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To: rsgb_lf_group@blacksheep.org
Date: Thu, 6 Jul 2006 08:17:29 -0400
Message-ID: <20060706.081730.428.0.CW4EVER4@juno.com>
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From: R obert H Reif <cw4ever4@juno.com>
Subject: Re: LF: Loading Coil Q
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Hello Gary,
      Just a few additional thoughts on the antenna loading and loading
coils.  There is a way to measure the Q of a coil that involves
resonating the coil with a "good" capacitor and coupling in and out with
a much smaller capacitor.  Then you tune a signal generator connected to
one of the coupling caps and look at the voltage across the other
coupling cap.  I use 50 ohm instruments but a scope can be used.  The
idea is to measure the 3 dB bandwidth of the tuned circuit.  And the
circuit Q is the Center Freq. / 3dB BW.  The Coil Q is greater than the
measured Q depending on the Q of the capacitor and the degree of the
coupling. But the intent is to keep the coupling very low.  The problem
is the bandwidth is very narrow so you need a very stable signal source
you can measure the frequency of very closely.  A Q of a thousand is only
136 Hz bandwidth.  So you see the problem.  
     This method was described by Wes Hayward in the 1977 classic, "Solid
State Design" by Hayward and DeMaw,  ARRL Pub.  I think this is out of
print.  The technique is described on page 240.  He states that with a 50
ohm source and load the voltage loss due to the small coupling caps,
which are equal, input to output should be 30 to 40 dB.  ( a voltage
ratio of 30 to 100)  With high Q coils like what should be used for LF
loading the higher this ratio the better.  It is also a good technique to
measure coil inductance assuming you have a know capacitor. Then just use
the formula for resonance.  Again the cap should be high Q (low loss)
because you are really measuring both the coil and capacitor.  The loss
resistances of the coil and capacitor are in parallel. If you know the Q
of the cap you could remove it for the Q measurement.   
     Also on the antenna effectiveness, you might consider putting the
loading coil at the top of the antenna.  This is a messy thing to do with
all the problems about tuning and weather proofing and supporting etc,
but it can raise the radiation resistance.  But probably the best thing
you could do is put down more copper in the ground system.  That is where
your loss is.  But I'm not telling you anything there.  But it is still
the most important part of the antenna at these frequencies.  Good luck
and 73  Bob  W1XP

On Thu, 6 Jul 2006 10:38:18 +0100 "Gary - G4WGT" <g4wgt@tiscali.co.uk>
writes:
> Hi All,
> 
> Thanks to all for additional comments on the previous thread 'LF 
> Loading
> Coils'.
> 
> Continuing on the theme of loading coils, is there a simple method 
> of
> calculating the 'Q' of an LF loading coil without using a 'Q' meter 
> (a
> device I have not got access to). I would like to carry out my own
> tests/experiments on my antenna system, it can only add to my 
> knowledge
> base.
> 
> 73
> 
> Gary - G4WGT.
> 
> 
>