[RFI] RFI from variable speed drive 1.1 miles away (very interesting case).

Don Kirk wd8dsb at gmail.com
Tue Nov 15 20:57:24 EST 2022

Today I tracked down new RFI at my QTH on 160 meters.  I was able to hear
the RFI from my driveway using my portable flag with the DX Engineering
preamp and it indicated the RFI was arriving from the SW (similar to what
my stationary pennants indicated).  Using the portable flag I was able to
quickly track the RFI down to a variable speed drive located at a Living
Historical Museum located 1.1 miles from my house and it was pretty amazing
how strong the signal was at my QTH considering the distance.

This case was very interesting because the variable speed drive is for a
fan that blows air into the bottom chamber of a 105 foot tall tethered
Helium balloon that takes passengers to a height of 377 feet.  The air is
blown into the bottom chamber of the Helium balloon to maintain a constant
pressure in the balloon regardless of the outside temperature.

I had never encountered this RFI before and it turns out their old variable
speed drive system was recently replaced with a new variable speed drive
system.  We replaced the new system with their old one and this stopped the
RFI.  They will now follow up with the manufacturer of their system to
investigate a solution, and I will help support their troubleshooting as
needed and until then they will use their old system.  It does appear a 3
phase powerline filter has been installed on the input of the variable
speed drive on both their old and new systems.

For those interested here is a link to a youtube video I just uploaded that
documents this case.  https://youtu.be/Qnz4E9r5vj0

Note: the more I use my portable flag which is unidirectional the more I
appreciate how much time it saves me compared with my bidirectional tuned
loops as I instantly know the direction the RFI is arriving.  It often
saves me hours if not days locating the source of RFI.

Just FYI and 73,
Don (wd8dsb)

More information about the RFI mailing list