There comes a time in every engineer's life when the station manager
or program director announces there is a signal problem in a particular
area. Questions about the exact location and date of occurrence are
usually answered with a vague “I don't know, it's been a
longtime.” The engineer's response depends on whether it's an AM
or an FM station and whether it's an audio problem such as distortion
or low signal level, or perhaps unusual interference.
If it is an AM station, my first reaction, after verifying that the
station was operating at full power, would be to grab a field strength
meter and the complainer, go to the location in question and listen to
the signal on my car radio.
If the level appears to be down, a quick check with the field
strength meter should verify the condition. If the field strength is in
fact down, roughly define the edges of the diminished coverage, and
make a check of the related monitor points if the station is
Have your coworker show you the area that suffers from the signal
Some engineers may question the suggestion to check monitor points
if only a small section of the general service area is affected. My
feeling is that I want to be sure that the transmitter is operating
properly and power radiation is normal. If it's a non-directional
operation there will be fewer checks to make because the signal
strength is theoretically more or less the same in all directions. A
quick check with a field strength meter or even a car radio will give
an idea of the extent of the problem — if there is one.
If audio distortion is observed and the poor quality area is in a
directional antenna (DA) pattern null, (which should not occur because
a null, or low signal azimuth is not normally in a desired listener
area) pay attention to the DA adjustment. This is not likely because a
major change in antenna tuning would be required to produce what would
be a serious antenna problem. However, this is something that should
not be dismissed without consideration.
If the antenna operation is found to be correct and poor audio
persists, the audio chain should be examined. It doesn't happen often
these days, but changed component values in the feedback loop should
not be overlooked, and the usual audio distortion correction methods
should be followed. Some of the older, high-efficiency tube-type
transmitters that use third-harmonic tuning to return third harmonic
power to the system can introduce distortion when out of
Reradiation from other towers is a common problem for AM antenna
A technique used to pinpoint low signal areas is the diagonal
radial. This is more commonly used when a directional antenna system
produces unanticipated low signal areas during a proof of performance.
The diagonal radial is used to pinpoint unanticipated nulls in a new
antenna system. It is not commonly used today because computer
programming has greatly refined directional antenna pattern design so
that unanticipated nulls show up in the design phase long before
construction, and therefore should never occur. This radial is laid out
so that it crosses the DA radials more or less at right angles. Any
pulling in of the signal along this radial could point to an unexpected
null. However, this is not likely to occur in an established antenna
system absent undesired major antenna system changes.
Pay attention to any new structures between the AM transmitter and
the area of interest. Any new towers in the vicinity of the AM antenna
system should be investigated thoroughly.
AM interference takes several forms ranging from power line
transformer problems to commutator buzz, and even fish tank
thermostats. A good directional radio or a field-strength meter will
usually find the source of interference quickly. Unfortunately, curing
this interference is not always as easy, especially if it's a pole
transformer. Interference from these beasts often spreads a long way
from the source, and is hard to track down.
If the problem occurs with an FM station, spot field strength
measurements usually are not helpful, mostly because of the receiver
design. Typically, there is not much noticeable level change as an FM
receiver is moved around, provided that the antenna is oriented
correctly for each move.
Unlike AM, FM transmission uses line-of-sight conditions. In cases
of diminished reception, see if there are any new towers or buildings
close to the antenna on this azimuth, or if there is a new metallic
construction in the vicinity of the affected area. If this question can
be answered yes, the problem may be local shadowing.
If the complaint is distortion and poor quality, determine if the
distortion changed on leaving the area, in case it was a local
phenomenon. If picket fencing occurs in a previously clean area, the
problem becomes hard to solve, especially if it is caused by new
construction. There is always the possibility that the new construction
is reflecting a previously unknown side lobe. It's also possible that
this problem could be fixed by changing or redesigning the antenna
after the possible side lobe has been identified. Unfortunately, this
would probably be expensive and time-consuming.
When designing an FM antenna installation, make sure to obtain the
coverage to the radio horizon, and also make sure there is adequate
coverage of listener areas between the transmitter and the radio
horizon. This point can be overlooked, and will result in desirable
listener areas close to the transmitter receiving low signal strength
because the main beam passed overhead.
In AM operation, the vertical width of the signal rarely causes a
problem because the ground wave originates on the ground and travels
horizontally across the earth, normally hugging it. On the other hand,
the FM signal originates several hundred feet above ground. It is
usually in the form of a concentrated beam that increases in vertical
depth as it travels over the immediate and near vicinity of the
radiator. Thus it could pass over the desired listening area. This
problem is normally taken care of in antenna selection by specifying
the necessary null fill or beam tilt that will serve that area.
E-mail Battison at email@example.com.