[RFI] Computer RFI problem

[Rob Atkinson, K5UJ]

>Remember first that the outer shield of the coax acts as an >independent 
>conductor (due to the skin effect). In this case it's >acting as a "long 
>wire" antenna, picking up the computer noise and >then carrying it up 
>towards the antenna. The noise is radiated from >this "long wire" and 
>couples to the main antenna element; then it >comes down the *inside* of 
>the coax in the normal way, and into your >receiver.
>
>A roof-mounted vertical is pretty much a worst case, because the coax 
> >feedline also drops away pretty much vertically - which maximizes the 
> >coupling to the main antenna element. So one thing you could try is to 
> >run the coax away horizontally along the roof, as far as possible >before 
>you have to turn downward.

I must say that I find this somewhat far-fetched. I'd like for someone to 
please explain how this can be, in particular the part about a feed line 
dropping down below a vertical antenna, in line with it "maximizing the 
coupling to the main antenna element."  If you want to couple your feed line 
to your antenna as much as possible I would advise wrapping it in a spiral 
around the antenna all the way to the top.  Since letting it hang down below 
the antenna is the physical opposite of this as much as possible I would 
think it would be decoupled as much as possible.

A line isolator is essentially a 1:1 unun.  I would recommend using one at 
the antenna connection point in lieu of an air coil coax rf choke, if the 
antenna feed is unbalanced and the impedance is the same as the feedline, 
i.e. you have for example, a commercially made antenna that's designed to be 
hooked up to a coax cable.  I think it will work better and be easier to 
install.  You would be surprised at how much feed line you will need to make 
a 6 turn choke by wrapping the feed line on a plastic bucket about 8" in 
diameter.  Someone correct me if I'm wrong but I think that's about 12.5 
feet.

To save time and be really sure your noise problem is within your house try 
this if you can:  Run your rig off a 12 v. car battery (fairly easy to do if 
you have a modern solid state rig.  If you don't, use a battery operated sw 
receiver and hook that up to your antenna). Kill the ac service to the whole 
house by flipping the main breaker.  If you still hear the noise it's from 
outside and you have saved yourself from a lot of messing around in your 
house.  Be sure all your home electric appliance goodies are off before 
turning the juice back on.  It seems to me that if you find the source is 
internal you would be better off finding it and fixing the source instead of 
spending time on coax line isolators and/or chokes.  In the case of 
computers, besides the aforementioned flat screen lcd monitors, I think a 
lot of problems have been eliminated by using line filters (the W3NQN 
filters have a good rep.), breaking up ground loops around the pc and 
monitor, wrapping computer cables around ferrite chokes, and using shielded 
leads with only one end grounded.  Good luck.

Rob Atkinson
K5UJ
k5uj@hotmail.com

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