Message-ID: <002001c17798$650befe0$8ae086d4@ericadodd>
From: "g3ldo" <g3ldo@ukonline.co.uk>
To: rsgb_lf_group@blacksheep.org
References: <5.1.0.14.0.20011121102933.00a7b730@gemini.herts.ac.uk> <001401c176b3$3c5f8100$b6a1153e@w8k3f0>
Subject: Re: LF: Measuring Q
Date: Tue, 27 Nov 2001 22:49:02 -0000
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Hello Dick,

> I used this method and two other ones to measure the Q of my loading coil.
> The coil can be seen on page 62  and on the cover of the LF Handbook (the
> caption of Fig. 4.17 at page 62 describes the coil on the cover).
>
> With a 47nF 2% capacitor in parallel the coil resonates at 11.56kHz. From
> this follows an inductance of 4033 microhenry (the large value of the
> capacitor eliminates the influence of the unknown self-capacitance of the
> coil). At 137kHz the inductive reactance of the coil is 3470 ohms.
>
> Using Jim's method I found a resistance of 9.17 ohms for the coil. This
> yields a Q of 3470/9.11 = 378.
>
> I also measured the resistance at 137kHz of the series resonant circuit,
> consisting of coil and vacuum capacitor,  using an admittance bridge.  I
> found the resistance to be 9.11 ohms. From this follows a Q of 381.
>
> I finally made a parallel resonant circuit at 137kHz, again using the
vacuum
> capacitor. The bandwidth between the -3dB frequencies was 340Hz. This
yields
> a Q of 137000/340 = 403. To avoid damping of the circuit by the tracking
> generator and selective voltmeter I excited the coil with a single turn
> loop, located 36cm below the bottom end of the coil. I connected a 1:10
> oscilloscope probe to the voltmeter and clipped it on the insulation of
the
> top end of the coil.

Your three Q measurements indicate that your measurement techniques are
sound - unlike mine!
It occurs to me that I could use my HP vector impedance meter to measure Q
using the series resonant method. Unfortunately it only goes down to 500kHz
so cannot measure Q at the operating frequency. If I did the measurement at
a higher frequency would the reactance of the series resonating capacitor be
significant and give a higher value of perceived resistance?
I could also use my 3M bridge (page 86 of LF book), which can measure R at
the operating frequency (136kHz and 73kHz). The resonance curve could then
be plotted using the S/W shown on Fig 6.12.
More experimenting to be done when the weather gets dryer.

Regards,
Peter, G3LDO

e-mail <g3ldo@ukonline.co.uk>

Web <http://web.ukonline.co.uk/g3ldo>