Thank you Ian, for your very thoughtful response ! The antenna is about
50 feet from the operating position. Interestingly, it replaces a R7
vertical on which the fiberglass coupling at the base failed during a
thunderstorm and the whole thing came down ! The newer R7000 has the
problem (and is using the same coax)! Anyway, I was thinking along the
current balun idea and actually installed one of RadioWorks 'line
isolators' but at the antenna switch. This had the effect of lowering
the intensity of the interference but it was still S9 (not at a current
maximum - interesting). The logistics will be interesting to do as you
suggest - crawl into the rafters above in what will be approx. 115
degrees F to a point about 15- 20 feet down the coax and coil up loops
of coax and tape/bind them up - and hope I have enough coax to do it
(ah, but easier to add coax at the END !!). Well, I'm still linber
enough to do this. One question - what diameter should the loops be ?
Jamie
WB4YDL
On Sunday, August 4, 2002, at 03:34 AM, Ian White, G3SEK wrote:
> 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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