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Message-ID: <ADF84B9BB28548DA8CDFC802B77477FD@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <4CB4C5CC.4000505@iup.uni-heidelberg.de>
In-Reply-To: <4CB4C5CC.4000505@iup.uni-heidelberg.de>
Date: Wed, 13 Oct 2010 03:02:36 +0100
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Subject: LF: Re: Questions and resonating tests and measurements on a 700m wire antenna at 137 kHz
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Dear Stefan, LF Group,

DK7FC wrote:
...>What does these values tell us? I have not yet completely understand the
whole dependency i think. _So i am looking forward to your ideas and
comments!

I think that, as far as impedance measurements on your antenna are 
concerned, the radiation resistance will make a negligible contribution, 
even though the length of the antenna is a substantial fraction of a 
wavelength. So instead of an antenna, it could be modelled as a length of 
transmission line, with either an open-circuit termination (inv-L 
configuration) or a short-circuit (loop configuration) termination. My 
transmission line text book says that Zo = sqrt (Zsc.Zoc) where Zsc, Zoc are 
the input impedances of the line with short and open circuit terminations. 
Using Stefan's impedance values, Zo works out to 729ohms <-33degrees - the 
complex Zo implies a fairly high loss in dBs per wavelength, which is not 
unexpected, and the magnitude of several hundred ohms is not unreasonable 
for a thin wire quite a long way from a ground plane. Further reference to 
the text book allows calculation of the attenuation factor (alpha), which 
gives about 7dB loss over the full length of the antenna. If we assume that 
the propagation velocity is slightly less than c in a vacuum, the phase 
propagation factor (beta) can be calculated too, which eventually gives an 
electrical lenght for the transmission line of about 169degrees, implying 
velocity factor of about 0.85, which is not unreasonable, since a 
significant part of the E and H fields will be in the ground, with a 
relatively high dielectric constant.

One way to check if this is reasonable would be to terminate the far end of 
the antenna with an RC combination equal to Zo, measure the feed point 
impedance - it should be also equal to Zo. The voltage or current at each 
end could then be measured to determine the attenuation.

Cheers, Jim Moritz
73 de M0BMU