>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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