Test your RF system

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Construction standards for most transmitting equipment today is generally so high that testing of individual components is uncommon and unnecessary. However, the overall testing of an RF transmitting system is necessary because the FCC has mandated annual NRSC mask compliance tests for directional and non directional AM stations. FM transmitters nowadays only require testing when putting a new one into service. The old days of running channel separation measurements and other proofs have been eliminated.

I can't help wondering whether it's the penny-pinching efforts of management that have communicated themselves to the FCC and resulted in this reduced number of tests that station engineers are required to perform. Or is the FCC finally beginning to realize that modern equipment is reasonably stable, and capable of performing in accordance with its specification, so that repetitious equipment tests are no longer mandated?

No matter what problems may be encountered, a recorded set of operating parameters will make troubleshooting and repair much easier.

Apart from the two mandatory tests, there are various RF tests that conscientious chief engineers should perform on a regular basis to keep their transmitters operating on the top line, and that will add to the overall performance of their stations. Unfortunately the bottom-line attitude of too many managers has greatly reduced the time that most chief engineers can spend on this rewarding work.

A time to act

Unless you are lucky, the time will come sooner or later when something goes wrong with your RF system. It may be a small thing that is easily fixed, or something that puts the operation so far out of normal parameters that it is necessary to close down or reduce power. No matter what the problem, a recorded set of RF system operating values always makes the process of correcting the problem much easier.

When joining a new station, look for this information and if it is not available, compile your own set of operating conditions. A loose-leaf binder with suitable tags and dividers works well for initiating this information record, and keeping it up-to-date. Even if you are not lucky enough to inherit one, your successors will bless your name.

For AM stations, DA and non-DA, the mandatory annual NRSC mask proof is important, not only because it is an FCC requirement, but also because it gives the engineer an idea of how his transmitting system is performing.

A spectrum analyzer and recorder are required to perform this proof properly. It is possible to make these measurements without this equipment, but it is time consuming and difficult to substantiate. The necessary equipment is expensive, and for a single station or just a few stations, it becomes difficult to justify the expense. Find a contract engineer service group that will make scheduled calls to do these tests. In some states, the state broadcasters association has made arrangements with one of these groups to offer this service for a reasonable fee. The charge is usually only a few hundred dollars. If a number of stations need to be tested, it might be more economical in the long run for a station to own the equipment. However, this depends on individual corporate circumstances and philosophy.

An OIB is a vital piece of equipment for a directional AM station and can be used to quickly diagnose antenna system operating problems.

The FM station engineer has even fewer mandatory tests or proofs to make. In the past annual stereo separation proof requirements were imposed as well as audio proofs. Today, unless an FM station is under construction or a new transmitter is being installed, there are no mandatory FM tests required. However, this lack of FCC requirements should not encourage engineers to let systems run for long periods of time without checking the overall performance. Components change, adjustments change, “knob twiddlers” come along, the quality changes and no one realizes that the overall signal has suffered.

The combination of operating impedance bridge (OIB) and generator and receiver detector is probably the most versatile tool in the average AM station. These instruments are the best set of equipment that an engineer can have when AM antenna problems develop. A non-directional station antenna operating impedance doesn't usually change, but when it does, the OIB is invaluable in tracing the cause, whether it is in the ATU, transmission line or antenna itself.

An OIB is essential for a directional AM station. The common point operating impedance seems to be susceptible to change from many causes including cracked guy insulators, ATU and phasor capacitors changing in value, adjustable inductances that adjust themselves and animal life that inhabits ATUs.

Like most test equipment, it is possible to rent OIBs and associated equipment, but after a few rentals it usually becomes cheaper to buy it outright.

A time-domain reflectometer (TDR) is useful for AM and FM stations, however transmission line problems don't seem to develop as often in AM stations. Fortunately the backwoods marksman is less attracted by the ground supported AM lines than the beckoning lights on the FM tower. As a consequence, FM lines suffer more from straight bullets. Luckily, many electric power companies include TDRs in their tool inventories. It may be possible to borrow one from a power company employee, and frequently this will supply sufficient information to locate a damaged portion of FM transmission line on a tower.

For the AM station with a damaged transmission line that is buried, a TDR can be worth its weight in gold. When not buried deeply enough, underground transmission lines are sometimes damaged by traffic passing overhead, from manufacturing faults or simple deterioration throughout the years. Digging up a buried line is almost as bad as trying to repair a damaged FM line on a tower. The TDRs used by the power companies are invaluable when dealing with a suspected faulty buried AM transmission line, and faults are usually traced to the nearest foot.

Being ready

Throughout the years, the FCC has reduced its mandated RF and other tests. Gone are the after-midnight audio proofs. Does 24-hour operation have anything to do with it? Taking two hours from midnight to 2 a.m. or more, for FCC tests, when those hours could have been earning money may have rubbed many managers the wrong way. Has management's unhappiness influenced FCC thinking about unnecessary RF tests that decrease a station's total earning hours? Whatever the cause, routine tests after midnight by engineering should be fewer these days.

E-mail Battison at batcom@bright.net.

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