Ian White GM3SEK gm3sek at ifwtech.co.uk
Tue Jan 29 07:06:12 EST 2008

K4RO Kirk Pickering wrote:
>On Tue, Jan 29, 2008 at 07:54:47AM +0000, Ian White GM3SEK wrote:
>> A clamp-on RF current meter will SHOW you the answers to all of those
>> questions. It really is like taking the blinders off - suddenly you can
>> SEE what's happening!
>I can testify to Ian's comments.  I still have a lot to learn
>about WHY I have so many common mode problems, but it sure is
>helpful to SEE where the biggest problems actually are. I built
>this ridiculously simple sniffer in about 2 minutes from parts
>on hand. Perhaps your junk box already has what you need.

Great idea to use a field-strength meter!

Just one thing: to avoid disturbing the setup that you're trying to 
investigate, don't forget to include a terminating resistor for the 
secondary winding. (I can't see whether Kirk is using one or not, so 
this is intended as a general comment.)

A  clamp-on probe will work without that resistor, and it will even be 
more sensitive when used as an RF "sniffer"... but it will also change 
the current that you're trying to measure, which can be quite confusing.

Without the resistor, clamping on the ferrite bead will  "insert" the 
normal choking impedance of that bead into the circuit.  This could be 
anything up to 100 ohms, which may reduce the current quite a lot.

With the terminating resistor, the bead will act as a current 
transformer which inserts only a small impedance into the circuit. The 
value of the resistor isn't critical, around 50 ohms for a 10-turn 
secondary winding (0.25W miniature carbon film, metal film or metal 
oxide). The impedance will now be the value of the resistor divided by 
(secondary turns)-squared, eg for a 47-ohm resistor it would 
theoretically be only 0.47 ohms. In practice it will be a little higher, 
but still only a few ohms.


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

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