Use Lasers for Close-In Points

The Author Says Laser Range Finders Enable More Accurate Measurements for AM Proof Reports
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The Author Says Laser Range Finders Enable More Accurate Measurements for AM Proof Reports

While reading "Close-In AM Field Intensity Measurements" (March 16) and the "Right Tool for the Job" side bar, I noted the discussion of measuring distances accurately by way of the knotted rope.

Please consider the following as a follow-up to that subject.

If there is one thing the FCC wants in AM proof reports, it is accurate data. Accurate data also is in the engineer's best interest - for future uses, and to avoid causing problems with the FCC.

Swamp thing

Many years ago, I was involved in making measurements in the Everglades in Florida. The problem was there was no easy or legal access to the swamp and no useful landmarks, especially so for the close-in points. The station was a multi-tower array with a central transmitter building, surrounded by miles of swamp.

We had to turn to helicopter measurements. And to this day, the FCC refers to that proof report as an example of acceptable helicopter measurements.

Such measurements are acceptable by the FCC if you can prove the relationship of the aerial measurements to the values that would be measured on the ground; and prove you knew where you were for each measurement. Both tests were satisfied and the latter will be addressed below.

The swamp area and the modestly tall transmitter building yielded an area and vantage point from which to direct the measurements. A theodolite and laser range finder were set up on the roof of the building. This was just at the dawn of laser distance measuring procedures, but a procedure and equipment were worked out.

The theodolite was set up on the building flat roof and sighted on the few easily identified landmarks, like water towers, and the true north azimuth was thus identified. Then each radial azimuth could be dialed in. The range finder was set next to it. The non-directional tower and the array center were a short distance away and close-in corrections for distance were easily made by simple trigonometry.

The helicopter was equipped with the measuring meter in a window mount and retro-reflectors were attached to it. The ultimate ranging distance was in the range of four to six miles, depending on heat mirage conditions.

The small and underpowered helicopter could maintain a slow forward speed near ground level - in the area of ground effect - and was surprisingly stable in that mode of flight. Further out, higher speeds and altitudes worked better.

Each radial was run from the furthest point moving inward. When it was seen in the theodolite scope and good ranging data was obtained, the measurement process started.

The helicopter was advised to fly right or left to keep it on line, and at relatively uniform time (distance) points a "mark" command was given by radio. That was the cue to read the meter, relay the reading back by radio and simultaneously read the range finder and write down the data. The process was repeated at short intervals with the helicopter speed adjusted to make the distance between points less for closer points.

As the array was approached the pilot had to maneuver so as to not hit towers while redirecting his path from the observation point toward the array center or non-directional tower.

The data was analyzed and the following processes were applied. A trigonometric correction was applied to the distance measurements to reflect distance from the non-directional antenna or array center accurately. The measured distance was truncated to the nearest foot and then converted to miles to tow decimal points. Normal plotting and analysis then followed.

For much longer radials, 10 miles or more, a similar process was used except a landmark visual navigation process was substituted.

Easily identifiable landmarks, such as canals, hammocks and occasional roads, were marked on topographic maps near each radial. This yielded known distance and guidance points for each radial path as the flights commenced. A recorder was coupled to the field meter and an event marker was actuated as each landmark was passed. Such paths were flown at higher elevation and speed, and an entire radial could be measured in about 30 minutes flight time.

You should see the beautiful Field Strength nulls that occurred when the aircraft passed over the top of the non-directional tower.

Seen and lased

So, what is this all about? I strongly suggest using a laser range finder for measurements at close-in points. They are now inexpensive, small and easy to use. Accuracy is not a problem at all - they are accurate to +/- a meter or two and repeatable.

If the non-directional tower can be seen it can be lased. A monopod on the meter makes it easier to hold to make measurements and at the same time makes for a stable point on which to rest the laser range finder.

If necessary a temporary target can be attached to the tower. I suggest about five feet of Scotch reflective tape at an easily seen location will give good signal reflection.

If practical, the observation point process we used could be applied with the helper, data writer and laser operator situated at the station and the reflecting surface attached to the meter case. Where intervening terrain, buildings or foliage occurs, intermediate reflective markers can be established. But when such conditions begin to become common, the area will no longer be "close in."

Today, the right tool most likely does not include knotted ropes or surveying chains.

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