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Close-In AM Field Intensity Measurements

In the March 2 issue, author Buc Fitch discussed field intensity measurement methodology. Here, Cris Alexander expands on Fitch's discussion of ND surveys.

Right Tool for the JobIt was December of 1995, about 15 degrees outside at the transmitter site just north of Denver International Airport.

An engineer we’ll call Burt and a helper were doing the close-in, non-directional field strength measurements on a new 50 kW four-tower directional array. I was at the base of the tower used for ND test operation doing something or another when Burt and his helper came walking up at the end of their radial measurements. I noticed there was steam coming off Burt’s head, and I asked him why.

Burt said he and his helper had come to the edge of the Denver-Hudson Canal about half a mile from the site. This canal is 40 feet wide and eight feet deep; it runs with 100 cubic feet per second of irrigation/drinking water, fed from a reservoir a mile or so away.

Burt said they’d anchored the rope on the far bank with a stick, tied a rock to the other end of the rope and tossed it across. The idea was that when they got to the other side and pulled the rope taut, the distance would be right. They could then pull hard on the rope and pull the anchor stick out of the ground and keep going.

Burt and helper walked along the canal for half a mile or so to the county road, where there was a bridge. There they crossed the canal and walked down the near bank to the rope.

Or where it should have been, anyway. They could still see the far end of the rope secured to the stick poked into the far bank; but the rope itself trailed into the water. The current obviously had been too strong for the rock.

Burt, mad as a wet hen, refused to walk a mile or more back over the bridge to the other side just to retrieve the rope. So he stripped down to his birthday suit then and there, dived in and retrieved the rope! He got dressed and (shivering and steaming), finished the radial.

The moral: Use a bigger rock.

— Cris AlexanderIn the March 2 issue, author Buc Fitch discussed field intensity measurement methodology. Here, Cris Alexander expands on Fitch’s discussion of ND surveys.

Traditionally (and as specified by the FCC), a full proof begins with a set of non-directional or ND measurements starting at five times the height of the ND element. Measurements are generally required at approximately equal sets of intervals so that 15 to 20 measurements are established within the first three kilometers. Often, these close-in measurements must be made while walking the entire distance through brush, dense woods and other inhospitable terrain.

Close-in ND measurements are used to establish the ND inverse-distance field, or IDF, of an antenna on each radial. This is sometimes expressed as the antenna’s efficiency. What it comes down to is a multiplier by which the DA-to-ND ratio on each radial is multiplied to determine the directional IDF on the radial.

Such measurements are valuable because they provide a great deal of information that is not colored by ground conductivity. The more points you have, the better the analysis will be and the more confidence you will have in the ND IDF number on each radial.

Ample data

Over the years, I have learned that it is useful to make a lot of measurements inside 3 km. When submitting ND IDF data to the FCC, where I have provided a large number of points inside 3 km, there has never been any question about the analysis. With a lesser number of points, the analysis may be subject to a different interpretation.

A good set of close-in measurements will start at exactly 3 km and work in toward the tower. A GPS receiver, topo map or combination of the two can be used to establish the 3 km starting point on the radial. A two-man team is needed to make each set of close-in measurements.

A 50-meter rope is used to measure distance. Use a permanent marker to mark off 10-meter increments on the rope. To start the radial, the end of the rope should be located as closely as possible to the 3 km point. A measurement is made at that point; then the rope is used to measure off 0.1 km (two rope lengths) to the next measurement point at 2.90 km.

The two men on the team leapfrog, taking turns holding one end of the rope while the other walks the loose end toward the station. (It is important to always pull the rope and not try and push it; pushing the rope does not work well!)

If the towers are not visible, the FIM is used to direction-find (DF) the antenna, keeping the measurements on the radial.

One measurement is taken every 100 meters from 3 km in to 0.5 km. At that point, a measurement is taken every 50 meters (one rope length) from 0.5 km in to 0.2 km. At that point, one measurement is taken at each of the 10-meter marks made on the rope until the FIM reads full scale.


Once the last point has been measured, the exact number of rope lengths and fifths to the ND element is noted. If this doesn’t jibe with where you thought you were at the last point, the difference is noted. This correction factor is then applied to all the measurements out to the end of the radial.

For example, if your last measurement should have been at 0.04 km but you measured one full rope length to the tower, your last measurement point was actually at 0.05 km. The difference, 0.01 km, is then added to all the measurements to get the corrected distance.

One may ask why, in the age of GPS, we need a rope at all. The answer, simply, is accuracy.

A rope measurement will far exceed the resolution and accuracy of all but the most sophisticated handheld GPS units. This is particularly important at very close distances, where an error of just a few meters will move the points above or below the inverse distance line. Errors in this close-in data will have a significant effect on the value of the measurements.

If the ND radiator is other than 90 degrees tall, it will be necessary to correct for proximity factor once inside a radius of 3 times the tower height. At that point, the tower ceases to behave as a point source. Radiation from different points on the antenna no longer adds up in phase. A correction factor is applied which normalizes the data. Simple computer programs are available for calculating proximity correction factors.


Once all the close-in data is in hand, the data is plotted on a piece of log-log field strength graph paper and the inverse-distance line is adjusted up and down for the best fit to the data. With so many close-in measurements, you will find that the points usually line up right down the inverse distance line, leaving no doubt about the IDF of the antenna on that particular radial. If a standing wave is evident in the measurements, that is evidence of reradiation, either from an unused but improperly detuned array element or some other object.

Close-in measurements are a real pain, sometimes quite literally. Their value in evaluating the performance of an antenna or directional array is tremendous. Not only will they eliminate any question of the ND IDF on any given radial but they will also provide a point of reference for future measurements, should the condition of the ground system ever come into question.