James C. Hall, MD wrote:
>Here is an interesting problem:
>
>I put up an old Cushcraft R-7000 on the roof as a secondary antenna. My
>primary antennae are on a tower 150 feet from the shack and perform very
>well without RFI difficulties. When receiving with the vertical on 20
>meters, I hear S9 computer generated interference. Changing antennas to
>either the beam or an inverted V yields no such interference. This only
>occurs on 20 meters - at least very audibly - and only with the vertical. I
>used to have trouble with the lower frequency bands ie. 75/80 meters until I
>switched out the computer power supply with a better filtered and shielded
>one. Now no problem on those bands.
>
>I'm thinking about adding some length to the coax off the vertical to see if
>that would help ... but I doubt it.
>
Extra length of coax can help... but only if you do something useful
with it.
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.
At the same time, discourage RF from flowing on the outside of the coax,
by wind some of the coax into flat coils to act as feedline chokes. See
below for dimensions. The first choke should be right outside the
connection box on the R-7000 - there is already a feedline choke in
there, but it could probably use some help. If 20m is the band that
concerns you most, place another coil (optimized for 20m, see below)
about a quarter-wavelength away from the box.
You may have to repeat these chokes at intervals along the line. The
best way to find out the most effective locations is to *transmit*. This
reverses the pickup situation, and induces RF currents onto the
feedline. Now you can check the outer-surface currents at various points
along the feedline, using a clamp-on RF current meter (an easy homebrew
project - see a fairly recent QST, and also my 'In Practice' pages).
Look for the places where the current indications are greatest, and wind
a choke coil right there. That won't stop the current altogether - it
will pop-up at another place instead, but probably at a reduced level.
(Hint: if you wind alternate choke coils in opposite directions, as you
go along the line, that will help to take the twists out of the coax in
between.)
------------------------
Feedline choke coils are also known as common-mode chokes or
current-mode baluns - those are all different names for the same thing,
but "balun" only applies when it's used at the feedpoint of an antenna).
Basically it's a flat coil of coax, whose self-capacitance is parallel
resonant with its self-inductance to maximize the impedance at a
particular frequency.
Optimum values are given in the ARRL Antenna Handbook:
14MHz: 10ft RG213 in 4 turns, or 8ft RG58 in 8 turns
14-30MHz: 8ft RG58/213 in 6-7 turns.
------------------------
Thanks, Jamie - you've just handed me a nice lead feature for my next
'In Practice' column!
--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek
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