Re: [RFI] TVI

["Ian White, G3SEK" <G3SEK@ifwtech.co.uk>]

Tom Rauch wrote:

> > there are 6(!) TVs in the
> > house, 4 of them - flat screen TVs. Three different DVD players, CD
>>players
> > and video (VHS) players, each connected to three of those 6 TV sets, and
>>one
> > big flat screen TV in a basement has 5 speakers (surrounded sound) with at
> > least 10' of speaker wire each and this TV set is also connected to a
> > satellite receiver (the dish is on the roof) in addition to regular cable.
> > All TVs are connected to the main cable line by means of 6-dB splitters.
> > Wow!
> > Probably good kilometer of audio, video and other wires all over the
>>house.
[...]
>>All I managed to do was to put
>> High Pass filter which I put between the cable coax and TV antenna 
input
>>and to wind
> >  3 or 4 turns of cable TV coax on the mid-sized thoroid core. Also I put
> > common mode chokes (3 turns of speaker wire on thoroid) on the 3  of 5
> > speakers. Nothing seemed to help.
> > I suspect that his TV signal from the cable could be very weak due to the
> > multiple splitters and his TV input is simply overloaded when I point my
> > beam on his house. In this case - can a UHF amplifier help? Should I still
> > put HPF and common mode chokes on the cable coax? Does he need AC
> > "brutal-force" filter?

> Hi Yuri,
>
>
> Small common mode chokes like you used are also almost not worth installing.
> You need many passes through a 73 mix core to have a high series impedance,
> since the bypass impedance designed into the equipment is unknown.
>
> I'd go back and put some real chokes on cores that you know the
> characteristics of in the system. Maybe 10-20 turns on a large 2" 73 mix
> core. The coax can be a little less, but it still needs several turns
> through a high permeability core.

[And other good advice]

Yuri's problem brings up two important and general points.

1. Many people with RFI problems get stuck at "I tried ferrite, and it 
didn't work."

But as Tom says, an *effective* ferrite choke needs to have:
 * A large enough core
 * The grade kind of ferrite
 * A large-enough number of turns.

If you don't have all three of these, then "trying ferrite" probably 
won't work... not even if you tried it in exactly the right place, where 
an *effective* ferrite choke actually could cure the problem.

There is a big, big need to educate people past that first-grade level 
of "I tried ferrite". (That could make a very good QST article... Ed, 
are you listening?)


2. Many home entertainment systems are becoming hugely complex and 
cross-wired - and Yuri has found an extreme example! You could search 
forever in that house, and still not find how your RF is getting in.

What you need is a way to cut out guesswork and *see* where the largest 
currents are, because those are the places to "try ferrite".

The right tool for the job is a clip-on RF current meter. You can buy 
one from MFJ or Palomar (and maybe others) or very easily make your own 
using info from QST or my website, at 
www.ifwtech.co.uk/g3sek/clip-on/clip-on.htm



--
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)

http://www.ifwtech.co.uk/g3sek
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