[RFI] Chasing RFI

David Cole dave at nk7z.net
Sun Oct 18 15:35:46 EDT 2015

Jim is most likely correct... 

Do this:

Borrow someone's mobile Low Band rig in a car.  Add enough attenuation
to get the RFI to about S5 or 6.  Drive around listening to it as the
highest frequency possible, keeping it at no higher than S6, using a non
directional antenna, on say 40 meters.  

Take your Air band AM receiver and keep it on with a good outside
antenna on the car...  Use the two meter antenna on the car in fact...
At some point in this process you will start to hear the RFI on VHF,
make sure the VHF radio is in AM mode.

Start doing a spiral from your house out, listening on the Low Band
radio, watching the S meter.  Have a friend watch it while you drive, it
is dangerous to drive and watch S meters...  

Keep the VHF AM radio on and up at all times, make sure you are hearing
background hiss, i.e. don't let the squelch suppress the sound.

At some point, as you get closer and closer to the signal, you will find
the S meter going higher and higher on the low band rig, when it gets to
about S8 or 9, move the Low Band radio up in frequency until you are are
back at S5 or 6, or add attenuation.  I have a screwdriver antenna, and
I just detune it to reduce levels at this point.

As you get closer to and closer to the RFI source, you may start hearing
the signal on your VHF AM rig, that is what you want!  Once that is
done, write down where you are...

Now go home...  Download a Google map of that area, give yourself a lot
of extra space on the map then print it out...  Use the Satellite view,
not the map option, you want to SEE what the area looks like, not be
looking at just a map, more on that later...

Now return to the same area.  Tune everything so you can hear the RFI
again, at S5 to S6 using the VHF radio.  Now write down the S meter
reading you are seeing on the Google map, AND the settings it took to
get it on a separate piece of paper, keep the settings paper, and from
now on always return your radio to that setting when you return to that

Now get a small 2 meter 3 or more element beam, and connect it to an AM
VHF radio...  

Now you are able to point at the source using the beam...  Take a few
readings by rotating around with the beam, looking for the highest
readings under S7 or 8.  If you start getting higher than S8, add
attenuation to bring the signal down to S5 or 6.

Look at the Google map, and see where you are pointing, this is why you
wanted the photo map, not the street map...  Draw a line from where you
are standing to what you see on the map, when you look down the

Move several hundred feet, and do the same thing again...  Draw another
line...  Move another few hundred feet, and do the same thing...  Do
this until you start to see the majority of your lines crossing in an
area...  That is probably the area your RFI source is located at.  If
they are not crossing after 5 or 10 tries, then you are doing something
wrong...  Maybe several sources...

Now move to the area where the majority of your lines crossed, and using
only the two meter radio, and an attenuator start the line drawing
process again...  

Keep the S meter low, maybe S3 or 4, by adding more attenuation.  You
will eventually get to the RFI source this way...  

If you have a TH-F6A, (I use this for all RFI hunting), keep adding
attenuation and drawing more lines.  At some point you will find
yourself in front of the RFI source.

I use a very small magnetic loop, as opposed to a beam, and I go for
nulls, not peaks, but if this is your first time DFing, use the yagi

That will get you started on DFing RFI sources...  Also, this all
assumes that everything is perfect, and an easy to locate source is the

There is a sort of art in listening to the RFI...  You want to make sure
you work on only ONE SINGLE source at a time...  Take your time to
insure this at each reading location.

Listen to the RFI carefully, be one with it...  If possible have a cell
phone on your home rig, and have another cell phone in your ear,
connected to the phone on your home rig...  That way if there is some
form of modulation on the RFI, you can see if it matches, that way you
know you have the same source... 

It is easy to get off on some other source...  Take the time to make
sure you are on the same source you hear at home...

All of the above assumes a perfect world, and an easy to lcate source...
But it will get you started...

I have a few tips, and links to various RFI resources at:


Good luck, please keep us posted ion your location efforts...

Thanks and 73's,
For equipment, and software setups and reviews see:

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For MM-SSTV see:

On Sun, 2015-10-18 at 11:06 -0700, Jim Brown wrote:
> On Sun,10/18/2015 10:30 AM, Charlie Gallo wrote:
> > OK  -  but the fun was I could only hear it on 10m in the general area
> > of  that  pair  of poles, got more than 1/2 block away, NOT audible on
> > 10m,  but  never  was able to hear it on Aircraft - which is why I was
> > wondering
> I suspect that you haven't gone far enough to find the primary source.
> 73, Jim
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