Roy,

It is amazing how a simple concept such as impedance can be made obscure
but most current probes are spec'd this way by the manufacturers and
test procedures for DO-160E (FAA aircraft testing) call out probes the
same way. By saying that a current probe has a transfer impedance of
0dB-ohm means 0dB relative to 1 ohm, but R = V/I so 0dB-ohm means that
for 1A of current thru the primary of the probe produces 1V across a 50
ohm load. Similarly a -20db-ohm transfer means that 1A produces .1V
across a 50 ohm load. For most of us, this means a 1 turn primary and a
50 turn secondary which yields a theoretical primary impedance or 0.02
ohms. One of the probes I have says the primary impedance is less than
..1 ohm.

Larry Benko, W0QE

Roy Lewallen wrote:
Larry Benko wrote:

Hi Roy,

I have a couple of RF current meters with freq. response from 10KHz to
100MHZ that are the types used for FCC part 15, DO-160E testing etc.
and have a transfer impedance of 0dB-ohm. They have an opening of
about 1.6" dia. and I sqeeze the feedline together (spacing about
1.5") temporarily and clamp over both conductors for measurement #1
and clamp over just 1 of the conductors for measurement #2. I read
the output of the current probe with an RF power meter. In my case
the current in a single conductor was approximately 15dB above the
differential current which seemed to be pretty well balanced. Is
there anything wrong with this approach?

Larry Benko, W0QE


Not that I can see. In my opinion it's the best way to make the
measurement. A homebrew version of this is entirely adequate, though, as
described in the earlier thread. It's very important to terminate the
secondary with a fairly low impedance so the transformer doesn't present
a significant impedance to the line. I assume your meter does this
internally -- maybe that information is in the transfer impedance you
mentioned and which I don't really understand.

Roy Lewallen, W7EL