Date: Tue, 01 Mar 2005 12:34:58 +0100
From: Dick Rollema <dickrollema@casema.nl>
To: LF-Group <rsgb_lf_group@blacksheep.org>
Cc: "W.F. Oorschot" <pa0wfo@chello.nl>
Message-id: <6.1.0.6.2.20050301115700.03662eb0@mail.casema.nl>
MIME-version: 1.0
Subject: LF: Current "lost" in loading coil
Content-type: text/plain; charset=us-ascii; format=flowed
Content-transfer-encoding: 8bit
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org

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