[RFI] Re: Radio Frequency Interference (RFI) Digest V3 #87

[J. Bradshaw]

: Date: Tue, 05 Dec 2000 20:40:03 -0600
: From: Jon Ogden <jono@enteract.com>
: Subject: [RFI] Television RFI Problem
: 
: Howdy all,
: 
: Well, about 2 months ago I went off and got married.  Now with a second
: person living in the house it's time to start cleaning up some of that RFI
: stuff that never bothered me.
: 
: My wife's TV is in the living room just down the hall from the shack and
: just about directly below the tri-bander on the roof.  We have CATV...
: 

You may indeed have cable leakage as well as poor shielding on 
several linear IC circuits in the set.  With many sets, solder joints do 
not have the longevity they used to because of the newer non-toxic 
water soluble fluxes now required for environmental protection.  
They are generally weak citric acid based fluxes.

You should first fully inspect the coax, insuring you have quad 
shield rg-6 type with tight gel filled connectors outdoors.  If you 
have any splitters outdoors, be very critical with your inspection as 
They disintegrate from the inside out from corrosion.  Also be sure  
your VCR cable is quad shield too.  I am often shocked to find that 
some expensive gold types are nothing but fancy rg-59 after all, with 
mediocre shielding.  Also, it would be a good idea to use premium 
double shielded cable throughout the radio installation, especially 
interconnecting the exciter, filters, and amp and for the run up the 
tower.

After all that, the quad shield should have at least 90db of isolation 
from the outside world even with an antenna running alongside of it!  
At this point I would like to see a good quality 75 ohm high pass 
filter just to try out (but who makes them?) but common mode 
filtering would probably be necessary to keep the RF on the shield, 
out of the chassis.  I personally haven't had much problem with RF 
on the TV line cord, since there is usually a substantial common 
mode line filter right where it connects to the board.  There is more 
problem though with RF at the radio or from the antenna being 
induced into the line, then being radiated throughout the house.  
Sometimes moving the TV around the room will help.

Next, open up the TV and start resoldering grounds on the shields 
and chassis points that may have deteriorated, or fractured because 
of flexing of the chassis.  Of course if you are savvy and have lots 
of time and equipment at your disposal, now would be a good time 
to find what circuits are most susceptible and make appropriate 
engineering changes and verify the improvements.  The channel 2 
and 6 hash are probably from harmonic spurs either from your 
transmitter, or created in the set from overload or even created in 
a faulty cable system or discontinuities anywhere in the RF field 
(I have a problem in my carport structure but is quite localized).  
The loss of color could be from agc action caused by front end 
or IF overload or by some rectification in the color processing 
circuitry itself.

After all this, I wonder what TV models are least susceptible in 
RF fields.  I have personally seen clean clear TV reception (on 
cable) Immediately across the street from a major broadcast and 
commercial radio site sporting numerous buildings and towers.  
I find that my little Radio Shack portable LCD pocket TV on its 
little rod is less susceptible than my Maggotbox 25" with outside 
antenna far away from the HF vertical.  Channels 2 and 6 are the 
only ones affected at all on 10 and 20 meters with or without the 
amp, but with the portable TV more than 10' away from the 
carport, the problem goes away.  The 100w Vhf radio with the 
antenna 2' away from the TV antenna is a bit more of a problem 
on some channels though.


Jim Bradshaw, AC6TK
Kenwood TS-130s and 30L-1 amp
25" Maggotbox on RG-6QS and outside antenna


--
FAQ on WWW:               http://www.contesting.com/FAQ/rfi
Submissions:              rfi@contesting.com
Administrative requests:  rfi-REQUEST@contesting.com
Questions:                owner-rfi@contesting.com