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Re: [RFI] distance to rfi source?

To: <rfi@contesting.com>
Subject: Re: [RFI] distance to rfi source?
From: "Frank Haas KB4T" <kb4t@arrl.net>
Reply-to: kb4t@arrl.net
Date: Fri, 7 Jan 2011 00:29:28 -0500
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>
I am an Interference Investigator for an electric utility in Florida. Others
on this list have more experience than I do and I have no argument with
statements made here on this topic. I will share my experience with
"distance" and HF interference sources. 

In the 7 years I've been a utility interference investigator, the absolute
farthest any source has ever been from an affected Amateur Station is 3
miles and that was reported by one of my colleagues. The farthest any source
has ever been from any of the cases I've investigated is 1.4 miles as the
crow flies. The offending device in that case was a faulty lightning
arrestor that was arcing internally and acting like a pretty potent
transmitter blanketing the HF spectrum and easily copyable up to 350 MHz
from more than a 1/2 mile away. The affected Amateur Station was about 1.4
air miles away and noticed the interference mostly on 20 meters despite my
hearing it from 1 MHz to over 350 MHz. 

I'm not an Engineer. I've earned the FCC 1st, numerous industry
certifications and the Amateur Extra Class ticket. I do have decades of
experience. The way I see it, there are an infinite number of variables and
every case is different. If a source has 1000 feet or more of power line as
an antenna, the low frequency components are likely to be heard a long way
off. In my experience, "a long way off" is usually less than 2 miles and,
most often, is 1/2 mile or less.  

But is this really important? I think it's wasted effort to try and assign
particular distances to certain frequency bands and signal strengths
because, in my experience, it always varies. 

The vast majority of cases I investigate involve sources that are usually
1/4 mile or less from the affected party. A much smaller percentage are 1/2
mile away or less. A tiny, tiny fraction of cases involve sources that are 1
mile away or so. This includes sources that are copyable only to about 5 - 7
MHz and those copyable to 150 or more MHz. 

One of the first questions I ask when I first speak with an affected Amateur
station operator is, "What is the highest frequency at which you detect the
interference?" If they tell me they can hear the interference at 28 MHz, 50
MHz or 144 MHz, my first thought is that the source should be fairly easy to
locate (simply because directional antennas for these frequencies are easy
to carry and use.) 

If the affected Amateur tells me that they only detect the interference up
to about 10 MHz or less (typically about 7 or 8 MHz,) I know my search is
going to be more challenging (simply because it's so hard to carry a 40
meter yagi down the sidewalk!!!)

I don't think about distance. I'm much more concerned with the frequencies
involved, particularly the highest frequency. Those sources with significant
VHF content are usually found in less than 1 hour. (It ain't braggin if it's
true!!) Those with no VHF content can take much longer. 

Some instances, where the affected party states that the interference can't
be heard above 7 MHz, end up with the source far enough away that the higher
frequency content of the interfering signal simply isn't propagating to the
affected station. As I search and get closer to the source, I'm consistently
increasing my listening frequency until, as I approach the source location,
I can easily detect the VHF content and pinpointing becomes a simple
exercise (with the right equipment.) 

For me, the bottom line is: If I can hear it, I can find it. It doesn't
really matter how far away the source might be. If it can be heard, it can
be found. If it's causing interference, it's got to be found. My experience
is likely different from those whose experience may be more varied and
extensive. While signal strength may be a factor, it hasn't correlated to
any specific distance in the hundreds of cases I've worked. 

I will say this: If the interference is strong it's usually, but not always,
easier to locate. Your mileage may vary!!!! (Ouch!)

Happy New Year!


Frank N. Haas KB4T
Electric Utility Interference Investigator in Florida

Original Message: 

Date: Wed, 05 Jan 2011 18:53:45 -0600
From: Sam Morgan <k5oai.sam@gmail.com>
Subject: [RFI] distance to rfi source?
To: "rfi@contesting.com" <rfi@contesting.com>

I just had a visit from my local AEP rep,
he was here to try to locate an rfi complaint I had called in.
I let him hear the noise (sounds like a noise floor)
(by that I mean no arcing just a steady high noise floor)
We listened from 160m up to 6m
I had my K3 set to AM and a 5khz bandwidth
antenna was a Fan dipole @ 25' (legs for 160/40, 80, 17m)
I tuned the K3 ATU to 1.1:1 on each band

160m = S9+10
80m  = S9+15
60m  = S7-8  (5.335)
40m  = S9
30m  = S6-7  (10.050)
20m  = S6
17m  = S6-7
15m  = S6-7
12m  = S5-6
10m  = S4  (28.027)
6m   = S4

his comment is what is worrying me
he said since the noise is louder on 160 than 10m
that showed the noise is closer to me
rather than farther away
(closer meaning within a block or so)

is that assumption correct?

GB & 73
Sam Morgan

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