On Tue, 15 Sep 1998, Sean Breheny wrote:

> Hi all,
>
> At 08:14 PM 9/15/98 +0000, you wrote:
> >The loop method tests only for current (especially if terminated as I
> >advocate). For voltage (E field) a prod probe is more useful. This can be
> >a unmodified scope probe set to x 10 with the clip-on gripper prolonged by
> >a 25 mm piece of rigid wire, as antenna. The x10 setting is essential
> >imho. The probe is "calibrated" in the same way as above.
>
> Maybe I m way off, but I don't see how it is possible to test only for
> "current(B field)" or only for "voltage(E field)" Maxwell's equations
> require that an EM wave in free space have the same E field strength as B
                             ^^^^^^^^^^

You have answered your question yourself. The probes described count as
'small probes' (which mostly means that they are much smaller than the
wavelength of the signals). They do not really test for E/B, but the
antenna probe tests for capacitive coupling of signals among other things
(i.e. voltage), and the loop for current coupling. They are also pretty
useless for EMF measurement, but they allow radiating components and
traces to be sniffed out quickly.

If you want to feel more politically correct vs. physics theory with this
you can say that the two types of probes have a very different Z and due
to this, the high Z (antenna) probe will have a better energy transfer
efficiency from sources with a higher Z0 while the loop will have better
energy transfer from sources with low Z0. Note that the space where one
measures this is NOT free space, it is something referred to as 'near
field' in some texts, i.e. d << lambda (or lambda/2 ?). The probe method
used here is a variant of something known as 'small probe method' in
literature. I don't know how up-to-date this is.

Just remember that neither of these probes measures Electro-Magnetic
fields, the loop is actually a current transformer secondary winding and
the antenna is the armature of a small capacitor... in both cases the
capacitive, resp. magnetic coupling induces much more energy into the
probe than the electro-magnetic field reported to the point of measure (by
proper measurement at a distance and scaling). THIS is why it's called
near field I think. This will put you at ease vs. theory. For a while at
least ;)

Peter