[TowerTalk] RF current meter

[Rick Karlquist]

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.  What was needed was a small iron powder ferrite in a clip-on
> which will give low loss, but no one makes these in a clip-on (there is
> no application for these).  I ended up splitting a toroid and
> fabricating a spring mechanism to make it function like a clip-on.
> Worked pretty good.  I have two calibrated ranges on the meter and can

Tektronix sells various current probes, some AC coupled, some DC
coupled, some clip on and some where you have to pass a wire through
them.  I own various models of these.   They do not add any significance
impedance to the circuit.  For example, the Tek CT-2 adds .04 ohms
to the circuit.  You can easily make your own CT-2 or clip on version
by simply winding 25 turns on the core and terminating it with a 50
ohm resistor.  Now it is true that if you don't use enough turns
or don't use a low enough termination resistance, then you will
introduce impedance into the circuit.  In that case, using powdered
iron reduces the problem, but is not the real fix.

Rick N6RK