Message-ID: <3BFC1300.B9B3B91B@att.net>
Date: Wed, 21 Nov 2001 15:48:00 -0500
From: "Dexter McIntyre W4DEX" <dmcintyre@att.net>
Organization: JDM Communications
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To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: RE: Measuring Q
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Can the -3db bandwidth measurement of the complete antenna system be
used to determine the Q of the coil?

Dexter


Talbot Andrew wrote:
> 
> If you use a larger capacitance resonance will naturally be at a lower
> frequency so you will be measuring the Q there instead.  Selectivity will be
> higher, but when the bandwidth at the 3dB points is divided into the centre
> frequency the Q will still come out at the right value, albeit at this new,
> lower, freq.
> 
> Incidently, when measuring low Qs you should use the geometric centre
> frequency in this calculation rather than the mid point between the two 3dB
> points.  ie.   Fc = SQRT(Fu * Fl)  where Fl and Fu are the 3dB frequencies.
> It won't make much difference when the Q is in the hundreds, but will be
> noticable when used on small coils.
> 
> Andy  'JNT
> 
> > Is this the right way of going about it? If I use a larger
> > capacitor across
> > the coil won't this give a sharper selectivity and improved
> > apparent Q?
> >
> > Regards,
> > Peter, G3LDO
> >
> > e-mail <g3ldo@ukonline.co.uk>
> >
> > Web <http://web.ukonline.co.uk/g3ldo>