From: "Terrell W. Fritz"
Date: Wed, 14 Feb 2001 19:00:28 -0700
Subject: Re: [TSSP] Top voltage testing
Hi Paul, I did some tests on my small coil. It is not "perfectly well known" but here are the best estimates I have of its parameters: Winding length = 26.125 inches Diameter = 4.25 #24 wire = 0.0201 diameter Turns = 1180? Rdc = 34.0 Ldc = 22.1mH Height off ground plane = 0.35 inches This coil has a fixed terminal stud on the top which will add to the top C a bit. It also has some steel 1 inch diameter washers for support of the stud at the top (should have been split brass...) I calibrated all the measurements against the HP meter using similar amplitude signals. Using the transformer box I get for open top terminal: F1 = 358.3kHz Iin = 0.505912mA Using the Tek 5100 probe on the top I get: F1 = 311.3kHz Iin = 0.20815mA Vtop = 7.9465V I calculate that the H/D is 6.1471 Vtop / Iin = 38168 ohms Zes = 19.5135mH or 88.3% of Ldc Cheers, Terry At 08:25 AM 2/14/2001 +0000, you wrote: >Hi All, > >This might be a good time for some transfer impedance measurements, >since I've been talking about effective series inductance (Les) on >the pupman list, trying to get these ideas into better circulation, >and at this time plenty of solid verification would be most >appropriate. > >Les relates directly to the transimpedance by > > Les = Zft/(2 pi f) > >For large h/d, Les is around 70% of Ldc, eg for Marc Metlicka's >coil #2, > > h/d = 10.11, Ldc = 29.12 mH, Les = 22.3 mH > >Terry's recent measurements support this for a more modest h/d, >where > > h/d = 2.94, Ldc = 75.1 mH, Les = 67.8 mH, f1 = 105.087 kHz > >are the figures for the probed coil, and the last set of measure- >ments were > > Vtop = 53.1V, Ibase = 1.1742 mA, so Zft = 45.2 K ohms. > >From this, the measured Les = 45200/(2 pi 105087) = 68.5 mH, > >which is 1% above the predicted Les, and nearly 8% below Ldc, which >is a comfortable result - Terry's measurements here are accurate >enough to distinguish clearly which inductance is in effect. > >The measurement of top voltage using a regular scope probe does not >seem to be introducing any uncontrolled error, and by adjusting the >probe capacitance representation in the model to match the observed >f1 of the probed coil, the resulting predictions of Les seem to >agree. > >Marc informs me that when coil #2 is loaded by a 10 Meg 11.2 pF >scope probe, the f1 drops from its bare coil value of around 276 >kHz down to 189 kHz. Modeling this I find 13.5 pF of probe C is >necessary to match the depressed f1, which is reasonable. > >For Marc's #2, probed, at 189 kHz, we have > > Ldc = 29.12 mH, Les = 26.2 mH > >As you can see, the effect of the probe capacitance is to increase >Les above the bare coil value (22.3 mH), so that it is now only 10% >below Ldc. A simple way to measure Zft might be to use the >arrangement shown in > > http://www.abelian.demon.co.uk/tssp/tmp/zftmeas.gif > >in which two almost identical probes pickup the top voltage and >the base shunt voltage. The Zft is then just > > Zft1 = Vtop/Ibase = Rs * Va/Vb > >where Va and Vb are the two scope probe readings. These two >readings will be subject to the unknown calibration factors of the >probes and vertical amplifiers in the scope. Therefore the probes >should be interchanged, so the probe B (and scope amp B) now >measures Vtop and probe A measures the shunt voltage. This time > > Zft2 = Rs * Vb/Va, > >and we can then eliminate both of the unknown scope calibration >factors by taking the geometric mean of the two readings, ie > > Zft = sqrt( Zft1 * Zft2) > >from which Les can then be calculated. > >Marc, perhaps you can try these measurements on coil #2, and see >if you get a Zft of around 31K ohms? For Zft measurements, which >are largely independent of Q, the shunt resistor Rs does not need >to be very small, up to 150 ohms would be reasonable. > >This approach disposes of channel-to-channel gain variation of the >two probes, but does not allow for gain error within each channel >occuring when the range is altered, but hopefully that will be >small. > >The reduction of Les below Ldc is disguised to a large extent in >these tests by the effect of the probe capacitance. If we go to >the other extreme and look for a coil for which we expect Les > Ldc >we might be able to obtain a more equivocal demonstration of Les. >If we can find a short, fat coil, equipped with a large toroid, we >might expect Les to be getting on for 15% greater than Ldc. I have >a coil h/d=1.36 and a toroid made from a tractor tyre, so if by >some miracle it ever stops raining for long enough, I'll set it up >outside for a test. My predictions are > > h/d = 1.36, big toroid, Ldc = 40.9 mH, Les = 45.1 mH > >a +10% increase. I've just finished putting together a new 18 sq m >ground plane, made from layers of foil sandwiched between sheets >of thick poly, so I'm desperate for a break in the weather. > >Cheers, > >-- >Paul Nicholson, >Manchester, UK. >--
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.