Method of Moments Modeling
If using base sampling, you are required to perform base impedance measurements on each tower with the other tower(s) open or short-circuited. The modeled tower impedance must agree with the measured impedance ±2Ω and ±4 percent of resistance and reactance.
Impedance measurements must also be performed on sample lines with the sample device connected. 47CFR73.155 (a) describes the requirements for both base current and sampling loop methods.
Typically MoM modeling of an antenna is represented by one or more vertical wires, which represent the actual radiator. The FCC permits a variation of the modeled tower to be 75 percent to 125 percent of the physical tower height.
Adjust the antenna monitor to agree with the data derived from the measurements performed for the MoM matrix. 47CFR73.151(c) (2) states "The computer model, once verified by comparison with the measured base impedance matrix data, shall be used to determine the appropriate antenna monitor parameters."
The antenna monitor must be calibrated per the manufacturers test procedure and a certificate or statement of calibration must be submitted with the application.
The appropriate fees must be submitted with the application for approval to use the MoM proof of performance going forward.
You will still need to make some measurements
Even after you have been approved to use the MoM proof of performance, there is still a requirement to establish and document a minimal amount of monitor points along radials that define the minimum and maximum areas of the pattern. 47CFR73.151 (3) states "Reference field strength measurement locations shall be established in directions of pattern minima and maxima. On each radial corresponding to a pattern minimum or maximum, there shall be at least three measurement locations. The field strength shall be measured at each reference location at the time of the proof of performance. The license application shall include the measured field strength values at each reference point, along with a description of each measurement location, including GPS coordinates and datum reference."
You will also need to recertify the antenna monitoring system every two years in order to be in compliance with the current rules.
The question of whether the expense of converting to the MoM proof is worth it depends on many factors, including the age of your system. Typically this would make the most economic sense for newly constructed systems or older systems that are updating RF infrastructure; however existing operations with limited technical resources may also find the savings in time alone worth the initial effort.
McNamara is president of McNamara Associates, Cape Coral, FL.
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