From: Finn Hammer
Date: Thu, 07 Jun 2001 21:46:07 +0200
Subject: Re: [TSSP] Interesting article on Medhurst, Wheeler, modeling,etc.
Paul! A close count revealed that I had 505 turns. Stripped off those 5 turns, the coil now measures 144.5 mm long, and 28.8 ~ 28.9 mH. (1kHz out of the Isotech LCR.) DCR is measured to 87 ohms. Material: 5.5 mm thick PVC Measured on 1 by 1 metre groundplane,(one meter from walls and ceiling) with coax all the way, antenna pickup, i now get: F1 = 359.5 f3 = 839.2 (not so sure about this one, the Grundig generator is real hard to adjust precisely enough to get a stable lissajous pattern, so i skipped the higher modes for now) Are data from these small coils valuable? I would rather move up to larger coils with frequencies that I can count. You mentioned that you need time to code, so perhaps it would be a good idea that you make a wish list of coils that you want data from, then I can make them and we can pick up the thread when I have gotten the generator and counter stuff sorted out, and you have got the code sorted out? Cheers, Finn Hammer Paul wrote: > > Hi Finn, > > Thanks for the info. Up to now the smallest h/d examined in detail is > h/d=1.36, so therefore you are taking us into new territory. > > Finn Hammer wrote: > > > The wire diametre is 0.25mm, and with 20m insulation, it ends up > > occupying 0.29mm per turn. The coil is closewound. > > Gosh, that makes a high spacing ratio, 0.29*500/146 = 0.99!! > You must have a superb coil winding machine! Have you really > got exactly 500 turns? > > The measured inductance, 29.4mH, compares with 28.9mH from > Nagaoka, +1.7%, quite a bit on the high side - we need to resolve that. > Can you take a DC resistance reading? Do you know what frequency the > Iso-tech runs at when measuring the inductance? If not, perhaps you can > observe it on the scope? > > > The generator is 50 ohms. impedance. > > Fine. > > > I had no groundplane under the coil, but have PCB sheets 1 by 1 metre > > square, which I can use. Is that big enough?. > > Do you have four of them, to make a 2m x 2m square? In view of comments > below, this may not be a big issue at the moment, so don't worry too > much about this right now. > > > Is it ok to just pick up the oscillations from an antenna, or > > should I rather measure the voltage drop across a resistor in series > > with the coil at the base? > > Either. It's important to make sure the field pickup is not perturbing > the coil. With plenty of signal from the generator and a sensitive > scope input, you can put the antenna a long way away, with little risk > of upset. Confirm this by taking set of readings, then bring the > antenna in significantly closer and take another set. They should change > by less than say, 0.2%, if the pickup is far enough away. > > If the generator provides insufficient power for a distant pickup > reading, then the base current is a good alternative. With a resistance > in series with the base feed, just tune for a dip in the voltage > downstream of the resistor. > > With careful measurements, you may notice that the frequencies obtained > by tuning for maximum antenna pickup are not quite the same as the ones > measured by looking for maximum base current. > > > Nordmende SRG 389 (2mHz) > > Grundig TG 11 (300kHz) > > > Fluke 87 > > Iso-tech 9053 handheld LCR meter > > > Gould 10000 (20mHz) > > Tektronix 485 (350 mHz) > > OK. > > > By creating lissajous figures on the scope, it is possible to > > determine the relationship btwn 2 frequences. > > That will work fine, providing the Nordmende doesn't drift too fast. > We need the frequencies to about 0.1% accuracy, which should be no > problem for this method. > > > Pls. allow me into the weekend to finish the measurements, I need some > > coax and hardware, and time to take the whole setup out into the > > backyard. > > Well, no rush. These things take their time, and it might take quite a > while to fully understand the behaviour of this coil. We need first to > resolve the discrepancy in the DC inductance. > > Here are some preliminary comparison results for your coil, > > fh1: bare d=0.160m