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I do agree. I did originally say "by
how much is another matter " , because I do not suggest for a moment that the
current proportions are the same when coiled up as when stretched out . The
bottom turn radiates energy so the current entering the second turn is a
little less than that entering the bottom one and so on all the way up. At the
same time the bottom turn is inducing an extra current in the second one to
counteract the effect !! It seems clear that if two coils of equal
inductance were tried then the current reduction would be less with a
close wound coil than with a double-spaced one. We should not be
calling this "Lost current" ;; we havn't lost it, we have radiated it
!.
BTW I think i(out) over i(in) could be
called "Bryan's Ratio"
G3GVB ( Bryan near Brighton ) ;-)
Sent: Wednesday, March 02, 2005 9:41
AM
Subject: Re: LF: Re: Current "lost" in
loading coil
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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