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Environmental, ground system problems
As the FCC continues to delete mandatory measurements and checks from the Part 73 rules, some unintentional traps are opened for the unwary engineer. Over the years old-timers have become accustomed to making rather comprehensive log entries of such things as basic current, common point current, monitor point measurements and similar operating parameters. As a result of this, values are checked regularly and therefore operation should comply with the Commission's requirements. Should an inspection occur, the odds are that the transmitting system will be found to be operating in compliance with the rules. In any case, a comprehensive operating history of the station is available to successive engineers.
Use a field strength meter to not only take monitor point readings, but also to track problems due to reradiation.
On the other hand, many newer engineers may read the revised FCC rules and fail to register the fact that a rule says “must be within limits,” or similar words. Sometimes the assumption is made that the system is operating within limits and regular checks are not made because they were not man-dated. The end result is that an inspector arrives to find an operating parameter is out and a pink ticket is issued. Or, after several years of normal operation the engineer finds one or more of his operating parameters out of limits. The unfortunate thing is that no one knows when the change occurred, because no one kept a log or even checked the meters on a regular basis.
Let's assume that the chief engineer decides to do a monitor point check on a DA system. He finds that one point is way out of limits. When he checks old logs, he finds that this particular point has been going out for a long time. This actually occurred on one station where I was called in, and we found the point had been going out slowly for three years, but the original proof value had been entered from time to time.
We examined the area of the questionable monitor point and discovered that a large pile of rusted iron, mainly consisting of iron girders and iron pipes, had accumulated within several hundred feet of the monitor point. A quick trip to the dump area with a field strength meter quickly confirmed my suspicions that the high field strength measured at the monitor point was due to the accumulation of this rather tall pile of iron. It was a relatively easy matter to select a new monitor point and file the necessary paperwork to change location. As a precaution, a skeleton proof was run on this radial and showed that some points were up slightly, but only the monitor point had been out of limits.
Detriments to conductivity
Sometimes environmental changes cause field strength to decrease because conductivity has changed at the location of the specific point, or between the point and the transmitter. This may be caused by urban spread as built-up areas intrude over a radial, or maybe by the construction of interstate highways, or even large commercial and manufacturing construction.
The result of all this construction is that large areas of concrete wall, paving and similar material extend along the radial between the station and the monitoring point. Ground conductivity decreases and signal strength drops.
Cases of reduced monitor point value or lowered radial values that have constricted and decreased critical coverage contours are much harder to deal with. The loss of signal strength has probably been caused by an actual change in conductivity that is beyond the control of the licensee.
If the problem concerns only a single monitor point, the best solution is to pick a new point. If the whole radial is down, there is not much that can be done, short of moving the transmitter. If a radial is badly damaged due to environmental changes in conductivity, it might be possible to augment a DA pattern to increase radiation in that direction to restore the original service. This would be an expensive undertaking and might require a new proof.
Such a situation would be more likely to occur to a long-existing station. The ground system of a station could be contributing to the reduced field strength due to deterioration. Look at the radials, especially those in the direction of reduced signal strength, as well as the copper around the base of the tower.
DA field-strength readings may change during hot or cold months.
Many stations place a large expanded copper screen around the base of the tower where the current is highest. Sometimes traffic, digging and other construction work damages ground systems. Take a field strength meter and walk the ground radials in the reduced field strength direction to see if they are intact and still there. The current at the end of the radials is usually low. However, if there are no indications of signal in the end sections of the radial it could indicate radial breaks closer to the tower base.
Avoiding potential problems
I have often wondered if a variation of the sacrificial anode system, such as those used on boats, might be useful in protecting a ground system. There are different metals, such as iron guy wire anchors and metal ATU Cabinet supports, within the radial system's region. A ground system covers an extensive area and the copper wires may pass through several regions of different chemical composition. Electrochemical currents could flow and erode sections of copper. Such a system is available for automobiles as well as boats, and is reputed to work quite well.
Assume there will be a small ground-system resistance loss when calculating radiated power. Keep in mind that a comparatively small ground system resistance can make a large difference in radiation. When measuring base operating resistance, include any losses in the ground system and also any other losses in the antenna circuit.
If salt or chemicals have been used to improve the ground connection, renewing the salt or chemicals may solve the problem. At the same time the ground electrodes should be examined for deterioration.
If we have a 5Ω ground system resistance and an antenna current of 10A we have 500W of RF dissipated in the ground. If the total measured operating base antenna impedance is only 35Ω the useful 30Ω would radiate only 3kW of RF. Ground system losses often exceed this value, especially in older stations where there has been substantial deterioration due to radio damage and copper erosion.
Temperature can also cause misleading and troublesome results. I have always felt that it is best to make initial proofs of performance in cold weather. In such weather ground conductivity seems to increase. It is not unusual for a DA that was performing properly during hot summer months subsequently to become out of tolerance on one or more points during winter when the ground is covered in snow.
E-mail Battison at email@example.com.
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