tower and damage monitor lines.
If an antenna monitor or tower monitor input is suspect it can be verified by changing the inputs to the monitor and comparing readings on different inputs.
Transmission lines are normally safely buried or mounted on adequate supporting posts. However ice has been known to damage lines in exceptional conditions and so have vandals, so don't be too quick in dismissing these items in the long-term examination.
It's important to remember that RF current transformers in ATUs can be damaged by lightning or even RF arcs, therefore they should be examined for obvious damage and electrical performance. In this connection it is important to ensure that the lightning protection single turn ring in the RF connection to the ATUs be restored after work on tower bases. If this is omitted the next storm may put your station off the air.
Sometimes the insulators holding the tower-mounted RF current loops become cracked and change operating indications. A strong wind can move such loops so that misleading voltages are picked up. Sometimes too, a gale may move just one such loop, possibly the reference loop. This can produce strange antenna monitor readings that tend to lead one away from the actual mechanical problems. Anything that affects the reference tower loop (or current transformer) will impact the readings for the other towers because it provides the reference voltage against which the other towers are checked.
It is not unusual for towers in directional arrays to support other devices such as FM or STL antennas. If the AM radiator is not shunt fed some form of feed line isolation will be used. This can take the form of a horizontal, or vertical, quarter-wave isolating stub or an isocoupler to carry the line across the base insulator. It is not unknown for isocouplers to develop faults and they should not be ignored if serious problems occur that are not amenable to other solutions.
In the past a surprising number of quarter wave sections have been finely tuned by means of an air capacitor across the section. This is an acceptable method of adjustment. However it is surprising how many newer engineering entries into the wonderful field of radio have not come across these little gimmicks. Such encounters have sometimes resulted in the removal or re-adjustment of these useful �gimmicks� and caused considerable work in readjusting the decoupling section.
Finally, don't forget the humble field mouse. Doghouses and even metal ATU cabinets have an especially strong attraction for these little animals in the winter. There is protection from other animals and warmth from the RF energy. We've all come across cozy mouse nests located in the ATU inductances. Sometimes we've found roasted mice that have short-circuited coil turns. So, when regularly cleaning out the ATU cabinets or doghouses, be sure to look for animal nests in RF areas. Snakes also sometimes come in out of the cold.
In summer wasps and bees can often be found happily building nests in ATUs and doghouses. Systems that use RF contactors in doghouses to change antenna patterns have found that the contactors themselves seem to have an attraction for these insects. Perhaps the buzz from the operating coils attracts them? In any case, be sure that your ATUs and doghouses are cleared of other natural life styles. The best way is to seal and close every conceivable entry point.