Directional antenna maintenance is an ongoing process based on
experience and adequate records. With the FCC's reduced logging
requirements, the paperwork involved is considerably less than in years
past. Therefore, there is no excuse for a directional array not to have
adequately written and readily available maintenance logs. Not only
does a well-maintained log book provide a guide to daily operation, but
it can give a great deal of diagnostic help when a crisis occurs
— and even with the best of maintenance crises do occur. A
quick glance at operating parameter records will often show that a
situation has developed.
Physical changes, like this shifted
tower base pier, can easily influence a directional array. Photo by
A properly recorded, correctly taken and regularly examined monitor
point log is the best maintenance tool. The Commission no longer
requires routine spaced monitor-point readings, so it behooves the
conscientious engineer to have these readings taken regularly. Even a
small change will be noticeable when compared to previous measurements,
and will lead 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 lead to 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 out-of-limit readings are observed, and before
assuming that the array has returned to operation in accordance with
Even though regular inspections are a thing of the past, there is a
lot to be said for the old-timer's hand check of capacitor and inductor
temperatures at sign-off, or even pattern change. A hot capacitor, a
high temperature or a discolored inductor is one of the easiest checks
to make to correct undesired and excessive RF current.
Look for warm spots in the transmission line. It's not unusual for a
line to be 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 tolerant of
standing waves, but VSWRs high enough to cause heating are an
indication of an improperly adjusted antenna system. This means getting
out the operating-impedance bridge and checking the phasor and ATU
lines to find the mismatch. Check actual base operating currents as
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
Tuning houses must be clear of debris
inside and out. The components inside must also be corrosion free.
Photo by Tracey Liston.
If the monitor points are in, the RF currents are normal and the
antenna monitor is out, check the sampling lines. They should be buried
and any excess also buried. There should be a record of the original
sampling line impedances and dc resistances. Checking the immediate
operation against the original values will offer a good idea of their
condition. Trucks may 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 can damage lines in exceptional weather
conditions and so can 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 RF arcs, therefore, they should be examined
for obvious damage and electrical performance. Ensure that the
lightning-protection, single-turn ring in the RF connection to the ATUs
is restored after work on tower bases. If this is omitted the next
storm may take the 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
loops so that misleading voltages are picked up. Sometimes, a gale may
move a 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 should be used. This can
take the form of a horizontal, a vertical, a quarterwave isolating stub
or an isocoupler to carry the line across the base insulator.
Isocouplers can develop faults and they should not be ignored.
In the past, quarterwave sections have been finely tuned by an air
capacitor across the section. This is an acceptable method of
adjustment. However, many newer engineers have not come across this
little gimmick. Sometimes a new engineer will remove or readjust these
types of useful gimmicks, and cause themselves considerable work in
readjusting the decoupling section.
Finally don't forget the humble field mouse. Doghouses and even
metal ATU cabinets attract these little animals in the winter, because
there is protection from other animals and warmth from the RF energy.
Sometimes an engineer will find roasted mice that have short-circuited
coil turns. Regularly clean out the ATU cabinets or doghouses, and be
sure to look for animal nests in RF areas. Snakes also like to come in
out of the cold. The best way to keep wildlife out is to seal and close
every conceivable entry point.
E-mail Battison at firstname.lastname@example.org.