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Re: [TowerTalk] RF current meter

Subject: Re: [TowerTalk] RF current meter
From: Ian White GM3SEK <>
Reply-to: Ian White GM3SEK <>
Date: Sat, 16 Jun 2007 14:41:31 +0100
List-post: <>
K4SAV wrote:
>The thing that surprised me when looking at the designs others have 
>done was that many people used the clamp-on ferrites that are intended 
>for RFI suppression.  These add a large series impedance into what you 
>are trying to measure and will mess up the current you are trying to 

It is important to terminate the secondary winding with a low-value 
resistor. If you do, then even an RFI suppression bead will act as a 
transformer with a very low insertion impedance in the main line. For 
example, if a 10-turn secondary is terminated in 47 ohms, the insertion 
impedance in the main line is theoretically 0.47 ohms; and in practice 
it is only a few ohms.

If you leave the secondary with a high-impedance load, *then* you will 
see the full insertion impedance of the ferrite bead. (A simple 
impedance meter like the MFJ-259B will show this effect quite well.)


73 from Ian GM3SEK         'In Practice' columnist for RadCom (RSGB)

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