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Message-ID: <7D653C9C42F5D411A27C00508BF8803D06A1AC9C@mail.dstl.gov.uk>
From: Talbot Andrew <ACTALBOT@mail.dstl.gov.uk>
To: rsgb_lf_group@blacksheep.org
Date: Thu, 27 Jul 2006 15:43:23 +0100
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Subject: RE: LF: Tapped loading coil design sprea	dsheet
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Status: O

And if the two coils were infinitely coupled,  you would effectively have an
auto transformer matching the transformed parallel equivalent resistance of
the antenna to the desired load impedance.

I've always been fascinated by tapped and link coupled air wound coils, and
the sheer number of variables needed to try to design them. Building my 700
Watt 137kHz switch mode PA, an overwinding on the tank for the overload
protection circuitry (Decca Tx style) had to have a transformation ratio of
equal to the loaded Q of the tank, ie about 5:1.   I ended up needing a
final turns ratio of around 3.5:1 for the 5:1 transformation, suggesting the
close overwinding gave a coupling coefficient > 0.5.  the tank coil was
close wound with Decca Litz wire, 100 turns in three layers so it was a lot
more compact than any antenna loading coild would be.

Andy  G4JNT
www.scrbg.org/g4jnt/


-----Original Message-----
From: owner-rsgb_lf_group@blacksheep.org
[mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of james moritz
Sent: 27 July 2006 15:20
To: rsgb_lf_group@blacksheep.org
Subject: RE: LF: Tapped loading coil design spreadsheet

Dear John, LF Group,

This design approach would work fine if there was no or negligible coupling
between the portion of the coil "below" the tap, and the portion "above",
i.e. if the loading coil was divided into two separate coils at the tap
point. Then you would indeed have an L network as you describe. This would
be approximately true with a "long, thin" coil, with large length/diameter
ratio and well-spaced turns. The limitation is that it can only match to
antenna resistances less than the TX output resistance. But the important
difference in the amateur type of tapped loading coil is that it has
substantial coupling between the two sections of the coil, and the resulting
mutual inductance allows an antenna resistance substantially higher than the
TX output resistance to be matched.

Cheers, Jim Moritz
73 de M0BMU

-----Original Message-----
From: owner-rsgb_lf_group@blacksheep.org
[mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of John Andrews
Sent: 27 July 2006 14:58
To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: Tapped loading coil design spreadsheet

Jim,

I have always viewed tapped loading coils as a high-pass "L" network, where
the upper part of the coil leaves a small amount of capacitive reactance,
representing the "C" in the network. The shunt inductor is the part of the
coil below the tap. The issue is then to design the coil for the inductances
above and below the tap. In the couple of these calcs I have done, I had
accurate R+jX readings of the antenna in advance. That may be more difficult
for amateur work.

John Andrews, W1TAG


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