[RFI] RFI clip-on probe?

[Ian White, G3SEK]

Cortland Richmond wrote:
>
>You can make a tolerable clip-on probe by putting a one-turn (preferably
>shielded)
>pickup winding through a snap-on ferrite bead. You could choose a bead to
>not
>adversely affect current in the lead it's on (less than 5 ohms) or don't
>worry about
>it if you only need relative readings or pickup for an already undesired
>signal.
>
There's a very important point here: in order to have a low impedance at 
the primary (the sampled wire), you *must* terminate the multi-turn 
secondary with a resistor. For example, with a reasonable grade of 
ferrite, a 10-turn secondary terminated with a 47 ohm resistor looks 
like a series impedance of 0.47 ohms.

If you don't terminate the secondary (or terminate it only in a high-Z 
detector) then the same core looks like a plain old ferrite bead with a 
series impedance of tens to hundreds of ohms.


-- 
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
                            Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek