Message-ID: <3BFBF830.25EA2595@netscapeonline.co.uk>
Date: Wed, 21 Nov 2001 18:53:36 +0000
From: "gii3kev" <gii3kev@netscapeonline.co.uk>
Organization: Netscape Online member
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To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: Measuring Q
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g3ldo wrote:

> Recently I described some experiments with multilayer coils based on a Decca
> coil.
> The resultant coil is now similar to that shown in the LF book, page 13 (See
> the second coil back) except my coil has two layers, double wound. I had
> been advised by some of you that the Q of a multi-layer coil is low compared
> with a single layer solenoid. The question is just how much lower is the Q
> and how significant is it in practice.
> The coil is at this moment being tested with a full kW on a T/A test and I
> get just as good results as I did with the single solenoid although the
> single solenoid was a much smaller coil wound with thinner wire.
> To progress I need to measure Q.
>
> It is quite surprising how little  there is in general Ham literature on
> measuring

Hi OM
Like I said many times before , one has to rely on past knowledge and
experience
themselves, and on LF learn from commercial operators who have done it
all
before. I have not found anything useful in the COMIC, in fact articles
published predicted that only a couple of miles or less if you were
lucky could
be covered on 73 and 136 khz. Others thought different and have proved
the
point.
For experienced proficient radio operators all that is required is a
good
antenna suitable for the frequency, good operating procedures and be
expert at
CW and appropriate rf output for the distance to be covered.
The coil to which you refer seems to have a very low Q. With 3mm litz
wire on a
suitable former I would expect at least twice the value you quote.
G3KEV


>
>  Q. I found something in the 'ARRL Electronics Data Book' and Wes Hayward's
>  'Radio Frequency Design', although the latter is related to filters and
>  measuring loaded Q.
>
>  I made a test rig up using a signal generator (with frequency counter) and
>  the Pegelmesser D2155 level meter set on wide band.
>  I connected the coil under test with a parallel air-spaced capacitor to the
>  signal generator via a variable capacitor. I coupled the coil to the level
>  meter with with just a clip on the insulation of the coil. I reduced the
> coupling to the signal generator with the variable capacitor to minimum
> while maintaining a good signal on the level meter.
>  The parallel capacitor caused the coil to resonate at 182kHz . Using the
> 3dB B/W points (down from resonance) on the meter I measured the Q as 185.
> With the variometer in the coil set to minimum the coil resonated at 209kHz
> and the measured Q was 167.
>
> 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>