Hello Jim,
Thanks once again for your comments.
Am 25.07.2012 04:28, schrieb James Moritz:
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.
Well i want that the antenna works as good as possible. If is imagine
the transmitter and battery and all the TX stuff as very compact and
floating, then does the position of the feed point make a difference to
the radiation pattern? For example, i simply use the MFJ analyzer as the
TX, floating in the air. Then i think there should be no difference in
the currents. Or am i wrong?
Sure, if i would use 50m RG58 on the ground between antenna and TX then
there should be an unsymmetrical component.
I want to try different feed points anyway and see what the feed point
impedance will be.
Last night i started to prepare the antenna. I want to start with a feed
point at 85% length. The short wire is ready, it is about 47m long. This
shorter wire is connected to the inner conductor of the coax. In
practice i may have a 3m long coax between antenna and TX. Here i
thought that the effect of such a short wire relative to Lambda and its
capacity (wire +TX stuff + battery+key+operator :-) ) against ground,
relative to the high capacity of the longer wire to ground, is negligible.
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.
Why does EZNEC not show "problems" in such an easy antenna configuration?
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.
Isn't that the case when the complete dipole is driven on its resonance
frequency?
These reactances will be quite large compared to the resistive
components - intuitively,
So the BW of the antenna is quite small then. EZNEC showed only a few
kHz BW. Normally i would expect a high BW from a full size and lossy
dipole... I will show it in a table later.
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?
Then i think the resistive component of the wire will be quite high but
due to the losses it will be limited. The question is if the current
distribution will be the same as if i choose the feed point in ther
center of the dipole. And if not, what will the ERP loss be!?!
We all know the extreme of that asymmetric dipole. It is for example the
J antenna which is a dipole driven at high Z at its end. And J antennas
work very well, even in a horizontal arrangement. So the effect of the
very close ground of a MF dipole is the speciality here. It will cause
many losses, that's clear. But we do not expect to have many 10%
efficiency but hopefully 1% or a few %.
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.
Probably you are right once again.
Anyway we will see what happens and as the discussion goes on i am more
and more looking forward to the first results, what ever they are :-)
Best 73, Stefan/DK7FC
PS: From the experiments of VK1SV (which are done on LF!) we see that
something will be possible :-)
Cheers, Jim Moritz
73 de M0BMU
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