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: [email protected]
[mailto:[email protected]] On Behalf Of John Andrews
Sent: 27 July 2006 14:58
To: [email protected]
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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