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Author Topic: HF DF System for Locating RFI  (Read 18413 times)

Posts: 236

« on: February 21, 2012, 07:43:17 AM »

Does anyone know where to obtain, or have plans for an HF DF loop/receiver handheld system which I could use for HF RFI location?

Posts: 17483

« Reply #1 on: February 21, 2012, 08:09:37 AM »

If you do a search for "ARDF" you'll find a lot of information on hand-held DF receivers for 80m.
There are international competitions that involve running several miles through the forest
looking for up to 5 transmitters on both 80m and 2m.  Receivers need to be compact and
easy to use.

Plans for a simple 80m DF receiver were published in QST for September 2005.  (Ignore the ugly guy
on the cover who is modeling it.)  It's direct conversion, and certainly not the best circuit,
but mostly adequate.  Unfortunately the kits are sold out and I don't know if the board is still
available.  I've built several versions using Ugly construction, though my recent versions
have been a superhet design that is, if anything, easier to build and works much better.

I've used such receivers several times for tracking down RFI.  I've managed to tune them
up as high as 40m.  Above that I built a simple 5 - 15 MHz loop that plugs into a receiver such
as the VR-500 and allows me to DF on other bands.  Building a loop for separate receiver is
pretty simple.  (And if you can hear the noise on the AM band, just use the build-in loopstick
antenna on a pocket portable receiver.)

When I was first asked for the loan of such a 80m DF receiver to track down RFI there were
only 3 that I knew of in North America - and I happened to have 2 of them at the time.  Now
there are many more, over 100 at least.  They are scattered around the country - I know there
are several in the Boston area.  For other bands you'll have to modify them or build a custom

Meanwhile, this page has a list of resources for receivers and some links:

There are many more circuits available - I have paper copies of some European articles, and
there are always newer designs appearing.  I can help you track down an 80m receiver to
borrow, or come up with a design if you need it for other bands.


Posts: 236

« Reply #2 on: February 21, 2012, 08:41:00 AM »

Thanks for the info...I've downloaded the article.
That looks like what I need, except 80M is one band I DON'T have RFI problems with, mostly 20, 15 M and sometimes 40M. Most of the problems have been related to part 15 devices, typically battery chargers, etc. I have a portable HF receiver, so just need to build a small loop I think.

Posts: 17483

« Reply #3 on: February 21, 2012, 09:49:30 AM »

I can't find my loop at the moment, but it was built on a length of 1 x 2 wood perhaps 18" long.
I wound 3 turns of wire for the main loop (about a foot in diameter?) tuned with a mica
compression trimmer.  If you drill 3 holes across the top of the wood and 4 across the bottom
you can wind 3 turns of #14 or #12 solid insulated house wire through them and they will be
stiff enough to maintain the shape without a cross spreader.  Then just wire the variable
capacitor across the free ends at the bottom.

The end of the coax is formed into a Faraday loop about 1/4 to 1/5 the diameter of the main
winding, again supported by passing it through two holes in the wood.

I'm just guessing at the dimensions, and you may want to make it a bit smaller for the
higher bands.  Frequency range depends on your variable capacitor.  A dip meter is very
handy for checking the tuning range, but if you hook it up to your receiver you should hear
a clear peak in background noise when it is tuned.  A quick estimate for the inductance of
a large diameter 3-turn loop in uH is the radius in inches, so a 8" loop would be about 4uH,
and would tune 10 - 25 MHz with a 10 - 60pf variable capacitor.  That's a good starting point
at any rate.

Drop me an email at ARRL dot net and I'll send you a photo of the one I built.

Using such a loop for location RFI sources isn't always as simple as it might appear.  You are
looking for the sharp null broadside to the loop, and if the offending radiator is spread out
over some distance (such as a power line or computer network cable) then you won't find
a null because it is coming from multiple directions.  (Bearings are often inaccurate indoors
due to coupling to the house wiring.)  In that case, just using the loop to sniff around a
building to find places where the signal is strongest sometimes is the most effective method.
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