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Date: Thu, 03 Dec 2009 11:12:08 +0000
From: Peter Dodd <g3ldo@ukonline.co.uk>
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Subject: LF: LF, reducing common-mode noise
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A couple of weeks ago I asked how can one measure the mu of ferrite 
rings. I received very good information via this reflector for which I 
would like to thank those who contributed.
I was experimenting with various common-mode chokes, particularly on how 
to measure their characteristics using the AIM 4170.
I was advised that the capacitance of the measuring setup might make the 
measurements invalid. I introduced various values of capacitance to 
measure these effects and by extrapolation concluded that my measurement 
method was acceptable.
Inserting large values of capacitance across the common mode choke 
destroyed its characteristic as a choke and turned it into a selective 
tuned circuit. This arrangement appears to reduce common-mode noise at 
the frequency to which it is tuned but is only effective at that 
frequency. The main advantage is that you don't need such large values 
of inductance.

Regards

Peter, G3LDO





Paul-Henrik wrote:
> Hello Stefan, LF
>
> I can not overemphasize the possible need to keep the feedline "clean" and free
> of RFI that may otherwise make it's way to any active whip antenna.
>
> My first attempts at deploying the PA0RDT Mini-Whip here were unsuccessful and
> disappointing due to the fact that some of the RFI I tried to avoid was
> conducted from the shack out to the antenna along the coax. This may among
> other things depend on what kind of radio the antenna is connected to.
>
> If in doubt, my advice is to use one common mode choke where the coax leaves the
> house and to ground the coax braid just behind it using an independent
> earth-stake and then further a second common-mode choke just before the
> feedline goes vertically up to the antenna.
>
> For common mode chokes I use large high-mu ferrite rings that can take 20 turns
> of RG-58 or up to 45 turns of RG-174 but smaller rings can be used with RG-174.
> There is a picture here of the first test-choke I used for tracking down my
> noise: http://peditio.net/utility/Attachments/choke_sml.jpg
>
> This may be "overkill" in most installations but I'm sure some users can gain
> from these extra measures just as I did.
>
>
>
> BR
>
> Paul-Henrik, OH1LSQ
>
>
>
>
>