Triplett Mitigator Loop Coil – 3232 - Manual (Page 16)

In the MITIGATOR Applications Manual, we noted that in order for noise to appear on a phone line, there
had to be Influence, Exposure, and Susceptibility. The magnetic field is the Influence. The Exposure
refers to how much of the telephone line runs parallel to a power line. And Susceptibility describes the
ability of the cable sheath and pair balance to suppress or reject noise. The magnetic field measured by
the Loop Coil only provides the Influence. Exposure and Susceptibility must also be present for noise to
occur on a phone line.
Consequently, the craftsperson should not assume that a strong magnetic field is the cause of noise on
a pair. While this usually is the case, don’t overlook the possibility that noise may be induced onto a pair
in a location where the magnetic field is much less intense, but the Exposure or Susceptibility is much
greater.
SECTION 11: INTERPRETING LOOP COIL MEASUREMENTS
A strict interpretation of specific frequency harmonics at particular levels is not possible when using a
Loop Coil as the measuring device. This is because the absolute level of the measured frequencies are
greatly affected by the positioning of the Loop Coil in relationship to the power lines. Not only is the
distance between the two a factor, but the angular position of the coil affects the level. Also, magnetic
field distortions caused by metallic objects in the vicinity distorts the strength of the field.
Fortunately, none of these things affect the accuracy of the frequencies. Consequently, we can note the
presence of the frequencies themselves, without giving undue significance to the levels of the frequen-
cies. After the craftsperson has some experience using the Loop Coil, he will be able to determine some
general guidelines for the expected levels based upon his test technique and the prevailing method of
power line construction in his area.
For the purpose of our discussion, we will divide the levels into three ranges: Low, Medium, and High.
THESE LEVELS AND THE FOLLOWING DISCUSSION ARE BASED ON READINGS TAKEN WITH A C-
MESSAGE [PMSG] FILTER.
Just to give the craftsperson some idea of the expected levels, although these may not correlate with
the levels in his area, the following general guideline is given:
Low: less than 30dBrnC [P]
Medium: 30 to 40dBrnC [P]
High: greater than 40dBrnC [P]
“Low” signals generally do not induce any audible noises onto a telephone line. If noise is heard on the
telephone line when the Loop Coil signal level is Low, there may be a problem with bonding and ground-
ing of the cable sheath.
“Medium” signals may produce noise on a telephone line if long Exposures (several miles or more) exist.
“High” signal levels will produce noise on a telephone line, even when the exposure is short and the
telephone cable shield is in good condition.
11.1: SPECTRAL SIGNATURES
In general, the spectral signatures of the Loop Coil signal are similar to those identified in the Contel
Chart (see MITIGATOR Applications Manual). Since the Contel Chart was developed for use with metal-
lic measurements on a “pair”, and not a Loop Coil, we must ignore the graph part of the Chart and look
only at the Matrix. A few typical signatures will be examined.
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