Test your RF system
Dec 1, 2002 12:00 PM, By John Battison, P.E., technical editor, RF
Construction standards for most transmitting equipment today isgenerally so high that testing of individual components is uncommon andunnecessary. However, the overall testing of an RF transmitting systemis necessary because the FCC has mandated annual NRSC mask compliancetests for directional and non directional AM stations. FM transmittersnowadays only require testing when putting a new one into service. Theold days of running channel separation measurements and other proofshave been eliminated.
I can’t help wondering whether it’s the penny-pinching efforts ofmanagement that have communicated themselves to the FCC and resulted inthis reduced number of tests that station engineers are required toperform. Or is the FCC finally beginning to realize that modernequipment is reasonably stable, and capable of performing in accordancewith its specification, so that repetitious equipment tests are nolonger mandated?
No matter what problems may be encountered, a recorded set of operatingparameters will make troubleshooting and repair much easier.
Apart from the two mandatory tests, there are various RF tests thatconscientious chief engineers should perform on a regular basis to keeptheir transmitters operating on the top line, and that will add to theoverall performance of their stations. Unfortunately the bottom-lineattitude of too many managers has greatly reduced the time that mostchief engineers can spend on this rewarding work.
A time to act
Unless you are lucky, the time will come sooner or later whensomething goes wrong with your RF system. It may be a small thing thatis easily fixed, or something that puts the operation so far out ofnormal parameters that it is necessary to close down or reduce power.No matter what the problem, a recorded set of RF system operatingvalues always makes the process of correcting the problem mucheasier.
When joining a new station, look for this information and if it isnot available, compile your own set of operating conditions. Aloose-leaf binder with suitable tags and dividers works well forinitiating this information record, and keeping it up-to-date. Even ifyou are not lucky enough to inherit one, your successors will blessyour name.
For AM stations, DA and non-DA, the mandatory annual NRSC mask proofis important, not only because it is an FCC requirement, but alsobecause it gives the engineer an idea of how his transmitting system isperforming.
A spectrum analyzer and recorder are required to perform this proofproperly. It is possible to make these measurements without thisequipment, but it is time consuming and difficult to substantiate. Thenecessary equipment is expensive, and for a single station or just afew stations, it becomes difficult to justify the expense. Find acontract engineer service group that will make scheduled calls to dothese tests. In some states, the state broadcasters association hasmade arrangements with one of these groups to offer this service for areasonable fee. The charge is usually only a few hundred dollars. If anumber of stations need to be tested, it might be more economical inthe long run for a station to own the equipment. However, this dependson individual corporate circumstances and philosophy.
An OIB is a vital piece of equipment for a directional AM station andcan be used to quickly diagnose antenna system operatingproblems.
The FM station engineer has even fewer mandatory tests or proofs tomake. In the past annual stereo separation proof requirements wereimposed as well as audio proofs. Today, unless an FM station is underconstruction or a new transmitter is being installed, there are nomandatory FM tests required. However, this lack of FCC requirementsshould not encourage engineers to let systems run for long periods oftime without checking the overall performance. Components change,adjustments change, �knob twiddlers� come along, thequality changes and no one realizes that the overall signal hassuffered.
The combination of operating impedance bridge (OIB) and generatorand receiver detector is probably the most versatile tool in theaverage AM station. These instruments are the best set of equipmentthat an engineer can have when AM antenna problems develop. Anon-directional station antenna operating impedance doesn’t usuallychange, 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 pointoperating impedance seems to be susceptible to change from many causesincluding cracked guy insulators, ATU and phasor capacitors changing invalue, adjustable inductances that adjust themselves and animal lifethat inhabits ATUs.
Like most test equipment, it is possible to rent OIBs and associatedequipment, but after a few rentals it usually becomes cheaper to buy itoutright.
A time-domain reflectometer (TDR) is useful for AM and FM stations,however transmission line problems don’t seem to develop as often in AMstations. Fortunately the backwoods marksman is less attracted by theground supported AM lines than the beckoning lights on the FM tower. Asa 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, andfrequently this will supply sufficient information to locate a damagedportion 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 passingoverhead, from manufacturing faults or simple deterioration throughoutthe years. Digging up a buried line is almost as bad as trying torepair a damaged FM line on a tower. The TDRs used by the powercompanies are invaluable when dealing with a suspected faulty buried AMtransmission line, and faults are usually traced to the nearestfoot.
Throughout the years, the FCC has reduced its mandated RF and othertests. Gone are the after-midnight audio proofs. Does 24-hour operationhave 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 moneymay have rubbed many managers the wrong way. Has management’sunhappiness influenced FCC thinking about unnecessary RF tests thatdecrease a station’s total earning hours? Whatever the cause, routinetests after midnight by engineering should be fewer these days.
E-mail Battison at[email protected].