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Re: LF: Current "lost" in loading coil

To: [email protected]
Subject: Re: LF: Current "lost" in loading coil
From: Dick Rollema <[email protected]>
Date: Wed, 02 Mar 2005 10:53:33 +0100
Cc: "W.F. Oorschot" <[email protected]>
Reply-to: [email protected]
Sender: [email protected]
Hello Rik and others,

William tells me that the measurement of the aerial capacitance was not a very accurate one. So it is probably lower than 210 pF.

The coil sits on a wooden stool of about 50 cm high and not too near a radiator. The coil is wound on a 40 cm diameter PVC pipe and contains 200 m of wire.

73, Dick, PA0SE

At 14:35 1-3-05, you wrote:
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

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