On Fri, 23 Jan 2009 02:26:48 -0000, "christofire"
wrote:


"Jim Kelley" wrote in message
...
Richard Clark wrote:
On Thu, 22 Jan 2009 13:43:19 -0800, Jim Kelley
wrote:

separately the magnetic and electric fields associated with a radio
signal
I perceive that the quote says nothing about "field separation" -

Now that is getting "précis."

73's
Richard Clark, KB7QHC

c'est exact

Do you claim to have separated voltage from current whenever you measure
one or both?

ac6xg


Isn't the point that an electromagnetic wave can be considered in terms of
the E or H fields associated with it, or indeed both at the same time? If
any power is extracted from the wave then this will involve E and H, or
voltage and current, simultaneously. And when the wave encounters a region
of space with effective relative permittivity or permeability different from
the free-space values, the ratio of E to H changes; that is, the intrinsic
impedance, Zo changes locally. The work I described earlier contributed to
the development of propagation prediction methods for medium and long wave
transmissions and an example of a region of space that exhibits a
particularly inductive effect is a built-up city with many tall buildings.

Well, this removes us from the Byzantine parsing between "separation"
and "separately."

If by the addition of these buildings you propose that characteristic
of "space" has become intrinsically different - I suppose. However,
your having made that observation, what of it?

I'm aware of issues involved in claiming generation of separate E or H
fields, as has been described by Kabbary et al in their 'crossed-field
antenna', but surely the issues concerning a receiving antenna are
different?

Ah! We return to -separate- fields as if there were some distinction
in their labels that now devolves to the discussion of the cfa. As
the cfa lacks credibility beyond having an invented nomenclature, this
does not elevate the discussion of -separate- fields to any great
distinction.

A very short monopole attached to a high-input-impedance
amplifier, for example (i.e. an 'active' antenna), should have very little
effect on the local intrinsic impedance, yet it should produce a signal
proportional to the magnitude and sign of the local E field, whatever the
local H field strength.

However? That adverb generally proceeds from a premise and introduces
a counter argument which is not developed here.

"should have very little effect on the local intrinsic impedance?"
This drops into the dialog without any sense of proportion as it is a
qualification and the rest of this lacks quantification. How much is
the effect, how much is little, and more so, how much is very little?
Without quantifiables your impression of "little" may in fact be quite
large and fully expected.

Equally, a small-diameter well-screened loop should
be capable of measuring the local H-field strength without altering the
local Zo.

Should? You write these as mandates, but using weak verbs. This is
the writing of disappointment about vague expectations. Why not use
the verb "must?" Perhaps because you would then be expected to
provide a quantified value instead.

All measurements disturb what they measure. If you cannot express the
degree, you don't have a measurement, it is a guess.

In these cases, 'short' and 'small' are relative to the
wavelength.

Rohde & Schwarz used to sell an HF diversity receiving antenna system based
on an array of small screened loops, the screens of which were applied
(separately) as active monopoles. This provided somewhat separate reception
of the E and H fields associated with the incoming radio wave and, from what
I've heard, it worked - it provided some degree of 'diversity gain'.
However, this was an array requiring a sizeable amount of clear land.

This still does not provide evidence of the supposed "separate
reception of the E and H fields." The performance described is rather
more ordinary and requires no elaboration.

Perhaps the difference is that what I described before was for use with
broadcast signals (following the topic of the OP), in which case short and
small antennas can be used for measurement purposes within areas provided
with adequate (or nearly adequate) field strength, whereas in amateur radio
applications the tendency would be to use as large an antenna as possible,
to maximise the possible range.

Well, this last apologia doesn't even hold up under scrutiny standing
by itself, much less supporting anything that came before it.
"the tendency would be to use as large an antenna as possible,
to maximize the possible range." is another qualified statement in
that "as large as possible" spans many interpretations, and simply
stating it much more simply in terms of wavelength would peg down both
the application and the validity of the claim. Is a tenth wavelength
sufficient to obtain the maximum possible range? How about 10
wavelengths?

The difference between the performance of a tenth wavelength vertical
antenna, and the optimally sized (roughly 5/8ths wavelength) is not
dramatic. Now compare the tenth wavelength vertical to its 10
wavelength distant cousin, and drama unfolds profoundly - subverting
the expectation that larger = maximum range.

73's
Richard Clark, KB7QHC