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From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
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Date: Tue, 16 Aug 2011 19:17:59 +0100
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Subject: LF: Re: ferrite coil
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Dear Mal, LF Group,

>Instead of winding a tight coil on to a ferrite rod to get 270 uH and 
>tuning with a 250 p variable capacitor...

The required tuning capacitance would be about 5000pF.

>...instead wind an air core coil and move the ferrite rod into/out of coil 
>for the required inductance necessary for frequency >of interest. This 
>would further reduce losses.

You could tune inductance downwards in this way, but unfortunately any 
reduction would be accompanied by reduced magnetic flux due to the received 
signal in the antenna winding. So the signal amplitude would be reduced. 
Also, I doubt if losses actually would be lower - certainly, the coil 
without the ferrite rod will have a much lower Q than with the rod in place, 
so a point must come as the rod is withdrawn where Q decreases rapidly.

Cheers, Jim Moritz
73 de M0BMU