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Message-ID: <B85ADB2D0072444EB341FF73A7A20326@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
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Date: Thu, 3 Feb 2011 19:22:15 -0000
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Subject: LF: Re: Earth-mode or not earth-mode
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Dear Roger, LF Group,

Somewhere, G6ALB is applying some AC voltage to some conductor, and some AC 
current is flowing as a result, down some collection of cables and pipes. At 
your receiving site, there is presumably some nearby conductor, maybe on of 
the original ones, maybe a different one, in which some small fraction of 
G6ALB's current is flowing. This conductor will have some impedance level 
associated with it, so there will be some AC voltage present on the 
conductor too. The current will produce some AC magnetic field in the 
vicinity of the conductor, and the voltage difference between the conductor 
and surroundings will result in some AC electric field - you can't have one 
without the other. Some currents will also be conducted/induced in the 
ground, which has some conductivity, and in other conductors in the 
vicinity, so there will also be some distribution of currents, electric and 
magnetic fields associated with these. I have used the word "some" a lot, 
because all these things, although they surely exist, are rather indefinite 
in their actual nature, so it is hard to know which are significant and 
which not.

It appears your receiver can detect the electric field produced by G6ALB's 
signal which is somehow reaching you, but the magnetic field is hard to 
detect. Why this is so is unclear from the description you have given - it 
may be because the mode of propagation favours the E-field, or it may be 
that local noise sources have a stronger H field component. To do so 
directly, you could calibrate the antennas, but this is quite a lot of work. 
The experiment Stefan descibes, comparing G6ALB's signal with the relatively 
well-defined Alpha beacon signal, would be another way to shed light on 
this. E.g., if G6ALB was down 20dB on the Alpha signal on the E field 
antenna, but 40dB down on the loop, then it would indicate a 10x higher E/H 
ratio than for "normal" far-field propagation, and a predominantly 
high-impedance, "voltage" kind of propagation for G6ALB. It is impossible to 
say what the non-result for the earth electrode antenna means; I would guess 
such an antenna would respond to E, H and J to some degree, but without any 
detection of a signal there is no basis for comparison.

Another experiment that would be useful to do would be to track down exactly 
what utilities the signal was being propagated along. You could use your 
receiver and antennas /P in a "cable tracker" mode. You would then have an 
idea whether the E-field was generally favoured, or whether this was 
peculiar to your QTH.

Cheers, Jim Moritz
73 de M0BMU