Dear Stefan, LF Group,
...> there will be a real component of
Z=R (=resonance)of 46 Ohm. So i would not even need a transformer to match
the antenna to the PA.
...However, if you wish the antenna to operate in a "dipole" mode, you will
need some way of making sure that the currents into the 2 terminals of the
feed point are equal - otherwise, you will have 2 asymmetrical inverted-L
vertical antennas back-to-back! You could achieve nearly equal currents by
ensuring all the system, TX, power supply, etc., was "floating" and
completely insulated from ground, but the resulting unpredictable RF voltage
between the equipment and the actual ground might be a problem. So a
well-insulated transformer would probably be a necessity, even if not
required for matching.
85% means that one wire is "just" 47m. So if someone lives close to a
forest, there would be a chance to build such a TX antenna and run the
major part of the antenna outside the garden. Just a short rest has to be
inside the own property. For a 90% feed, R would be 96 Ohm. This could be
matched to 50 Ohm very easily and the shorter part of the antenna would
only be 31.5m...
I am sceptical about how this will actually work in practice. The short
section of the asymmetrical dipole on its own will no doubt behave as a
normal inverted-L antenna, and will have of the order of a kilohm of
capacitive reactance, measured between its terminal and a ground connection.
In order to end up with a resistive overal feed point impedance, the longer
section will have to have a corresponding large inductive reactance. These
reactances will be quite large compared to the resistive components -
intuitively, I have my doubts about whether a long piece of wire trailing
through the forest will behave as a fairly high-Q inductor... The concept
must fail at some point, e.g. what happens when you have only 1m of wire in
the short section of the dipole? Having used NEC antenna simulators in the
past, I am not at all sure that they handle the effect of a lossy ground
very accurately.
Cheers, Jim Moritz
73 de M0BMU
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