[RFI] Introduction - power line noise

N1BUG paul at n1bug.com
Sun Jun 24 15:56:33 PDT 2012

Thanks. At present I doubt I will need to use HF to track this down 
but we will see how it goes. I'm finding considerable ambiguity when 
tracking at 135 MHz with the MFJ-856 but I think it would be 
beneficial to go higher in frequency rather than lower. Even using 
all the tricks I'm aware of... peaks and nulls of the antenna, 
polarization of the antenna, attenuation, walking all the way around 
poles to take readings... I am all too often identifying adjacent 
pairs of poles as "noisy". This doesn't feel right to me. I suspect 
only one of each pair is the actual source. In three separate 
locations, the power company investigator found one, but not both of 
my "adjacent pair" poles noisy. Tips? Tricks?

In one case, I can easily hear noise with the MFJ-856 from 3/4 mile 
away with *nothing* but forest between me and the offending pole(s). 
Within a 1/2 mile radius, I can clearly hear the peaks and nulls at 
half wave intervals along the line as I walk with the antenna 
pointed upward toward the line. As I get within 1/4 mile, things get 
very confusing with what I currently believe to be re-radiation from 
most poles in the area. Very near the source I find several poles 
with essentially equal noise levels, but I don't believe they are 
all sources. At that point I am using 45 dB of attenuation to keep 
the S meter from being pegged at full scale regardless of antenna 
heading. Instinctively I feel there is one, or at most two offending 
poles, but at 135 MHz I just can't pinpoint it. This strikes me as a 
fairly serious noise. Even the local 2 meter FM repeater, 25 watts 5 
miles away line of sight is quite "rough copy" in that immediate area.

A better description of my RFI issue:

I operate 160 through 6 meters with emphasis on 160 and 6. I use 
directional antennas... Beverages on 160 and a 7 element yagi on 6 
meters. Ten years ago all was quiet, but noise levels have been 
creeping up for a long time. At least two new strong sources 
appeared this Spring, making things really ugly. On 6 metes my S 
meter is never below S9 when pointed southeast or northwest, except 
in heavy rain or after several hours of moderate rain. Once 
sufficient rain has fallen, all bands are absolutely free of noise, 
every time. There are now multiple sources, concentrated in those 
directions since that is looking up/down the lines passing my house 
(13+ kV distribution line with 46 kV transmission line above). I can 
hear various sources cutting in and out, as all but two are 
intermittent even in dry weather.

I'm not sure what else to add, but if anyone wants more information, 
just ask.

Paul N1BUG

On 06/24/2012 05:01 PM, Scott Yost wrote:
> N1BUG,
> I have a bit of experience grappling with power line interference. I
> can offer tips on use of the HFDF direction finder if you need to
> use HF to track it down, though I do note you have the MFJ VHF unit.
> Perhaps that has sufficed for your needs. Please describe your RFI
> issue so we can consider it. There are some folks on the list who
> are professionals in this field (I am not) who may weigh in as well.
> NM8R

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