I am not an expert at all but a feeling for the answer can often be found by
taking a predictable answer to a simple example and moving progressively to
the situation under consideration.
(A)If you pull the coil out into a straight wire (all in the mind of
course ) will the current be less at the remote end than the close end ?
Ans. Yes of course
If you make a single spiral turn of the original diameter but stretching the
full length of the wire will it be less ? Ans. of course it will.
How about two spiral turns? Ans, Yes
Three turns ? Ye-es.
Four ? Five ?
Is there some magic number of turns that the current suddenly becomes the
same at the top and the bottom? Ans. Never heard of such a thing
OK then ,When you have the coil all put back together (in your mind) to the
original configuration will there be less at the top than at the bottom?
Obviously yes!!
How much difference is another matter.
(B)
I always thought of a short loaded antenna as an inductance in series with
the capacitance of the "whip" to make a resonant "acceptor" circuit but I
was coaching a nurse for her american radio amateur exam so she could use
it on board a far-away sailboat. When talking about bottom loading of a
back-stay antenna with a coil of wire she said "well that's obvious , the
coil is just a winding up of the rest of the wire which should have been out
there in the first place" and ever since I have thought of bottom loading
as just a winding up of some of the antenna wire. On that basis the current
is sure to be less at the top than at the bottom.
G3GVB
----- Original Message -----
From: Dick Rollema <[email protected]>
To: LF-Group <[email protected]>
Cc: W.F. Oorschot <[email protected]>
Sent: Tuesday, March 01, 2005 11:34 AM
Subject: LF: Current "lost" in loading coil
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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