That is his "obvious" explanation. He should remove that from his webpage as
it is rather embarassing. Given that the magnetic field moves at the speed
of light, there will be no equipment in any hamshack that will measure the
delta between the field affecting coils spaced 1mm apart vs coils spaced
10mm apart or 1000mm apart.



I should think that many hams have things that can measure 3 ns (1000mm
light time), particularly in a repetitive system. That's one cycle at
300 MHz, or 36 degrees at 30 MHz.


Systems that rely on nulling or matching, with a variable line
stretcher, for instance, can do this fairly well.

For example of a measurement technique, say one put a LED in series with
the turn at one end, and another at the other end, along with enough DC
bias current to make sure they both stay lit, with the RF current
essentially modulating the brightness (Hmm. the LED has parasitic terms,
and you'd need a fast one.. but that's the general idea).

You could then observe the two LEDs with some system that compares the
modulated signal from the two in a nulling arrangement (for instance,
put an optical chopper wheel in front of one light path), then adjust
relative lengths of the optical paths (with a moving mirror).

Or, what about using a H field probe (i.e. something like a Rogowski
coil), fed back to a measurement system using resistive leads (377
ohms/square) that don't perturb the field.

If you have a LOT of RF power available for the test, you could use the
Faraday or Kerr effect to measure the magnetic field too.. Flint glass
has a Verdet constant of 0.11 radians/(Tesla*mm).
Rotation(radians) = V*B*l

Say your probe is 1mm long, and you can reliably measure a rotation of
0.11 radian (5-6 degrees), you'd need a field of 1 T, which is fairly high.
Biot-Savart is B=4piE-7*I/(2pi R) = 2E-7 *I/R
Say your probe is 1mm (1E-3m), to get 1T you'd need 1/2E-4 amps (5kA)..



Anyway... a sufficiently clever amateur probably does have equipment in
their shack that could be cobbled together to make this sort of
measurement, without needing exotic measurement gear.

(Mind you, having a fast sampling scope would make it easy).