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AM Field Intensity Measurements
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AM FIELD INTENSITY MEASUREMENTS, MAPS AND DATA SHEETS 73.186 FCC RULES AND REGULATIONS PLEASE READ THESE INSTRUCTIONS CAREFULLY: It is important that you accurately record the measurement points on the maps and data sheets, and any data pertinent to these measurements, in such a manner that the measurement points can be found again at a later date and your results verified. Use permanent and easily recognized land marks to identify measurement points.
1. Measurement Points
At least seven of the points need to be within the first 3 km. When parts of a radial are unaccessible, take more measurements along those portions of the radial that are accessible so that there are at least 20 to 30 total measurements. If the towers are visible, it is best to start at three kilometers, at a known reference point on the map, and walk back towards the towers. It is easier to stay on the radial if one is headed towards a visible marker like the towers. For distances beyond three kilometers, make measurements at intervals of approximately 0.5 to 1 km. The general rule for measurements beyond three km is to take as many measurements as possible out to a distance of 20 km. In practice this means measurements wherever roads cross the radial. We want at least 15 measurements per radial in the DA mode. A non directional measurement must be made at each location where a directional measurement was made. 2. Point Descriptions Provide a complete description of each measurement point so that the exact spot may be found at a later date by someone else. Use permanent land marks such as addresses, intersections of roads, utility facilities (poles, manhole cover, hydrants, etc.) to describe the location of each point. Use compass directions (north, east, south and west) not left and right descriptions. If you are using a GPS, it can be useful to provide the geographical coordinates but a complete description is required. GPS datum is assumed to be NAD 83 values. If more than one person is making measurements, it is helpful to mark points with surveyor's tape or spray paint. So that the other person can easily find the same point location or if the point is going to be revisited several times during the measurement program. It is not necessary to mark points if they are only going to be measured once or twice by the same person. Most points will be measured at least twice, once in the non-directional mode, and again in each directional mode for the radial. The measurements should be made as close as possible to the identical location each time (same foot print). An error in location of 10 to 20 feet is not acceptable. 3. Data Recording For each day that field strength measurements are made the station engineer should check and record a complete set of readings from the antenna monitor, common point current or antenna current for Non DA, and transmitter power to assure you are operating at the correct power level and antenna mode. All data sheets must have the name of the person taking measurements, the time, the date, the distance from the antenna, the reading, a description of the measurement point, the antenna operating mode (i.e. DA or non-DA), the station call letters, power for the operating mode used, radial being measured, and point number that is the same as that placed on the map for that point. Record the field strength meter make, model, serial number. Blank Data sheets are provided herein and are also available from www.sawyer.com/pdf/amdata.pdf. An Example Data Sheet is provided with instructions for filing out the form in the amdata.pdf file. All field intensities should be recorded on the data sheets in units of millivolts per meter. For full scale values shown on the meter in "volts per meter", multiply by 1000. For readings taken with the meter range switch at "100 microvolts per meter" full scale, divide the reading by 1000. All times listed on the form (time of measurement) are to be recorded on the data sheet using the standard 24-hour clock, i.e. 9:00 AM is 0900, 1:00 PM is 1300 and 2:12 PM is 1412, etc. All times are recorded as the local time. Keep the maps clean and smudge free; do not fold them if you receive them unfolded. It may be useful to transfer radials to current street maps. Use extreme caution when doing this. Street maps are not always accurate. Calibrate maps with landmarks from USGS maps. The USGS maps are the best. Write on the maps in pencil only (use a soft lead pencil, that allows for easy erasure, not a hard lead or colored pencil), placing an "X" at the exact point where the measurement was taken. Number the points on the map with a pencil corresponding to the point number on the data sheets. 4. Measurement Technique Do NOT take readings under power lines or near wire fences. Readings should be taken on the side of the power lines closest to the transmitter site. If this means walking out into a field (75 - 100 feet) or other clear area, do so. Measure between two hours after sunrise and two hours before sunset. If more than one meter is used, periodically compare the measurements made with each meter to assure the calibration. There is usually a pattern null on those radials where a Monitor Point is specified in the Construction Permit. When taking measurements on a radial where there is a null, the meter MUST be pointed in the direction of the towers even when higher fields are measured in some other direction. If you are unable to see the towers, use the radial line on the map to orient the meter to the correct position. The meter lid should be aligned (in-line) with the radial at all times. Reading should start at a distance from the tower, or center of the array, of about five times the tower height. For close-in measurements (under three kilometers) reading should be taken at about 0.2 kilometer (km) intervals. The meter should be calibrated with each measurement (see instructions inside of meter lid). When making measurements in high fields close to the towers, it is very difficult to calibrate the Potomac FIM series meters properly. Since the accuracy of the meter can be decreased by attempts to calibrate at the stations operating frequency, one of two procedures should be followed: (1) do not attempt to calibrate the meter in high fields; (2) detune the meter 10 kHz before calibrating (see the instructions inside of meter lid). Never enter "posted" property without contacting the owner. Always carry proper personal identification. If asked by a member of the public as to what you are doing, keep your explanation simple and courteous. A typical response might be; "I'm checking the signal level from radio station WXYZ to determine if their antenna system is operating properly." If you have any questions, it is always better to ask then guess. We may be reached at (301) 913-9287 or via e-mail at info@sawyer.com If interested in our Consulting Services, please see our contact page. How to Contact Us Page |
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