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From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
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Date: Thu, 12 Jan 2012 13:04:18 -0000
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Subject: Re: LF: Tuning inductance for 137kHz receive.
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Dear Martin, LF Group,

The simplest type of variable inductance I have used was permeability 
tuned - It consisted of a coil of about 150 turns wound on a plastic tube 
about 60mm diameter, with a "tuning slug" consisting of the ferrite core of 
a defunct SMPSU transformer that just fitted inside the tube. The tube was 
mounted horizontally, so the core just rested in place with no additional 
means of holding needed. The core was taped to a plastic ruler which 
provided a handle to slide it in and out, and also a convenient tuning 
scale!

This arrangement gave an inductance variation of a few hundred uH when 
connected in series with the main loading coil, and worked quite well with a 
few hundred watts of TX power. A fairly large core was needed, and a fairly 
small fraction of the total inductance, in order to keep heating of the 
ferrite and coil losses to a reasonable level. This would not matter for 
reception, where you could use a much smaller coil with higher inductance. I 
would suggest adapting an LW ferrite rod antenna so the rod can slide in and 
out, with additional fixed inductance as needed.

For receive only, Alan's suggestion of a variable capacitor works fine. 
Voltage rating is likely to be a problem on transmit. You can use a larger 
inductance, with the capacitor in series with the antenna to reduce the 
overall capacitance. Or you can connect the variable capacitor in parallel 
between antenna and ground, and use a smaller value of inductance.

Cheers, Jim Moritz
73 de M0BMU