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Method of Moments Directional Array Proofing
MoM proofing is a newly approved method of field
strength proofing a directional array. The old rules required
that an array be adjusted to a set of assumed perimeters and the a
set of external field strength measurements be made to verify
its tuning. For the most part, a set of measurements taken
within a relatively short distance (15-20km) of the array was being
used to determine the far field (interference potential) signal
strength to distant stations.
For a number of years, consulting engineers have been
using MoM techniques in the initial setup of arrays with VERY
impressive results. However, when the measurement phase of the
project began it was sometimes found that the measured results
didn't coincide with the modeled results. Upon considerable
study, it was determined that the measured near field signal
strengths were being influenced by external factors such as "nearby
conductors, reradiating objects, diffraction, and terrain anomalies"
without substantial change to the interference potential in the far
field. To make matters worse, the electrical environment of
those "nearby conductors, reradiating objects, diffraction, and
terrain anomalies" was an ever-changing kaleidoscope of
construction, development and weather conditions.
MoM Proofing uses a computer model based on
internal measurements to adjust and predict an arrays
operational characteristics instead of making hundreds (or possibly
thousands) of field strength measurements over and over (and over
and...).
A few (only 3-8) field strength reference
measurements are still required to verify that the array is
operating as predicted.
MoM proofing is not cheap but is MUCH less expensive
than a full proof. This is especially true on a
troublesome array. Time wise, a MoM proof requires only a
small fraction of the time required to run a full proof even on a
small array.
Monitor Point
ReRadiation Cure
MoM proofing can also be a very quick remedy for near
field reradiation that is disturbing existing monitor points.
Because there aren't any monitor points per se. There will be
a few pseudo monitor or reference points for array verification but
they will not have to be regularly measured. These "reference"
points will be measured when the final tuning is complete and may be
used for troubleshooting and/or verification.
The MoM model either accounts for the reradiation
source in the model (if it is modelable) OR simply ignores it since
near field reradiation typically is not a factor in the far field
(causing interference to other stations). Pursuant to the MoM
modeling rules the Commission is examining requests to standardize
the rules regarding construction of outside reradiation sources
(i.e., cell towers, etc.).
Restrictions
MoM modeling will currently not be allowed for arrays
using:
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Top Loading
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Sectionalized Elements
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Folded Unipole Elements
MoM modeling also requires that ALL base samples be
obtained using calibrated sample toroids and EXACTLY
identical electrical length & loss sample lines. This of
course precludes old arrays with unequal sample lines or sample
lines that can not be verified to be the exact same electrical
length and/or loss. This situation can be easily rectified
however.
More on this later...
MoM Preparation
Method of Moments Proofing entails very precise
measurement of array physical properties (tower size, height,
locations and other physical data), precise measurement of tower
impedances with other array towers in various base configurations of
open and shorted. All of this data matrix is used to build a
computer model that exactly mimics the measured data.
Additionally, the Sample System (antenna monitor,
sample transformers and sample lines) must be checked for exactly
identical output specifications. This includes: transformer
output phase and amplitude, sample line length and loss and antenna
monitor channel calibration.
Many older arrays will require that the sample system
be replaced to facilitate the precise measurement of the internal
perimeters.
THIS IS WHERE AM GROUND SYSTEMS COMPANY CAN HELP
Working with your consulting engineer, we can help
design and implement a new sample system that satisfies the exacting
requirements for an MoM proof.
Whether building a new turnkey transmitter site that
will be proofed using the new MoM technique or retrofitting an old
array with a new sample system, we can provide all physical services
needed to implement the new technique. This of course in
addition to our already comprehensive site construction
capabilities.
Great care must be taken during the cable
installation to ensure that the phase relationships of the new
cables is not altered. New sample transformers must be located
at similar locations using similar cable routing and often must be
mounted using custom built brackets. RF routing and equipment
between the sample transformer and the tower base must often be
modified to be similar on each tower. Finally, all of the existing
feed and control cabling MUST be accurately located and protected
and any damage to the existing ground system (if retrofitting an old
array) must be repaired.
AM Ground Systems Company can provide all of
these services with minimum disturbance and hassle to you and your
stations operation.
Contact us via emai now or call 1-877-766-2999 if you are
contemplating a MoM proof.
We are familiar with and understand the science,
software and math involved but currently are not equipped or
qualified to perform a complete MoM proof ourselves. We can,
however, assist your consulting engineer with all measurements if
additional on-site labor is required.
Please see our list of
suggested consulting engineers for a qualified consulting
engineer.
References:
FCC report and order
containing background and comments on the MoM procedure.
FCC rule (73.155)
allowing MoM Proofs (courtesy of the fine FCC rules Site
www.hallikainen.com).
Articles etc.
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