Return-Path: Received: (qmail 6327 invoked from network); 21 Nov 2001 12:50:44 -0000 Content-Transfer-Encoding: 8bit Received: from unknown (HELO murphys-inbound.services.quay.plus.net) (212.159.14.225) by excalibur.plus.net with SMTP; 21 Nov 2001 12:50:44 -0000 X-Priority: 3 X-MSMail-Priority: Normal Received: (qmail 17822 invoked from network); 21 Nov 2001 12:50:44 -0000 Received: from unknown (HELO post.thorcom.com) (212.172.148.70) by murphys.services.quay.plus.net with SMTP; 21 Nov 2001 12:50:44 -0000 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1106 Received: from majordom by post.thorcom.com with local (Exim 3.33 #2) id 166Wf8-0001ct-00 for rsgb_lf_group-outgoing@blacksheep.org; Wed, 21 Nov 2001 12:39:34 +0000 Received: from hestia.herts.ac.uk ([147.197.200.9]) by post.thorcom.com with esmtp (Exim 3.33 #2) id 166Wf6-0001cn-00 for rsgb_lf_group@blacksheep.org; Wed, 21 Nov 2001 12:39:32 +0000 Received: from gemini ([147.197.200.44] helo=gemini.herts.ac.uk) by hestia.herts.ac.uk with esmtp (Exim 3.22 #1) id 166WeO-00010w-00 for rsgb_lf_group@blacksheep.org; Wed, 21 Nov 2001 12:38:48 +0000 Received: from [147.197.232.252] (helo=rsch-15.herts.ac.uk) by gemini.herts.ac.uk with esmtp (Exim 3.33 #1) id 166WdD-00063I-00 for rsgb_lf_group@blacksheep.org; Wed, 21 Nov 2001 12:37:35 +0000 Message-ID: <5.1.0.14.0.20011121102933.00a7b730@gemini.herts.ac.uk> X-Sender: mj9ar@gemini.herts.ac.uk X-Mailer: QUALCOMM Windows Eudora Version 5.1 Date: Wed, 21 Nov 2001 12:38:25 +0000 To: rsgb_lf_group@blacksheep.org From: "James Moritz" Subject: Re: LF: Measuring Q In-reply-to: <001b01c17220$e9cc8820$531686d4@ericadodd> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group Sender: Dear Peter, LF Group, This is one of the well-known techniques for measuring Q; as Rik pointed out, the important thing is to minimise coupling between generator and detector and the tuned circuit under test, to ensure the Q is not being decreased by loading by the test gear. The way to check is to reduce the coupling so the signal amplitude is reduced, say by about half, and measure the Q again - if it is significantly different, there is significant loading. You need to do this with both generator and detector, since either one can cause loading. With very high Q coils, the coupling must be very small; the equivalent parallel resistance of a high Q LF loading coil can easily exceed a megohm, so almost anything actually connected to, or even anywhere near a high potential point in the circuit will really clobber the Q. What you really want to know is the equivalent series resistance of the coil. I usually do this as follows: Connect generator to meter and measure ampliude V1. Then connect a series resonant circuit using the coil to be tested in series with a suitable resonating capacitor across the generator terminals, and tune for a null in meter reading, ie. series resonance, and measure voltage V2. At resonance, the reactance of L and C cancel, and the remaining Rseries forms a potential divider with the paralleled source resistance of the generator (Rs) and load resistance (RL) of the meter. If you know what Rs and RL are, you can calculate Rseries: Rseries = (RsRL/[Rs+RL])*1/([V1/V2]-1) Typically, The meter needs to measure a reduction in voltage of 10 - 30dB, which should not be a problem for a level meter, or a scope. The main problem with this method is that harmonics of the generator signal will not be nulled out, and will produce an increase in apparent Rseries if that resistance is very low. However, this is not a problem if the generator output is clean (harmonics < 1%), or if a selective level meter is used to measure the voltages. It works well for series resistances less than the generator source impedances, which they should normally be. There is no other connection to the junction of L and C, which is the sensitive, high potential point in the circuit, and provided you know what it is, the source and generator impedance does not cause errors. A selective voltmeter with tracking generator is the ideal tool for this job. Once you have Rseries, Q = XL/Rseries, = 2pi*f*L/Rseries The resonant frequency of the coil for the large coils we are using will depend on stray C between windings and connecting leads, so the apparent L will wary with frequency, as will Rseries. So again I agree with Rik and Andy that it is important to measure at close to the desired operating frequency. There will always be variations in stray capacitance between measurement of Q and connection to antenna, so the effective L will be somewhat variable. Also, with large diameter coils, nearby conducting objects will absorb energy from the coil, and again affect Rseries and L. So keep coil as far as possible from ground, metallic objects, walls etc. both when measuring and in use. My 136k Loading coil has 80 odd turns of Decca litz wire on a sectional manhole former. L is about 4mH, and Rseries about 5ohms, making Q around 700. For 73k, another sectional manhole is stacked on top, wound with about 120 turns of 19/0.25 Teflon insulated stranded wire, which gives a total L of about 15mH, and a Q of around 300. The coils are wound in sections, with the total turns divided fairly evenly between the 14 slots on the former The required number of turns wound in to each slot before moving to the next, with the aim of minimising inter-winding C and maximising breakdown voltage, rather like the old-fashioned RF chokes. So G3LDO's Qs of less than 200 suggest either poor inductor performance or Q measurement errors. Having said that, with most combinations of antenna and loading coil, even reducing loading coil losses to zero would only lead to modest 10-20% increases in antenna current, because losses are dominated by the antenna itself. Other effects, like how wet the weather is, will produce similar variations. My main reason for winding big loading coils was to stop the things melting! Q measuring seems to have gone out of fashion in the last few decades - all the major test gear companies have stopped making Q meters, which is a pity because the impedance meters which have replaced them do not cope well with measurements on high-Q circuits. Older textbooks, like Scroggie's "Wireless Laboratory Handbook", discuss Q measurement at some length. Cheers, Jim Moritz 73 de M0BMU