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
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.
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
the situation under consideration.
(A)If you pull the coil out into a straight wire (all in the mind
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
full length of the wire will it be less ? Ans. of course it
How about two spiral turns? Ans, Yes
Three turns ? Ye-es.
Four ? Five ?
Is there some magic number of turns that the current suddenly
same at the top and the bottom? Ans. Never heard of such a
OK then ,When you have the coil all put back together (in your mind) to
original configuration will there be less at the top than at the
How much difference is another matter.
I always thought of a short loaded antenna as an inductance in
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
it on board a far-away sailboat. When talking about bottom loading of
back-stay antenna with a coil of wire she said "well that's obvious
coil is just a winding up of the rest of the wire which should have been
there in the first place" and ever since I have thought of
as just a winding up of some of the antenna wire. On that basis the
is sure to be less at the top than at the bottom.
----- 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
> loading coil is higher than at the top (aerial side) of the
> 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
> He measures 1.5 A at the bottom of the coil en 0.6 A at the
> My theory is that the "lost current" flows via the
capacitance of the
> to its surrounding (even a metal object in free space has
> The current at the bottom of the bottom of the coil divides between
> 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
> the coil and for the aerial 210 pF.
> But these values do not explain the large difference in current at
> and top of the coil.
> In a transmitting aerial the current increases going from the end of
> radiator towards the coil.
> Now to my question: does this increase in current also occur
> winding of the coil? My feeling is that the current at the
> end of a coil should be the same; apart from the current that flows
> capacitance to the surrounding.
> I also have read that the coil should be considered as an aerial
> length equal to the length of the coil. But on 2 km
> that would be an extremely small aerial, reckoned in
> radiation by the coil must be negligible.
> There are certainly experts on the reflector who know the
> welcome their views.
> 73, Dick, PA0SE