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Date: Thu, 27 Jul 2006 11:48:26 -0400
From: John Andrews <w1tag@w1tag.com>
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Subject: Re: LF: Tapped loading coil design spreadsheet
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Jim,

I knew about the mutual coupling between the turns, but never stopped to 
think about the effect on the upper limit of impedance match. 
Interesting! Please send me the file.

John Andrews, W1TAG

james moritz wrote:
> 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
> 
> 
> 
> 
> 
> 
> 
> 
>