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Re: [RFI] high noise level...

To: rfi@contesting.com
Subject: Re: [RFI] high noise level...
From: "Ian White, G3SEK" <G3SEK@ifwtech.co.uk>
Reply-to: "Ian White, G3SEK" <g3sek@ifwtech.co.uk>
Date: Thu, 11 Sep 2003 14:50:56 +0100
List-post: <mailto:rfi@contesting.com>
Tom Rauch wrote:
Paul,

I ALWAYS use a balun with any balanced directional antenna, because even
small amounts of common mode can cause loss of directivity by filling in
nulls. It just is a very good habit to get into, better safe than sorry.

Strongly agree! A choke balun at the feedpoint is the best and most basic precaution against RFI - either to your station or from it.

Also ground the cable (if you easily can) at the mast top and bottom for
lightning protection and reduced common mode problems. Otherwise tape it to
the mast very well, and bring it all the way to the grounded base.


My guess would be that Paul has a crankup tower (fixed towers are rare over here) so he may not be able to follow the advice about taping the feedline to the tower. That makes grounding the shield at the base even more important.


Understand this is all an educated guess, and real results can be
exceptions, but:
If the noise isn't terribly directive when you rotate the quad (assuming the
quad is working OK and has good directivity), and doesn't change when you
crank it up and down, then that is a strong indicator the noise is following
the feedline or mast up to the area of the antenna. Either that or the noise
is close to the antenna and in an area where distance doesn't change much
when the antenna is raised and lowered.

Another possibility is that it might be coming up the rotator cable, and crossing over to the antenna at the top of the tower. Remember, there is actually no "ground" at the top of the tower - there are only "common" connections.

One way to check for noise currents on the antenna and/or rotator cables might be to wind any slack into choke coils at the base of the tower, or to clip on a fairly large number of ferrite beads. Neither of those might be a complete cure, but if the noise level goes down, you'll know you're on the right track.

Yet another way is to try transmitting, and use a clip-on RF current meter to check for RF on the cables. There should of course be none - but where your RF can get out, noise can get in.


Did you try opening the breakers in your house one at a time, and seeing if
something in your house is causing the noise?? That's always the first thing
I do. Switching power supplies, lights, bad doorbell transformers, and
dozens of other things can cause noise. Telephone lines can conduct noise,
as can power lines. Arcs in power line also cause noise, and it can be
conducted for long distances.

Good points, all well worth following-up. Even so, it seems suspicious that this noise should suddenly appear (and be so loud) when you've put up an antenna that ought if anything to be "quieter" than most.


Think *very* carefully about what other changes happened you made at the same time - there may be a clue in there.

Can you still check the noise level from your previous antenna, Paul? (Also what kind was it, and where was it in relation to the new quad?)


-- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) Editor, 'The VHF/UHF DX Book' http://www.ifwtech.co.uk/g3sek _______________________________________________ RFI mailing list RFI@contesting.com http://lists.contesting.com/mailman/listinfo/rfi

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