A Simple RF Sniffer

Here is a gadget that can be used to detect current flowing on the outside of your coax shield.  You can also use it to make relative measurements of current flowing in radials, to see the effect of adding an RF suppressor to power or speaker leads, etc.  A split ferrite bead of the type used for RFI suppression serves as the core of a transformer.  The 'primary' is the wire or cable carrying RF, and the secondary is a 10-turn loop.  Voltage induced in the secondary is measured by a detector consisting of a 1N34A germanium diode in series with a 500K pot and a 50 ua meter.

The clip originally was part of a clamp-on reading light.  The split bead came in a plastic housing; I removed it and attached the two halves to the clip with hot glue.  The meter is also hot glued, as well as having a cople of cable clamps holding it for extra security.  Although the meter is calibrated in "Amperes DC x100" it is really a 50 ua meter!
Here's the side view.  There's an 0.01 uf capacitor accross the meter.
Sniffer side view
Here you can see the 10-turn loop of no. 24 enamelled wire and the hot glue that holds the bead halves in place.  The yellow glob on the far side of the top jaw is the 500K pot.

I taped the bead halves together so they would be in close alignment before gluing.
Sniffer jaws
This is my friend Nick demonstrating how to measure the current in one of the elevated radials of my 160/80 meter inverted L. 
Nick measures current

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