Taming a wild monitor point
May 1, 2008 12:00 PM, By John Battison, P.E., technical editor, RF
When a monitor point reading is out of tolerance, it's time to take action and determine the cause of the problem.
There comes a time in every AM radio engineer's life when he will be confronted by a wild monitor point. Sometimes it comes when joining a new station, or it may appear unexpectedly during a routine check. No matter how it makes its appearance it is always a big shock in an otherwise good day.
When monitor points suddenly exhibit surprisingly high field strength, one's first inclination is to look at the previous logs and ask the technical staff about the monitoring point's history. Sometimes the logs all show within limits operation. Frequently I have found, when called into a station with this problem, a fairly long succession of logs all showing this one point �out.� Sometimes no one seemed to care. In some cases, the situation may appear to be that of one monitor point being out consistently without any action being taken. On one occasion I was called into a four-tower in-line array with one monitor point that had been reading high for one year.
The first thing to do was check and know that the antenna and transmitting system were operating precisely as licensed and then read the monitoring points preceding and following the one in trouble. These were slightly higher than the original proof readings but still within limits. This was followed by a careful check of the directional antenna operating parameters including common point impedance. These figures all turned out to be fairly correct; the antenna was apparently operating more or less in accordance with its authorized characteristics. It goes without saying that monitor point description, if available, is accurately observed.
We returned to the monitor point causing the trouble and very carefully remeasured it, paying particular care to the orientation of the field intensity meter. It indicated a direction away from the station. Following the indications of the field strength meter we came to a heavy equipment junkyard a few miles away.
The heavy metal junkyard had a number of fairly tall metallic structures and the whole area was reradiating strongly. This dump had developed after the station was built and put on the air. No one knew exactly when the disturbance to the monitor point occurred, but it seems as though someone had just lived with it until the new chief arrived.
We reran the whole radial and all the readings were close to their original values, except of course, the wild monitor point. The cure in this particular case was somewhat drastic. The owner wanted to change his coverage to take population growth into account and it was possible to file a new DA pattern and take care of the unwelcome reradiation problem. This was a somewhat expensive cure and it's not one that is usually recommended or possible.
Generally, when a reradiating structure appears locally and affects DA operation it can be taken care of by detuning the structure and or selecting a new monitor point to replace the one that was compromised. The Commission has made provision with adequate rules to ensure that new tower construction in the immediate vicinity of an AM radio station will not affect the AM operation. All applications for new radio tower construction within specified distances from existing AM station must disclose the situation and receive conditions in their construction permit, which, when complied with, prevent reradiation problems. Unfortunately some applicants for various radiating systems fail to provide the information required by the Commission and as a result the protection provisions are not enforced. Because of this omission, unexpected towers often pop up like mushrooms within critical areas of AM stations.
Examine the area
Whenever an anomaly is discovered in monitor point readings, first examine the area within a mile or so of the antenna site. However, sometimes a weather check is all that is needed. If the temperature is close to freezing or below and the ground is snow-covered don't be surprised to find that all your monitor points are high. Ground conductivity increases in cold, snowy weather. It is important to note the weather conditions on the monitor point report for protection, in case of an FCC inspection at a later date. Some consulting engineers have reportedly had a preference for conducting proofs in cold snowy weather to provide leeway in future monitor point readings.
I recall in one particular case on a higher frequency, the proof of performance for the directional antenna was completed and being prepared for presentation to the FCC for license. The station's chief engineer made a quick run checking the monitor points and found one distinctly higher than in his original proof and out of limits. He checked with the field intensity meter and noted the direction of maximum signal. This did not coincide with the direction to the antenna site.
We checked in the indicated direction and found that a 100' tower for a VHF mobile phone system had been installed within the protected area around the AM antenna. A look at the application for the VHF phone system showed that its constructor had not disclosed the adjacency of the AM antenna and its location within the prohibited distance. Consequently no beforehand measurements had been made on the AM station and its pattern had been contaminated.
Discussions with the VHF mobile operator resulted in an agreement to pay the cost of detuning the 100' tower, and the extra work involved in redoing the proof of performance. In that particular case, right prevailed. Unfortunately not all random tower builders are as easy to convince.
Sometimes a monitor point located on a farm will cause a short-lived problem. I recall in one case the monitor point in question was located in a reasonably clean electrical area but when field strength measurements were made some time later the point was way out. Fortunately, when the original proof was made, detailed photographs were taken of the monitor point and the cause of the anomaly was immediately clear. A metal silo, about 100' from the monitor point, was reradiating like mad.
Accurate monitor point site descriptions, when making proofs of performance, are essential. It is important to remember that 10 or 15 years after the measurements were made local characteristics may change. A measurement that might have been described as �in the center of the garage drive� might now be adjacent to a pole or other metal object with subsequent contamination.
The radio engineer's standard of reference is his field intensity meter and it is essential that this instrument be treated with due care and properly and regularly calibrated. It is not unusual to find stations with field strength meters whose last calibration was four or five years ago, although most meters are surprisingly stable and hold their calibration very well. It is also very good engineering practice to check the calibration of all field intensity meters used in the proof on all scales. All should read the same value. It goes without saying that batteries must be checked and replaced as necessary prior to making a series of readings.
E-mail Battison email@example.com.