Message-Id: <5.1.0.14.0.20050301141747.02623b70@u0019445.kuleuven.be>
Date: Tue, 01 Mar 2005 14:35:48 +0100
To: rsgb_lf_group@blacksheep.org
From: Rik Strobbe <rik.strobbe@fys.kuleuven.ac.be>
In-Reply-To: <6.1.0.6.2.20050301115700.03662eb0@mail.casema.nl>
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Subject: Re: LF: Current "lost" in loading coil
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Hello Dick,

as you noticed there is always a small "current loss"in the loading coil, 
due to capacitive coupling to the surrounding.
Something in the order of 5..10 % seems normal to me. By the way, this 
current loss is not equal distributed over the coil but is larger in the 
upper part ("hot part") of the coil.
I am a bit fussed by the values of the loading coil and antenna 
capacitance, normally these should result in a resonance at 137 kHz. But 8 
mH and 210 pF give resonance at 123 kHz. Or the other way around : to bring 
a 210 pF antenna to resonance one would need only 6.4 mH. Also the 
capacitance of the coil to the surrounding (150-200 pF) seems very large to 
me.
All this might indicate that the coil is placed directly on the ground or 
very close to a large conducting (metal) object.
If that is the case I would suggest to place the coil at a certain distance 
(1..2 m) from ground / other objects and try again.

73, Rik  ON7YD

At 12:34 1/03/2005 +0100, you wrote:
>To All from PA0SE
>
>Several amateurs have found that the current at the bottom end of the 
>loading coil is higher than at the top (aerial side) of the coil.
>In my station the difference is of the order of 10%.
>
>William, PA0WFO, has a large coil of 8 mH and a 23 m long wire as aerial. 
>He measures 1.5 A at the bottom of the coil en 0.6 A at the top.
>My theory is that the "lost current"  flows via the capacitance of the 
>coil to its surrounding (even a metal object in free space has  capacitance).
>
>The current at the bottom of the bottom of the coil divides between the 
>capacitances of  coil and  aerial.
>
>I suggested to William he  measure the capacitance of the coil and of the 
>aerial. For the coil he found 150 - 200 pF, depending upon the position of 
>the coil and for the aerial 210 pF.
>But these values do not explain the large difference in current at bottom 
>and top of the coil.
>
>In a transmitting aerial the current increases going from the end of the 
>radiator towards the coil.
>
>Now to my question:  does this increase in current also occur in the 
>winding of the coil?  My feeling is that the current at the beginning and 
>end of a coil should be the same; apart from the current that flows via 
>its capacitance to the surrounding.
>
>I also have read that the coil should be considered as an aerial with a 
>length equal to the length of the coil.  But on 2 km
>that would be an extremely  small aerial, reckoned in wavelength.  So 
>radiation by the coil must be negligible.
>
>There are certainly  experts on the reflector who know the answers.  I 
>welcome  their views.
>
>73, Dick, PA0SE
>