Received-SPF: neutral (gmail.com: 193.82.116.20 is neither permitted nor denied by best guess record for domain of owner-rsgb_lf_group@blacksheep.org)
From: "james moritz" <j.r.moritz@herts.ac.uk>
To: <rsgb_lf_group@blacksheep.org>
Date: Thu, 6 Jul 2006 14:05:10 +0100
Message-ID: <000001c6a0fc$cfc5b040$e6a4c593@RD40002>
MIME-Version: 1.0
Importance: Normal
In-Reply-To: <NKEMLGOEKANPNJOBFBAFAELCCNAA.g4wgt@tiscali.co.uk>
Subject: LF: RE: Loading Coil Q
Content-Type: text/plain;
	charset="us-ascii"
Content-Transfer-Encoding: quoted-printable
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org
Status: O

Dear Gary, LF Group,

The technique I use is like this -=20

Select a low-loss capacitor (polystyrene, silver-mica, polypropylene, =
air
variable, etc) that will resonate the coil near the frequency of =
interest
and connect in series with the coil.

Connect this series L/C across the output of a sine-wave source with a =
known
output impedance (sig gen, function generator), Rs, usually 50R. Monitor =
the
output voltage (using scope, RX + millivoltmeter, SLM, etc.) and adjust =
the
frequency (or capacitance) for minimum output voltage (i.e. series
resonance)

Measure the output voltage at the minimum (Vmin), then disconnect L/C =
and
measure the open-circuit voltage at the generator output (Voc). The
equivalent series resistance of the inductor (RL) is then:

RL =3D Rs.Vmin/(Voc-Vmin)

RL is usually what you want to know, but you can find Q =3D =
(2.pi.f.L)/RL
where f is the frequency.=20

You can also calculate L if you know what C  and f is:

L =3D (1/(2.pi.f.sqrt(C)))^2

If your voltage measurement capability is not very accurate, or you are =
not
sure exactly what the sig gen output impedance is, you can use a
substitution method - note Vmin voltage L/C series resonant as before, =
then
replace L/C with different resistors until you find the value that gives =
the
same Vmin - RL is then the same as this resistor.

If you use a scope or wide-band voltmeter to measure Vmin, errors can be
caused for high values of Q (100 or more) due to harmonics in the output =
of
the signal source. This is fairly obvious on the scope screen. Using a
selective level meter, or the more cumbersome Rx + audio voltmeter,
eliminates this problem. The ideal tool for the job is a selective level
meter with tracking generator - I usually use a W&G SPM6/PS6. I have =
used
this technique successfully from audio to HF, with a wide range of
inductors. If testing a large loading coil, be careful to arrange things =
to
keep the coil well separated from the test gear, large metal objects, =
the
ground, yourself etc., otherwise Q can be substantially reduced by the
losses induced by the coil's field.

Cheers, Jim Moritz
73 de M0BMU