h/d=0.91 sr=0.86 b/h=6.85 turns=500 > measured modeled error = (modeled-measured)/measured * 100% > f1 340.0kHz 361.8kHz +6.4% > f3 725.0kHz 878.5kHz +21.2% > f5 1100.0kHz 1355.9kHz +23.3% > f7 1450.0kHz 1838.2kHz +26.8% > > By comparison, the lowest h/d tested so far (one of my big CW coils) > gives: > > pn1: bare d=0.590m h/d=1.36 sr=0.91 b/h=0.05 turns=356 > f1 150.7kHz 151.5kHz +0.5% > f3 360.0kHz 357.8kHz -0.6% > f5 543.0kHz 538.6kHz -0.8% > > You can see, we have some way to go. Can you let me know how thick the > coil former material is? Do you have available any polyethylene film, > suitable for wrapping several layers around the outside of the coil? > > Incidentally, if I switch off Cint in the model, and re-run the pn1 > coil, we get: > > pn1: bare d=0.590m h/d=1.36 sr=0.91 b/h=0.05 turns=356 > f1 150.7kHz 172.8kHz +14.7% > f3 360.0kHz 617.5kHz +71.5% > f5 543.0kHz 1232.8kHz +127.0% > > from which we can see that Cint is contributing around 30% of the total > capacitance at f1, and considerably more at the higher modes. As h/d > falls lower still, we expect an even bigger proportion of Cint. > > Can you see why I believe the long-range Cint theory? There are no > fiddle factors in the code - it is just Coulombs law and I have no > choice about it, so to get the pn1 readings to be all better than 1% as > they are, would otherwise require a miracle of coincidence! > > Why are your initial readings so far out? Not your measurements - that > is almost certain. Rest assured I have a number of other readings from > similar coils which have much the same sort of error pattern. For > example, two of Malcolm Watts' h/d=1.0 coils: > > mwa1-1hd0: bare d=0.168m h/d=1.00 sr=0.91 b/h=3.94 turns=272 > f1 600.0kHz 639.8kHz +6.6% > > mwa2-1hd0: bare d=0.168m h/d=1.00 sr=0.49 b/h=3.94 turns=272 > f1 605.0kHz 641.8kHz +6.1% > > also exhibit the same offset at f1 (higher modes not measured on these > coils, so diagnosis is limited). > > I can account for around 5% to 8% of the f7 error because the short- > range Cint calculation is not quite right. Shows up more at lower h/d. > We see the errors appear asymptotic to around 30% as the mode number > goes up, which is a sure sign that Cint is being under-estimated by the > model, for some reason. (You can reach f9 on the Nordmende - see if > it's getting on for 1764 kHz, (1764 + 30% = the predicted 2294 kHz). > Cint almost completely dominates the coil's capacitance at these high > modes - so they are important to look at. > > Why does my coil, at only a little larger h/d give a good account, > compared with yours? I'm beginning to suspect that this is due to the > coil former material - the model does not account for its permittivity. > In my coil, 1.5mm thickness of polyethylene is much less, in proportion > to coil length, than is likely for your coil. Lines of E-field > contributing to short-range Cint have to pass inside the material, and > we are not allowing for this. I think this may increase short-range > Cint by up to 50%, which would account for all our problems at small > radius, high elevation, and small h/d. All coils tested with diameter > less than around 20cm are exhibiting this Cint shortfall. > > In view of the initial errors from your coil, probably no need to run > it outside - if the external C was badly wrong, we would see a different > pattern of errors altogether - so stay indoors, at least for now! > > But can you let me know the distance to the nearest wall, and to the > ceiling? > > Anyway, Finn, I'll hope that you write back to say that your coil former > has a really thick wall, and that you can wind another using a > paper-thin cylinder of the same size! Of course, that may be not be > possible, so the plan (b) would be to wrap the outside of the coil in > plenty of plastic, which should - if my theory is correct - make the > higher mode frequencies fall even lower than they are now. That in > itself would give a good indication that the former material cannot be > ignored when computing Cint. > > Cheers, > -- > Paul Nicholson, > Manchester, UK. > --
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.