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RE: [RFI] shielding answer

To: "'RFI List'" <rfi@contesting.com>
Subject: RE: [RFI] shielding answer
From: "David Robbins K1TTT" <k1ttt@arrl.net>
Date: Wed, 24 Dec 2003 19:20:31 -0000
List-post: <mailto:rfi@contesting.com>
Well, two days of running cable, winding toroids, and adding ground
wires and I think the system is about as clean as its going to get.

Some more details of the setup and fixes.  This whole setup is on the
2nd floor, the shack is on the other end of the house on the first
floor.  The tv is a brand new 38" Sony Wega.  It sits on top of a stand
that contained a vcr, dtv receiver, and dvd player/receiver.  All the
components are connected with either composite video or s-video.  The
whole thing is powered by a ups in the cabinet, but needed an extra
power strip for all the plugs.  The 5.1 speakers were wired with the zip
cord that came with the dvd player.  

This tv generated horrible hash on 80 and 160m when I first used it.  To
cure that the tv was wrapped in aluminum (except for the tube front of
course), this cut the noise it generated to almost nothing and all was
well... until the fall contests when various bands created tvi.

I rigged up a long ptt cord so I could key the transmitters from up by
the tv while they keyers were sending morse at about 30wpm.  I used a
full 1500 watts on each band to the following antennas:

10m 4 ele at 40' about 150' from the set

15m 4 ele at 30' about 100' from the set(worst case, this is only about
15' higher than the set but rarely points this direction fortunately)

20m 4 ele at 60' about 250' from the set

40m 2 ele at 90' about 200' from the set

80m 4-square beamed at the house about 150' away from the set

160m phased inverted L's about 100' tall and 1/2 wave apart beaming
broadside, the set is just about at the mid point between the verticals
which should be about the worst case.

One additional point, there is a radio shack pcm-optical converter to
get digital audio from the dtv satellite box into the dvd player.  Oh
how nice it would be if all this stuff was optically connected!

In each of the tables below I tested all 6 bands against the direct tv
feeding the tv and the dvd playing.  When using dtv I tried both the
analog audio which goes from the tv to the dvd player to the speakers,
and the digital audio that goes from the dtv box through the converter
to the dvd player to the speakers.  The purely subjective interference
scale is 0=not noticeable, 5=annoying, 9=totally unusable.

Initial test results:

                        160m    80m     40m     20m     15m     10m
DTV Video               0       1       2       4       3       1
DTV Digital Aud 0       9       5       0       3       0
DTV analog Audio        0       1       0       0       4       0
DVD audio               0       0       0       0       2       0
DVD Video               1       0       1       4       5       2

Round 1:  run shielded cables for speakers and add ferrites at receiver
end, about 12 turns of both wires through a ft-114-77 core.  Connect
shields together and to the aluminum wrapping the cabinet. 

                        160m    80m     40m     20m     15m     10m
DTV Video               0       1       5       0       1       1
DTV Digital Aud 9       9       0       0       0       0
DTV analog Audio        0       0       0       0       0       0
DVD audio               0       0       0       0       0       0
DVD Video               0       1       5       0       2       0

Interesting.  Analog audio and audio direct from the dvd player are
clean, but the digital audio from the satellite is worse.  And even
worse, the video effect on 40m is much worse!

Round 2:  remove vcr and all its cables, remove extra power strip,
remove feed from satellite rf output to downstairs bedroom (now vacant
anyway). Replace old untyped ferrite on tv power cord (probably an old
tv focus coil ferrite) with a ft-240-77.  add two ft-240-77 cores (5
turns on each) in series on power cord of ups.  Add two ft-240-77 cores
in series (5 turns on each) on the two satellite antenna coaxes.  Add a
bunch of turns on ft-114-77 cores on the phone line in and out of the
ups filter jacks and at the dtv box.  Add as many turns as I could get
on the composite and s-video cables from the dtv box and dvd box up to
the tv on ft-240-77 cores.  Change the ground connection from the audio
shields to go to the ups case.  Add ft-114-77 cores on the digital audio
line and the power supply to it.

First test on 160m still destroys the digital audio so I wrap the
converter in foil and ground the foil to one of the rca jacks on it (not
grounding the foil to the rca jack didn't do anything!).

                        160m    80m     40m     20m     15m     10m
DTV Video               0       1       3       0       1       0
DTV Digital Aud 0       0       0       0       0       0
DTV analog Audio        0       0       0       0       0       0
DVD audio               0       0       0       0       0       0
DVD Video               0       0       3       0       2       1

Well, better.  All the audio is clean, but there is still some
bothersome video blinking on 40 and 15m and a bit on 10m, and slight
color bars on 80m.  Covering most of the tube opening and front of the
cabinet with foil doesn't fix it.  Unplugging the ups doesn't help.  The
effect is there even when there is no video input to the set...
unplugging the video leads from either the dvd or dtv box almost stops

Final round:  add ground wires from dtv box, dvd box, and ups to the
aluminum shielding.  Also add ground wire from a front panel rca jack on
the tv to the aluminum shielding.  Now there is only very slight
blinking on 15m, 40m is clean as are all the other bands... now to wait
for ARRL DX tests to see if multiple bands at once can break it.

What we won't do to watch our TV.

David Robbins K1TTT
e-mail: mailto:k1ttt@arrl.net
web: http://www.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net

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