[CQ-Contest] RFI Issue
k1ir at designet.com
Mon Aug 29 09:43:18 EDT 2005
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
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
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.
Jim Idelson K1IR
email k1ir at designet.com
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