Owen Duffy wrote:
Roy Lewallen wrote in
:

A very small loop responds less strongly to the electric field than a
very small dipole only within a fraction of a wavelength of the
antenna.

I have seen this expressed as a sensitivity to E and H that imply an
impedance that varies with distance from the antenna, and that it
"bounces around" (that is a technical term, you know) eventually
converging on 120*pi.

Is that correct?

They do converge, but only after one change in slope. More below.

Beyond that, it actually responds more strongly to the electric field
than the dipole does. So at HF, for example, it would be helpful only
in

Roy, accepting that the response of the loop and dipole to electric and
magnetic fields are different close the the antenna, do they not
eventually converge on sensitivity to E and H in the ratio of 120*pi
when immersed in the far radiation field?

Yes.

I don't know if I have put that sensibly.

My understanding was that when placed a very long way from the sources,
neither one had any advantage in response to the desired signal just by
virtue of their type (loop vs dipole).

That's correct.

E/H is the impedance of the field and, close to a small loop, the
impedance is small as expected. (As a receiving antenna, this means that
it's relatively more sensitive to the H field than the E field if the
source is very close.) However, the impedance rises rapidly as you get
farther from the loop, and at a fraction of a wavelength, it actually
overshoots 276 ohms. Then, after reaching its peak, it monotonically
approaches 276 ohms from the high side as you get farther and farther
away. A short dipole acts just the same, but with E and H reversed: the
impedance is high very close to the antenna, then overshoots on the low
side, and from there approaches 276 ohms at a great distance. So at all
points except very close, the impedance of the loop's field is actually
higher than that of the dipole's. In practice, the difference is
negligible except perhaps for a very small region, so they behave
virtually the same for signals coming from any distance of, say, a
wavelength or further away.

You can very easily see this behavior with NEC-2 or EZNEC modeling,
using the near field analysis. The free demo version of EZNEC is adequate.

Roy Lewallen, W7EL