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Re: [TowerTalk] RF current meter

Subject: Re: [TowerTalk] RF current meter
From: K4SAV <>
Date: Sat, 16 Jun 2007 10:09:37 -0500
List-post: <>
Ian White GM3SEK wrote:

>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.)
Thanks for the reply Ian.  Yes, this makes sense.  I guess I wasn't 
thinking too clearly when building this, however I did test some of the 
large ferrite split beads to see what they did to the circuit being 
measured.  I used a 20 turn transformer terminated in 100 ohms, placed 
on a 50 ohm line which was terminated in 50 ohms, and then measured the 
change in SWR on the line, using about 10 watts.  I also checked it with 
an antenna analyzer.  I found all of them produced an unacceptably high 
SWR. Maybe the reason is something other than core loss.  A very thin 
toroid that contains only a small amount of material worked much better.

Jerry, K4SAV

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