The road to antenna maintenance

November 1, 2006

It is a true statement that AM antennas are usually forgotten until something goes wrong with the signal. Then, finally, some attention is paid to the pile of steel rusting by the transmitter building. In the case of a nondirectional station it generally takes serious deterioration to affect the signal greatly. In the case of a directional station many factors such as current, phase, self and mutual impedance, monitoring system and transmission lines can interact to ruin a pattern and affect a service area.

I'll speak more directly at directional array (DA) maintenance than to nondirectional because a DA's additional complexity requires greater attention to several numerical values than a simpler nondirectional antenna. Before any useful maintenance can be performed it is essential to know how one's system is supposed to operate and also know the licensed operating parameters.

It''s important to know a station''s licensed operating parameters before any meaningful system maintenance can be performed.

A station chief engineer should have information on the operating parameters for the antenna system at the station including a copy of the latest proof of performance and the current license. If the station does not have this information, make it a priority to obtain them. Sometimes this takes a great deal of repetitious effort to find out the name of the consulting engineer or other person who performed the proof of performance and obtain a copy of the proof. At the same time the station files should be checked carefully to be sure that the license displayed is the correct current license.

These may sound like obvious steps to take. However, it is surprising how many times I have visited stations and had a great deal of difficulty in finding the current license and a copy of the latest proof of performance. Several times I have found that the engineer has several licenses in his file and is not quite sure which is the latest. This is not generally the engineer's fault. It is often the fault of management who has not passed on engineering papers to the proper department.

While searching for the proof of performance and license it is essential to ask if any special temporary authorizations have been issued and are currently in use. Also, check for any pending applications that may have been acted on without the knowledge of the chief engineer.

Worthwhile effort

The foregoing may sound like a lot of unnecessary work, but believe me it is not. Until you know that you have the current, correct operating parameters it is not possible to be sure that you are resetting the operation in accordance with licensed FCC authorizations. There is nothing more frustrating on completing a DA tune-up than to have the station engineer come along and say “I just found this — it was dated last week” and produce a new license.

While the chief engineer is gathering the above information examine the operating log. Also examine the maintenance log to look for obvious potential problems. A properly maintained maintenance log is a wonderful maintenance tool.

If you don't know what you're looking for and what the operating values are supposed to be you can't very well perform maintenance on a system. Once all the necessary information has been gathered and assimilated the engineer will know what he is trying to maintain. He will have all this information ready for the time when it's needed for maintenance work or an FCC inspection.

Maintenance procedures

The road to proper maintenance is the road to adequate record keeping when the actual licensed and anticipated operating parameters have been established The next step should be to write down all the dial settings on the phasor cabinet face (as well as in the maintenance log book) and all of the other indicating devices, including the antenna monitor. If, at this point, the antenna system happens to be properly adjusted according to the required operating parameters it will probably be a small miracle.

Compare the power output according to the antenna with the output of the transmitter to verify operating efficiency.

The old FCC requirements of weekly inspections and equipment operating logs, in my opinion, made maintenance a great deal easier. The regular equipment inspection contributed greatly toward trouble-free operation. The old operating logs were often used as directional signs pointing toward potential problems.

Comparison of RF power out of the antenna based on Icp2 times Rcp against power consumed by the final stage Ip times Ep provides an excellent way of checking the efficiency of the final stage. Check this figure from time to time to be sure that RF power is not being lost in unexpected places. The efficiency figure thus obtained can be useful if the engineer needs to operate under conditions of indirect power measurement, which is based on the use of previously measured or the manufacturer's rated efficiency.

Regular maintenance must include the examination of all connections and connectors in all circuits, tower-mounted antenna monitor loops, or current transformers at the output of the tee-matching network in the ATU. Lighting circuits and photo-electric controls must be checked and tested as well as pattern change contactors and control circuits.

If maintenance occurs immediately following transmitter shutdown, capacitors should be checked for unusual heat, which may indicate potential failure or excessive current through a circuit. Inductors should also be examined for signs of overheating. This often occurs at anchoring point connections and is especially important when clipped leads are employed because their clamps frequently work loose or develop poor contacts. Inductor overheating is indicated by discoloration of the plating.

Check tube operating hours if meters are provided, and don't forget to record the date of putting tubes into service. It's also a good idea to include details on actual or suspected reasons for tube failure.

Transmission lines and antenna monitoring lines normally have little to check provided that no visible damage has occurred and that there are no symptoms that would indicate transmission line problems. Unless there are erratic readings there is normally no need to make more than a thorough inspection of the visible portions of the lines and their connections.

If any of the lines are gas pressurized, or use a dehydrator individual gas pressures and the amount left in the tank, and dehydrator conditions should be checked. Of course if any damage to the lines has been noticed the line should be checked with a time domain reflectometer (TDR).

Guy tension and anchors must be examined and verified. Check for rust and galvanic action at guy anchors. Use field glasses to check for cracked guy insulators and look for cracks in base insulators. Water can infiltrate them and complete disintegration can follow in winter.

Ground systems and associated copper strapping must be examined for breaks and poor hard soldered joints. Unless there has been heavy traffic the soft ground radials should be OK in the absence of indicated electrical problems.

The common point impedance and current must always be verified after the completion of tuning the antenna. If a built-in operating impedance bridge is provided in the phasor it should be used, otherwise insert an in-line operating bridge on the transmitter side of the common point ammeter.

FCC monitoring points should be checked after all maintenance is completed. Maintenance results and work performed on the system should be recorded in the maintenance record book together with all measured operating parameters including the FCC monitor point readings and antenna monitor readings.

If any tubes have been replaced or major items taken from spare stock, replacements should be ordered or obtained, and other maintenance supplies replenished as necessary. There is nothing worse than realizing at midnight that the only spare final tube was used last winter and never replaced.

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