Return-Path: Received: (qmail 21181 invoked from network); 23 Apr 2002 09:14:42 -0000 Received: from unknown (HELO warrior.services.quay.plus.net) (212.159.14.227) by exhibition.plus.net with SMTP; 23 Apr 2002 09:14:42 -0000 X-Priority: 3 X-MSMail-Priority: Normal Received: (qmail 8441 invoked from network); 23 Apr 2002 09:14:40 -0000 Received: from unknown (HELO post.thorcom.com) (212.172.148.70) by warrior.services.quay.plus.net with SMTP; 23 Apr 2002 09:14:40 -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 16zxr3-0005lK-00 for rsgb_lf_group-outgoing@blacksheep.org; Tue, 23 Apr 2002 11:49:01 +0100 Received: from cmailenv2.svr.pol.co.uk ([213.218.77.54]) by post.thorcom.com with esmtp (Exim 3.33 #2) id 16zxr2-0005lF-00 for rsgb_lf_group@blacksheep.org; Tue, 23 Apr 2002 11:49:00 +0100 Received: from [62.21.145.56] (helo=w8k3f0.freeler.nl) by cmailENV2.svr.pol.co.uk with esmtp (Exim 3.35 #1) id 16zwK4-0000pn-00 for rsgb_lf_group@blacksheep.org; Tue, 23 Apr 2002 09:10:53 +0000 Message-ID: <5.1.0.14.0.20020423090723.00a20e00@POP3.freeler.nl> X-Sender: FRE0000086604@POP3.freeler.nl X-Mailer: QUALCOMM Windows Eudora Version 5.1 Date: Tue, 23 Apr 2002 09:43:37 +0200 To: rsgb_lf_group@blacksheep.org From: "Dick Rollema" Subject: LF: Measuring loop inductance In-reply-to: <5.1.0.14.0.20020422172434.00a89200@gemini.herts.ac.uk> References: <16z3F2-0asCcGC@fwd00.sul.t-online.com> MIME-Version: 1.0 Content-Type: text/html; charset=windows-1252 Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group Sender: Content-transfer-encoding: 8bit To All from PA0SE

Jim, M0BMU wrote;

One needs to be slightly cautious about this type of measurement - the inductance meter measures at a frequency somewhere in the LF range which depends on the inductance being measured - in general, this frequency will not be the same as the intended operating frequency of the loop. The inductance of the loop is fairly frequency dependent due to distributed capacitance, skin and eddy current effects, so the value at the measuring frequency will not be exactly the same as at the operating frequency. This is likely to give only very small errors for moderately sized, single turn LF loops, but could be serious for multi-turn loops with high inductance and stray capacitance, very large loops, or loops for use at MF or HF. Measurement methods which resonate the inductor at the intended operating frequency would then be highly desirable, although more cumbersome.

I fully agree with Jim.
If I had to measure the inductance of a loop in the LF band I would resonate it at 136 kHz by putting a variable capacitor in parallel, if necessary with extra fixed capacitors added. Only small power is needed; a signal generator or small transmitter will do. As detector for resonance an oscilloscope or VTVM can be used. After resonance is found the capacitor(s) can be measured with a capacitance/inductance meter. The inductance of the loop is then found from:
L = (159.154/f)squared/C, with f in megahertz, C in picofarad and L in microhenry.

The capacitance can be measured at a frequency different from 136 kHz, as a capacitance/inductance meter may do because - at least on LF - there are almost no disturbing stray effects in a capacitor so its capacitance is for all practical purposes independent of frequency.

The loop inductance so found is not the real inductance but the effective one, including stray effects. But that is what you are interested in.

If for some reason you want to know the real value of the inductance the capacitance in parallel with the loop can be increased so much that resonance occurs at a low frequency, something of the order of 10 kHz or so will probably be low enough. The distributed capacitance of the loop - the main disturbing factor - is then so much diluted by the very large capacitance in parallel that it that it hardly effects the measurement.

73, Dick, PA0SE