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 TM 5-684/NAVFAC MO-200/AFJMAN 32-1082
pole should be examined in detail. A slight delay
those used in more modern brackets, and the use of
a copper jumper with appropriate connectors for the
between the blow and the change in receiver noise
indicates that the problem is in adjacent structures.
connection to the pole ground.
(c) All connections to the neutral or to the
(1) Locating problem poles is more involved
pole ground should be made with jumper wires and
when standing waves are present. A series of occur-
connectors. This also applies to static wires used on
ring maximums and minimums in signal strength
transmission circuits. A pole ground wire placed un-
(as the truck is driven along the line) indicates
der galvanized hardware is unsatisfactory as a
standing waves. Maximum peaks will occur at in-
ground and should not be used.
tervals of 400 to 500 feet (120 to 150 meters) for 800
kilohertz and at closer intervals for higher fre-
16-6. Electrical interference during bad
quencies.
weather conditions.
(2) When comparing the strength of signal
Leaky insulators generate greater EM1 when it
peaks, the operator should consider the distance
rains as the assembly is grounded by water on and
between the receive antenna and the electric line.
in the pole. Rain can cause other leakage noises to
Signal increases due to overhead guy wires, ser-
disappear, since the resistance of the water over the
vices, and taps should be disregarded. Comparison
pole surface may be low enough to drain off electro-
of signal peaks can be made more easily if the noise
static charges before discharge gaps can develop. In
is tuned to the FM frequencies on the broadcast
most cases, wind increases the interference level,
band as the signal strength increases. Since FM
but in a very irregular pattern. Above 50 megahertz
broadcast signals die out faster than AM broadcast
(television frequency) noise emitted is usually very
signals along the electric line, this technique nar-
small during fair weather conditions, but will in-
rows the search area considerably. When the "hot
crease in bad weather.
zone" is located, taps should be momentarily discon-
nected while the operator listens for changes in the
16-7. Methods of electrical interference loca-
signal. If the noise stops, the tap should be recon-
tion.
nected and search should be continued from the tap.
A decrease in the noise upon disconnect may mean
Although there are many very good instruments
that the transmission characteristics of the line
available, no device has yet been invented which
have been affected by the removal of the tap,
will unerringly locate and identify the source of
thereby altering the level of the signal. The noise,
EMI. The most successful method appears to be
however, is not originating from the tap. After tap
narrowing the search area and isolating suspected
isolation has been effected, and the noise continues,
apparatus. Success in locating EMI depends on the
each pole in the noise zone should be struck with a
acquired skill of the operator to interpret instru-
hammer and subjected to visual inspection. A ham-
ment readings and to recognize possible sources of
mer blow will not affect staple noise and some other
noise by characteristic sounds.
types of interference. It will, however, show up poor
a. Sound characteristics. The intermittent noise
connections.
from loose connections is most apparent on windy
days, as is noise caused by tree contact with pri-
16-8. Instrument requirements for checking
mary conductors. Electric fences have a regular
electrical interference.
popping sound, whose frequency is governed by the
timing apparatus built into the electrified fence con-
A receiver suitable for power systems should have
troller. Insulators and bushings generally produce a
sufficient sensitivity to detect a signal that would
heavy, rasping buzz, while staple and hardware
interfere with an automobile radio or a sensitive
noise is higher pitched.
home receiver. It should be selective enough to iso-
late the noise in the broadcast band without inter-
ference from broadcast signals. It should be capable
cator) and/or meter that measure field-strength in
of tuning over the broadcast band in order to iden-
the audio and radio frequency range. Some interfer-
tify signals which are causing radio disturbances in
ence is easily found by driving along the road with
the area. Another necessary instrument feature is
the receiver volume turned up and the receiver
the inclusion of short-wave reception up to 20 or 30
tuned to a position near 800 kilohertz but free of
megahertz, for noise tracing at the higher fre-
any local broadcast stations. The offending pole is
quencies as previously described. The receiver
easily picked out and can be examined for sources of
should incorporate a meter in the audio circuit to
interference (unless standing waves are present). A
give the relative levels of noise signals. If the instru-
useful implement for the detection of loose connec-
ment is to be used in a truck for patrolling the line,
tions is a heavy hammer for striking a suspected
it should be shielded so as not to pick up vehicle-
pole. If the receiver noise changes with the blow, the
16-3
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