Rob Atkinson, K5UJ wrote:
>Very good; so then we are thinking of the same configuration. Once
>again, I would like an explanation as to why this causes a maximum
>amount of coupling of unwanted noise between the antenna and feedline.
>I have yet to see a reason given for why this is so.
My apologies, Rob. Now that we're talking the same language, that's a
very good question indeed... I thought I knew the answer, but you made
me see that I didn't!
To check this more carefully, I put together a highly simplified EZNEC
model of a typical "no radials" vertical, and then tried the effects of
running the feedline either straight downwards or away horizontally.
I assumed a perfect choke at the feedpoint, so that induction was the
only way for current to get onto the feedline. I also assumed a
half-wave resonant feedline, which is pretty much the worst possible
case.
On calculating the maximum current induced onto the feedline, it really
didn't make much difference at all whether the feedline ran straight
downwards, horizontally or at 45 degrees! The straight-downwards case
was worst, but only by about 20%.
I had been confusing it with the case of a center-fed vertical dipole,
where it certainly is desirable to run the feedline away horizontally to
reduce induced currents. But it's clearly very different case with these
"no radials" verticals which are off-center-fed.
So, it seems that I was wrong: it probably doesn't matter too much how
you route the coax away from the antenna. Preventing pickup or radiation
involving the outside of the feedline is therefore all down to the
chokes - the one the manufacturer provides at the feedpoint, and any
others you add yourself.
Jim, I hope you haven't been up in the loft re-routing the coax! If not,
Rob is the one you have to thank for saving you a lot of wasted time -
and once again, my apologies.
Even so... it's still a good idea to clean-up the currents on the
outside of the coax as much as you can, because RFI can travel that
route in both directions. RFI from computers and other appliances can
travel that route into your RX, and also RF from your TX can get into
those appliances.
I've certainly experienced it both ways. The cures were found by adding
chokes, and checking the results in TX mode using a clamp-on RF current
meter.
--
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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