Hi Ian,
Thank you for your investigation--no apology needed; I appreciate your
comments as they have been a big help in my understanding. They were
facinating, particularly the 20% increase with a downward feedline. I would
have never guessed that.
I was plagued by rf coming back on my coax shield on transmit last year when
I fired up a 1.2 kw amp for the first time (I am on a small lot so my
vertical antenna could not be sited far enough away from the radio shack to
eliminate the problem). by improving on the shack ground system, and
putting line isolators at the feed point and at the transmitter and amp,
plus wrapping just about everything in the shack around ferrite beads (mic
cord for example) I eliminated the problem and cleaned up my audio to boot.
Since then I've begun to discover that this is a fairly common problem.
Some hams try running amps, experience distortion on transmit, give up and
get rid of their amp and simply conclude they can't run high power. Others
operate with fuzzy audio not knowing they have a problem.
BTW this has been especially interesting to me because I operate with an
asymmetric 1/2 w. vertical dipole type antenna so the induced rf business
has had some light shed on it. thanks again.
73,
Rob Atkinson
K5UJ
k5uj@hotmail.com
----Original Message Follows----
From: "Ian White, G3SEK" <G3SEK@ifwtech.co.uk>
Reply-To: "Ian White, G3SEK" <g3sek@ifwtech.co.uk>
To: rfi@contesting.com
Subject: Re: [RFI] Computer RFI problem
To: <rfi@contesting.com>
Date: Wed, 7 Aug 2002 22:46:16 +0100
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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