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,
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
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
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
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
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.