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
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