"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.

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

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.

Chris