All this speculation is a huge waste of time and is irrelevant. There
is only one way to find this particular interference: Get the right
tools for the job and DF it like any other RF signal.
It is folly to assume that the utility has the skill and tools to find
an HF only source. Guessing about what the source might be based on
what it sounds like is a waste of time. Desperate people who have
exhausted all other options that they know about use the "sounds like"
strategy out of frustration but it's a huge waste of time and is
rarely useful (despite comments made to the contrary in this thread.)
I know this from real experience.
The reason this source has not been located is because too many
assumptions and guesses are being relied upon instead of the right
tools and an objective mindset.
If this HF interference can only be heard within a certain band of HF
frequencies, then the key to success is to use a tool that will give
you some directional information at that frequency.
If you really want to find this source, you or whomever you hire must
be equipped with the right tools. I am an interference investigator
for a large electric utility. I chase HF only RFI sources for a
living. After suffering through ineffective techniques to locate
HF-only RFI sources, I finally found the right tools to make the job
easier. I've written about these tools on this list more than a few
times. Yet the message simply doesn't sink in.
For me and professionals like me, time is money. I don't waste my time
guessing about the source. I just go find it. Of course, I have
equipped myself with the right tools to make the job much easier.
Essentially HF RFI sources are found much the same way, VHF/UHF
sources are located. Skill and the right tools combine to "direction
find" (DF) the source precisely.
National RF ( http://www.nationalrf.com ) makes a set of loop antennas
that work very well with a proper receiver to zero in on HF RFI
sources. I have used their Model HFDF Vector Gun with great success.
The loop antenna system is used with a frequency-appropriate receiver
equipped with some sort of RF Gain control and a good signal strength
indicator. There are many possible receiver options from simple
battery operated handhelds to professional grade specialty receivers
specifically designed for DFing RFI. The antenna kit costs about
$240. Receivers can cost $100 to $3500.
If solving this problem seems too expensive, ask around the ham
community in your area and ask if anyone has any experience DFing
HF-only sources. Ask if anyone has a loop antenna that might do the
job. Perhaps you might be willing to build your own loop antenna to DF
this source. I've seem hams in my local area do just that. They get
the data, build the antenna and DF sources on their own. Perhaps the
portable SW radio you use for "sweeping" may work. If not, perhaps
someone in your area has a receiver they might be willing to let you
use or they can join you in the search effort. Where there's a will,
there's a way.
Speculating doesn't work. Even if you knew what it was, you would
still have to DF it. Equip yourself with the right tools and you will
soon locate the source. Good luck.
--
Frank N. Haas KB4T
Utility Interference Investigator
Florida
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