[RFI] Broadband RFI...

Dave Cole dave at nk7z.net
Tue May 31 11:10:48 EDT 2016

On Tue, 2016-05-31 at 09:54 -0400, Frank N. Haas KB4T wrote:
> A few years back I investigated a broadband source that was noticeable
> from 1 to about 10 MHz. It turned out to be a privately owned
> device...the actual type of device won't be mentioned because what is
> important here is the search and not the result. Suffice to say the
> source was a power supply.
> When the complainant demonstrated the problem all that could be
> noticed was an elevated S meter reading and a high pitched tone whose
> pitch didn't vary anywhere the receiver was tuned between 1 and 10
> MHz. If one just listened to the receiver one would think everything
> was normal. In reality the noise floor hovered around S9 to 15 over.
> Searching for an elevated noise floor is challenging but not
> impossible.  Every source has a unique characteristic of some sort. In
> some cases, the unique feature can be seen on a receiver scope i.e.
> Radar Engineers Model 240 or equivalent. In others it may be some
> aural feature. The search then becomes a hunt for the location where
> the signal with the unique feature is strongest.
> In the case of the broadband source I chased, I had to listen
> carefully for a few minutes to learn all of the unique qualities of
> the source. I ended up doing an expanding circle search which brought
> me to the source.
> Frank N Haas KB4T


Thank you for the suggestions here, and you have described the RFI
exactly!  It looks like an elevated noise floor, by about 12 db.    I
also think I hear a single tone in there, that seems to be around 700 or
800 Hz.  This is shown on the audio FFT.  I will check to see where it
starts and where it ends after I have the start/end time pinned down

If only I had a Radar 240...  Does Radar Engineering device actually
pick a signature off of these sorts of broadband signals?  If I can tag
it in some way, I think I can locate it easily enough.  I have
successfully located maybe 20 or more sources over the past two years,
so I am somewhat familiar with the process, but this is my first
broadband RFI source, and I am having great difficulty in pinning it
down in the mobile, as opposed to normal band noise.  

My issue is my hearing.  It is not as good as it used to be, and that is
my primary way of tagging an RFI source, (no Radar Engineering
equipment), so I expect this one will be a pain.  That and it is only 12
db. over background...  

My plan of attack is to do a drive around, stopping, and taking
background S meter readings at every intersection for a radius of about
1/2 mile from my QTH when I know the RFI is off.  Then to it again after
it starts, and has time to stable down a bit.  Your tip about it being
unstable, might be why I could not get this process to work well
yesterday, I drove around right after the start time, and had S meter
readings all over the place, I suspect it was rotating at that time, and
I was seeing changes because of that.  I think I will give it an hour to
stable down, then do the post start drive around.  I think this process
will divorce my bad hearing, the small 2 s unit change in levels, and
and the lack of any sort of signature from the process, thus showing me
at least where it might be by elevated background noise.  Did your
source put out anything on 2 Meters, AM?

I am going to go to Sea-Pac this year, and it is a two night deal, so I
will leave things recording to verify the start/stop times...

Thanks again for the help! 

73's, and thanks,
Dave (NK7Z)
For software/hardware reviews see:
73's, and thanks,
Dave (NK7Z)
For software/hardware reviews see:

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