Message-ID: <500FEC24.9060600@iup.uni-heidelberg.de>
Date: Wed, 25 Jul 2012 14:52:52 +0200
From: =?ISO-8859-15?Q?Stefan_Sch=E4fer?= <Stefan.Schaefer@iup.uni-heidelberg.de>
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References: <500EB973.8080005@iup.uni-heidelberg.de> <3E9653200305422E8F12BCF9447F314F@JimPC>
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Subject: Re: LF: Re: Pretests for the 630m band dipole
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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
>