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Date: Thu, 25 Oct 2001 08:46:02 +0100
From: "Claudio Girardi" <in3otd@qsl.net>
Subject: LF: Re: Loading coils and shorted turns
To: rsgb_lf_group@blacksheep.org
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In an attempt to gain some insight on the
behaviour of coils with shorted turns I tried
the mathematical way and this is what I found
(hope it is correct...):

Said
L1 = inductance of the first inductor, R1 = its series resistance (losses), 
L2 = inductance of the second inductor, R2 = its series resistance
k = coefficient of coupling between the two,
(L1 and L2 can be two parts of the same coil or can be two distinct
coils, it doesn't matter)
when the second inductor is shorted the inductance
seen at the terminals of the first is

L1s = L1 * (1 - k^2)

and the losses

R1s = R1 + k^2 * (L1 / L2) * R2

...well, I'm not sure abot what can be inferred from these formulas,
but it seems that the losses will surely increase (and the inductance
obviously will go down) so the Q will be lower with the turns shorted.
How much lower depends (strangely enough, hi) on the amount of turns 
shorted (L2, R2) and on the coefficient of coupling between the coils
(coils or turns spacing). This doesn't take into account any variation
in losses due to different proximity effects / distributed capacitance 
with the second coil shorted.

By the way this suggest a (well known) method to measure the coupling 
of two high Q coils: measure the inductance of the first coil (i.e. L1)
then short the second coil and measure the inductance at the terminal
of the first coil (i.e. L1s), then the coefficient of coupling
k is the square root of (1 - L1s / L1)

Hope this helps...

                       73 de IN3OTD, Claudio
--
in3otd@qsl.net                  http://www.qsl.net/in3otd/

Peter wrote:
> Yes. If you look at the coil design by Dick, PA0SE on the front cover of
> LFEH he uses taps every 10 turns at one end of the coil and a tap every
> single turn at the other thereby having an incremental one tap per turn for
> over the whole coil length. See the description of an earlier version on
> page 62. Dick shorts out the unused turns.
> 
> Another method is to use the G3YMC coupled bucket variable coil. (I like
> this idea of Dave's).  I have used this method using coupled bins on 73kHz.
> 
> Regards,
> Peter, G3LDO
> 
> e-mail <g3ldo@ukonline.co.uk>
> 
> Web <http://web.ukonline.co.uk/g3ldo>