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Re: LF: Re: Current "lost" in loading coil

To: [email protected]
Subject: Re: LF: Re: Current "lost" in loading coil
From: Dick Rollema <[email protected]>
Date: Wed, 02 Mar 2005 10:41:15 +0100
Cc: "W.F. Oorschot" <[email protected]>
In-reply-to: <[email protected]>
References: <[email protected]> <[email protected]>
Reply-to: [email protected]
Sender: [email protected]
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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