[RFI] stealth RFI

Michael Tope W4EF at dellroy.com
Mon Jun 16 10:28:22 EDT 2003

Hi Ted

I am using a roof mounted vertical here on top of my two
story house. The TV is upstairs which puts it right under
the counterpoise for my vertical. Adding a bunch of ferrite
beads to the coax that runs between the CATV wall outlet
and the TV seemed to help (I used Amidon type
77 material), but it didn't get rid of the problem entirely. I
also added more radials on the roof in order to help create
a shield between the antenna and the electrical wiring in the
attic. I started with 2 tuned radials on each band (10, 15,
and 20). With this configuration, I was able to wipe out the
picture with 100 watts on 15 meters. I recently added an
additional 16 non-resonant radials (I just ran them out to the
edge of the roof whatever length that came out to be). I am
now down to light crosshatching, whereas I had really nasty
picture blackout and audio thumping before. Unfortunately,
I had to switch televisions at the same time I added the
extra 16 radials, so I am not entirely sure that the radials
were soley responsible for the cure (the old set failed for
reasons unrelated to RFI).

Also, don't rule out RF conducted through the AC line. If you
have a VCR,  you can test for this by disconnecting the cable
for the TV and then playing a tape with the VCR. If you still
have RFI, then its either conducted through the AC line, or
direct pick-up. If it turns out to be conducted through the AC
wiring, then you can try filtering the line with some kind of
low-pass AC line filter.

Most of these problems can be solved if you just hang in
there and go through the process of elimination. At my old
QTH, I ran 1200 watts to a 27' high top loaded vertical on
160 meters (27' high 80/40 meter parallel dipole with the
coax tied together for 160 meter operation). The feedpoint
of the antenna was only about 8 feet from my Panasonic 21
inch computer monitor, and it didn't show the slightest sign
of RFI while I was transmitting. This of course was after I
laid down about 25  40' to 60' radials and connected them
to the feedpoint of the vertical. Before that I had quite a bit
of RFI. In fact it was so bad on 40 meters where I used the
same antenna in a dipole configuration, I would get bitten
by RF in the shack. After I added the radials, the system
showed no sign of RFI at all. BTW, not that I am
recommending this, but the only grounding in the shack was
via the shielded internconnects between the equipment (coax
and audio jumpers) and the safety grounds
for the AC line connections. I had a big copper ground buss,
but I never got around to hooking it up. They key was the
radial system just out side the coax entry point. In fact the
radials where connected to the small metal coax feedthru panel
in the window of the shack. This created a low impedance
shunt path for any RF on my feedline thus keeping it out of
the shack.

Good luck!

Mike, W4EF.............................

----- Original Message -----
From: "Ted Bohrer" <N7QY at ARRL.net>
To: "Michael Tope" <W4EF at dellroy.com>
Sent: Sunday, June 15, 2003 4:34 PM
Subject: Re: [RFI] stealth RFI

> Roger, ground mounted about 25' from the TV.
> In fact, the Challenger is basically attached to the house (behind)
> where the neighbors can't really see it. The house is 2 story, and
> the TV and stereo are in the family room only separated from the
> antenna through sliding glass doors to the outside deck by the
> antenna.
> >Ted where is you vertical mounted relative to the TV. It sounds like it
> >ground mounted. Is that correct?
> >
> >Mike, W4EF...............

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