MFJ-852 Instruction Manual
AC Line Noise Meter
3
source. Length may be shortened to reduce sensitivity for extremely strong
RFI sources.
2. Headphone Jack:
Connect standard stereo or monaural type headphones
using a 3.5 mm plug. Also, the headphone jack may be used to feed a
portable cassette recorder (line-level input)--or even a portable oscilloscope
for complex waveform analysis.
3. Power Switch:
Press in to apply power to unit.
4. Power/Battery LED:
Illuminates when power is applied, extinguishes when
battery voltage drops to a marginal level.
5. Meter:
Semi-log metering system displays wide signal range with more
resolution at the low end of the scale.
HUNTING DOWN NOISE ON THE POWER LINES
Most utility-line noises comes from arcing or corona. Arcing (or sparking)
happens when 60 Hz AC swings to its extreme plus or minus value. This can
induce voltage in associated wiring and hardware that arcs across an imperfect
insulating medium. The resulting RFI often appears as a 120 Hz AM modulated
buzz or hum. Corona happens when a discharge occurs in ionized air rather than
through a discharge path to ground. Corona may produce a more constant noise,
frying, or even a high-pitched whine.
120 Hz Buzz
Off-Air RFI
60-Hz
Energy on
AC line
+
-
+
Arc
Arc
-
The distance powerline RFI propagates usually drops rapidly as monitoring
frequency increases. A noise that's received for a mile or more in the AM
broadcast band may only be detectable for a hundred feet or so at 130 MHz.
That's why utility companies use handheld VHF-AM receivers instead of HF
radios to zero in on line noise problems.
Distance from Fault
10 MHz
130 MHz
RFI
Strength
1 MHz
