[RFI] stealth RFI

Rob Atkinson, K5UJ k5uj at hotmail.com
Tue Jun 17 01:02:16 EDT 2003

Hi Ted,

RF is weird at times and can be pretty frustrating.  It seemingly defies 
logic.  I'm not sure if you are having the problem or your buddy but anyway, 
you left out a few things:  Are you running a low-pass filter in your 
feedline to your antenna?  If not, I'd get one and give it a try.  There was 
a LP filter survey article in QST a few months ago (it might have been last 
year) in which a bunch of filters were reviewed.   It is probably on-line 
from the ARRL web site.  You could look at it and select a filter.

You mention something about RF in the shack.  Do you mean, RF in your tx 
audio?  That's a separate problem from the TVI in some ways, (but some 
things you do about the TVI may help with the audio RF if you have any).   
Besides the LP filter, I'd try a few other things.  I hate to say it but you 
may have more work to do.  In my experience, a 1 kw amp needs a pretty good 
ground.  Depending on where you are, one rod may not be enough.  I'd put in 
at least two more, each at least 4 or 5 feet away from the one you already 
have in.  The one in now should be the center rod with the others coming off 
it, each with its own strap from the center, i.e. don't daisy chain them.  
It's better to have all this below grade also.

You don't say what band you are operating on when you cause interference.  
If it's 10 meters your 10' ground run is just about a 1/4 wave antenna.  If 
possible, try to make it shorter, as in about 5 or 6 feet (hard to do I 
know) so your gear sees a lower reactance on that band.  I would also 
recommend using either a heavier gauge cable such as 2/0 gauge from Home 
Depot, or better yet, some 3" wide copper strap which is available in rolls 
from AES and perhaps other vendors.   Try to avoid sharp bends in your run 
to the rods.  Use the ferrite beads on your audio cables into your rig (mic 
cord for example) if you have RF getting into your tx audio.

As always, do the easy stuff first such as trying the LP filter.  That may 
make a big difference.  You don't mention if the tv is connected to an 
antenna or cable service.  It's real hard if not impossible to keep 1kw of 
rf out of a consumer electronic product connected to an antenna that's near 
the tx antenna.  You have done a lot already--I'd try protecting the tv next 
by putting a high pass filter in the line and looping the tv feed thru some 
ferrite beads and putting the tv power cord on a line filter--it could be 
looped through ferrite beads also.  After all this, the tv front end may 
simply not be cabable of rejecting a real strong out of band signal.  Please 
report back to the list on the outcome.


Rob Atkinson
k5uj at hotmail.com

From: Ted Bohrer <N7QY at arrl.net>
To: rfi at contesting.com
Subject: [RFI] stealth RFI
Date: Sun, 15 Jun 2003 14:13:50 -0700

Well, All
My buddy has a RFI problem in the TV and home stereo. Here's the challenge.
(He has the Icom 756 Pro II, Icom linear and tuner, GAP Challenger)
"I turned on TV, sat the wife down, asked what channel was the worst, and 
did a baseline test with Icom linear on (500w), voice and CW. The TV screen 
does a little herringbone on key down full 500w and the speakers mumble on 
SSB, worse with processor on."

"So I tear out my grounding harness made of 1.5 inch braid bus bar and 0.5 
inch braid straps going to each piece of equipment and replace it with 
individual braid lines all going to the back of the Icom tuner, and the 
tuner uses #8 ground wire on a 10' run to a 8' ground rod just outside the 
shack.  Then I installed a Radio Works T-4G grounded Line Isolator at the 
Gap Challenger antenna hooking the isolator ground strap to the 10' ground 
rod at the foot of the Gap.Then back in the shack I put T-4 Isolators (The 
Radio Works: http://www.radioworks.com/clitop.html) between the Icom 756 Pro 
II and the Icom linear and another T-4 Isolator between the linear and the 
tuner.  So after much cussing, cutting of finger on who knows what, writhing 
in dust behind the rigs and multiple contortions working on the back of 
equipment from the front, I declare the project done and RFI hereby gone."

"So I sit the wife down in front of the TV again and fire off the rig to 
show her how OUR $100.00+   worth of Isolators and ground rods had banished 
the RFI.  So I lay on the key while we are both watching the screen . . . . 
. and.....

. . 5 X 9 !!!

much more blankety blank, blankety blank. . . It is easy 3 times worse than 
before I installed the line isolators!!!

So do I still have RF in the shack?  Darned if I know, how am I going to 
tell? Do I still have RFI on the TV?  Oh yeah, big time.

Maybe, just maybe the RF on the coax is now gone, and there is less RF in 
shack, but the result was more power to the antenna or a different pattern 
which is pumping more RF into the house wiring which is really the way the 
RF was getting into the system in the first place, thus more RFI at the TV.  
I still have 4 split ferrite squares that I was going to put on the 110v 
power cords of the TV and the Tuner box.  I almost hate to do it cause I 
have this sinking feeling it will make it even worse."

Can anyone make any suggestions for us?

Thanks and 73,
Ted N7QY

"What could possibly go wrong?"  Laurel and Hardy

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