[RFI] Line Noise - Ideas needed
Radio_Q at mchsi.com
Thu Nov 30 19:43:05 EST 2006
I have a rather unusual line noise problem that I need some help with. I
live in Springfield MO and Mike Martin was here helping CU to learn how to
find these things but I thought I might do a little leg work before I called
them. But, this is a very unusual problem. Here are some observations.
I have an intermittent, irregular line noise problem that has been going on
for several weeks starting before Thanksgiving so I am sure it is not
outdoor holiday lights etc. It is a sharp snapping noise and on my scope
which is synchronized with the AC line, it occurs exactly as the power line
reaches it's positive going peak. It never occurs on the negative going
peak which makes me think there is some kind of diode action. I call it
irregular as it rarely occurs on every cycle but perhaps every 5th to 10th
cycle of the waveform. Sometimes it only snaps 3 or 5 times a second or
even less. I call it intermittent as it will be on in the most evenings,
some in the daytime although daytimes seem to be the lower rep rate of
discharges. Sometimes it is on with a 'vengeance' and other times is of low
enough repetition rate that it is tolerable. It is quite unpredictable.
The good news is the noise blanker on my radio can almost completely
eliminate it. I do mostly CW so the effect is just a bit more noise in the
passband. Again, since it does not arc on every cycle of the 60 cycle line,
the characteristic Buzz is missing but the snap of an arc is quite
Now here are some of the findings that baffle me. I have a 135 ft, off
center fed, dipole which covers 80, 40 and even harmonics upward in the
bands. It is about 40+ feet above the ground. On 40M it is a full wavelength
and of interest, the noise is only heard on the 40 meter band. I can no
longer hear the snapping when I go below 5.7 MHz, it peaks around S9 at 7.2
MHz and is gone again around 9.2 MHz.
I took a small portable HF (AM only) receiver outside and can hear the noise
out there using a short vertical whip.. The higher I hold the radio in the
air, the louder it gets. It gets very strong when I get near the vertical
mast holding up the Windom antenna. But it is even stronger when I go out
in front of the house (100+ ft away) and hold it near one of the aluminum
street light poles. Both of the light poles in my neighborhood act about the
same. I can hear the signal near the power distribution transformer situated
in the front yard, although the signal gets weak rapidly as I leave the area
of the transformer. I can also hear the signal when I walk on the front
porch of any of my neighbor's homes and put the receiver near the outdoor
electric outlet on their porches. I have walked about 1/4 mile away in the
middle of a large (~5 acre) lot and when I get on the highest spot of the
lot, I can hear the signal. As I walked down off the crest towards some
houses, it got weaker.
There is a large substation about 1.5 miles away, straight line, and
although I heard all of the buzzing and other power line noises there, no
cracking like something shorting. Finally I went about 1 mile, straight
line, away near some large transmission lines. These again were out in an
open field. It was about the same strength but as I started walking down a
hill toward one of the other sets of support poles, the signal got weaker.
It's the strongest when I am on top of a hill.
I sometimes think there is a problem in the underground utilities
distribution and every house in the neighborhood is acting like a radiator.
The only problem with this theory is that about 35 ft away from my outside
Windom, I have a one wavelength long 40 Meter loop installed in my attic.
If the noise is there, it is almost unreadable. Of course if the problem
was in my house, I would suspect it would be even stronger on the loop
antenna but that is not the case.
I do have a ground rod outside my room and all of my coax shields are
grounded there and I have a ground rod at the base of my antenna mast and
the coax going to the Windom is again grounded there.
My next step is to get a VHF receiver and my 2 meter beam out and see if I
can get some kind of a bearing but I can hear nothing on the 6 meter
antenna, which is also in the attic so I'm not to busy pursuing that
I know this got way too long, but it is one of the more difficult situations
I have come up against and I would appreciate any insights from Mike or
others who have come up against this kind of arcing problems.
73's for now
Jim Baremore K5QQ
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