To All from PA0SE
I do like G3GVB's kind of reasoning. But I think the situation is
simplified a bit too much. Assume as an example that the coil is
wound on a toroid of high permeability material (e.g. for use with
a QRP transmitter....).
The magnetic flux in the core is proportional to the current in the
winding. When the current increases from one end of the winding to
the other end this would mean thatt he flux in the core would rise as
well along the core. But this is impossible. The flux must be
uniform. So current in must be equql to current out.
Now real loading coils are wound on a tubular former without a core. The
coupling between the turns will not be 100 % as in the toroid
case.
The coupling depends on several factors like the length/diameter
ratio. But some of the current equalising effect due to the flux as
in the toroid must be present.
So I think that the current in the coil indeed rises from the top to the
bottom but not at such a steep rate as when the winding was removed
and stretched into a straight wire.
Hw?
73, Dick, PA0SE
At 13:08 1-3-05, captbrian wrote:
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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