On 2/9/2026 2:31 PM, Don Kirk wrote:
On the first day I had limited time and Mike and I tracked the RFI down to
an area 2.6 miles away from Mike's house where we could also hear it on VHF
but we had a hard time believing this was correct as this was almost a mile
more than anything I had previously tracked down.
I have preached for years to understand the difference between impulse
noise and electronically generated noise, to identify what you have
using spectrum displays, and to use the appropriate method for each.
Electronically generated noise is seen as vertical lines on a waterfall,
impulse noise is seen as horizontal lines. Lightning and arcing on power
lines are the most common impulse noise, but there are other possible
sources, like failing doorbell transformers and electric fences that arc
to vegetation.
Virtually all noise generated by power systems is impulse noise, and the
best way to track it, by far, is at the highest frequency where you can
listen. In the case you're describing, I would be driving around with a
radio with an AM detector hooked up to a VHF/UHF whip listening around
160 MHz until it got really loud, then moving higher in frequency as I
get closer. I'd also try DFing it with a UHF yagi if I could hear it
with the whip. When I get close, I get out of the car with the talkie.
Held at my chest, it's a half-space antenna, with or without a duck
attached. When I get really close, I remove the duck. I learned this
technique from others in my Chicago ham club during fox hunts, and won
one using it. :)
My Kenwood mobile FM rig and Kenwood talkie have wideband RX to above
500 MHz with AM and SSB detectors.
By contrast, electronically generated noise is rarely that broadband,
must be chased on frequencies where we hear it, AND the spectral and 24
hour waterfall analysis developed by NK7Z is extremely useful for two
reasons. First, it lets us distinguish between multiple sources, so that
our DFing doesn't get confused. Second, it tells is time of day that
these sources are present, telling us when to chase them AND giving
clues about what those sources might be (for example, street lights,
industrial machines, power control devices). Search on his call to find
his work. I've used his techniques, which are detailed for an SDR RX
that HRO sells for about $140, with free software for Windows OS.
73, Jim K9YC
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