On Tue, 13 Jul 2004 17:57:21 -0500, "Crazy George"
wrote:

OH, for Pete's sake. Loops are sensitive to the H vector. Wires receive
the E vector. Most near field noise tends to be predominantly E field.
But, that seems to only be effective up to 3 or 4 MHz, due to the wavelength
factor, i. e. the near field shrinks as you go higher in frequency. Fully
formed far field wavefronts of noise sources will be just like wanted
signals, and unless some polarization difference is available, then
directivity is the only way to improve S/N. Only in special circumstances
can you see much improvement above 5 MHz due to near field/far field
differentiation.

In the _far_ field both the E and H fields are inversely proportional
to distance and have the 120 pi (377 ohm) relation (impedance) between
the fields. However, in the _near_field_ ( 1 lambda) the 377 ohm
relationship is no longer valid and the magnetic field is inversely
proportional to the square of the distance, while the electric field
is inversely proportional to the cube of distance.

Summarising the graph from an article by Lloyd Butler VK5BR in Amateur
Radio, August 1990: The output voltages from both E and H field
antenna system are calibrated to the same value at 1 lambda (i.e. in
the far field). The antennas are moved closer, when the E and H
antennas are moved to 0.05 lambda, the E antenna delivers 50 dB and
the H antenna 40 dB (relative to 1 lambda) i.e. the H-field is 10 dB
quieter. At 0.005 lambda, the E field antenna output is 110 dB and the
H-field 80 dB, i.e. the H field antenna is 30 dB is quieter.

Thus, with same far field sensitivity, the sensitivity to very local
interference is attenuated considerably when _only_ the H field is
used. However, at 3.5 MHz and 80 m wavelength, 0.05 lambda corresponds
to 4 m and 0.005 lambda to 40 cm, so we are talking about really close
noise sources. At even higher frequencies the number of potential
interference sources is dropping within the 0.05 (or even 0.1) lambda
radius from the receiving antenna, in which the H antenna has an
advantage.

However, on the 135 kHz LF band (lambda 2.2 km), the distances would
be 110 m resp. 11 m, thus much more unwanted interface sources could
be eliminated.

Shielding the H-loop simply prevents the stronger E field from
entering the loop and thus destroying part of the advantage of using
the H-antenna.

Paul OH3LWR