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
>>measure.
>>
>>
>
>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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