[CQ-Contest] RFI Issue

Jim Idelson k1ir at designet.com
Mon Aug 29 09:43:18 EDT 2005

Hi Joe,

Sounds like you are doing the right things to get started. I have a couple of 

-  Is 20m the lowest band you can hear it on?
-  Can you hear it in the AM broadcast band?
-  What frequency did you use when you made the measurements you reported here?
-  How strong is the noise when you are pointed right at it?
-  What is the strength when you turn the antenna away from it?
-  When you drive around, were you planning to listen on an HF rig or something 

The efforts of your power company are certainly welcome, but not likely to be 
effective without a better location of the noise. There are far too many 
components in their system to be able to go out and "tighten things up" and 
expect that to solve the problem. I would definitely not rule out the power 
company or anything else at this stage. You simply don't have enough 
information yet.

The best thing to do in this situation is to drive around with a map in hand 
and try to locate the source. You will want to listen for the noise in your 
mobile at the highest frequency that allows you to hear it well enough to get a 
good s-meter indication. Employ the assistance of a helper at your home station 
to tell you in real-time whether the noise is still there or not. Talk to that 
person on a separate link - cell or vhf.

There are some tricks to the driving game. At low frequencies, like the AM 
broadcast band, you will be likely to hear the noise at a great distance if it 
is coming from an arcing source [bad insulator, lightning arrestor or motor]. 
This is where you will probably want to start. But as the signal propagates 
around town over the power lines, the signal strength will vary not only with 
distance from the source, but with standing waves on the power lines. When 
listening at long wavelengths, you will hear some distracting peaks and nulls. 
So, you need to pay attention to AVERAGE strength over about a quarter to half 
mile distance. At this frequency, your location accuracy will not be very good. 
Your next step is to listen at higher frequencies. A shift to 10m, for example, 
will reduce the strength, but it will allow you to pinpoint the source more 
accurately. When you finally get close - within a few hundred yards, you will 
be able to hear it on a 70cm rig. Remember, you should be listening in AM mode 
at all frequencies, and you should keep the AGC from fooling you whenever 
possible by turning it off or using a variable attenuator in front of your rig.

When you finally get really close to the source, you can switch to a handheld 
radio and use DF'ing techniques [body shield, switching polarization, variable 
attenuation and directional antennas] to further pinpoint the source. You may 
not actually be able to get to an accuracy of less than 50 yards or so if it is 
an arcing source. At this point the pros will switch to a highly directional 
ultrasonic listening instrument - usually from a company called Ultraprobe. If 
you can get that close to the source, you should be able to get the power 
company troubleshooting team to come out with the Ultraprobe. You don't need to 
own one!

Most of the above applies to noises that are propagating over powerlines. The 
approach you will need to employ to track the source of the noise if it is not 
powerline related is basically the same, except you don't have the problem of 
the standing waves on the powerlines, so the signal strength variations should 
be more directly related to your distance from the source.

When you do this, make good use of the map. Use a grid square approach. Mark 
average signal strengths inside each square. Since the noise is intermittent, 
you will want to be able to remember which areas you have already explored and 
tested so you can cover the whole area in multiple sessions without 
backtracking too much.

Good luck!

Jim Idelson K1IR
email    k1ir at designet.com
web    http://www.k1ir.com

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