From: Paul
Date: Fri, 23 Feb 2001 09:37:52 +0000
Subject: Re: [TSSP] Problems with high elevation/small radius
Malcolm Watts wrote: > Again, the fact that a piece of wire sitting above the top of the > coil adds significant capacitance is well known. There are > corrections for this cited by Medhurst and others and I have seen it > occurring in old test. It is a remarkable fact that for a particular > coil geometry, a piece of wire as long as the coil itself and > sitting bolt upright on top (and obviously connected) can double > the effective Cself of the structure. Yes, looks like we'll have to take these lead-out wires into account when modeling. I have a tendency to assume that because the surface area is small compared with, say, the coil surface, that the additional C will be negligible. I'll have to take more care to calculate the effects of fittings and lead wires, especially on small coils. Kurt Schraner wrote: > When measuring the little coil, > > sk5b185: bare d=0.051m h/d=8.03 sr=0.91 b/h=0.45 turns=934 > f1 919.5kHz 1027.0kHz +11.7% > > there has indeed been a top end wire of length 27cm floating > around, which surely has influenced the result strongly. OK, if I assume it just sticks out horizontally, its capacitance is about 1.73 pF, and if I add this as a lump into the model, the result for sk5 is sk5b185: bare d=0.051m h/d=8.03 sr=0.91 b/h=0.45 turns=934 f1 919.5kHz 886.4kHz -3.6% which turns the situation around somewhat. > Probably it was just hanging in the air, some 5...10cm distant, > along the outside of the coil - I've not paid attention to it. > At the top end, before pointing away from the coil, the wire forms > one loose turn, with a height of the spiral of 2.2cm. I'll deduct 16cm for the circumference of the extra loose turn, and consider the effect of the remaining 11cm, at an elevation of 62cm, which comes to 0.7pF, with which we get sk5b185: bare d=0.051m h/d=8.03 sr=0.91 b/h=0.45 turns=934 f1 919.5kHz 963.5kHz +4.8% So it appears that we can bracket your measured error. That is encouraging. Does this coil have a similar wire lead at the bottom end, or can we use the trick of measuring it upside down? > You remember, regarding my larger long coil, > > Sk long: bare d=0.161m h/d=8.71 sr=0.89 b/h=0.389 turns=1976 > > I have written (Sun, 04 Feb 2001): > > "The measurements for this coil have been teaching me, how > sensitive to the (-capacitive-) environment such tests > behave: A 0.5m free wiping end of the winding wire, at the top > of the bare coil, was showing large movements of the resonance > peak weaveform-amplitude on my oscilloscope, when just pushed > to vibration by hand." Sorry, I remember reading this, but it didn't register that the wire was attached to the top of the coil. The additional C is around 3pF if the wire was taken out horizontally - a worst case perhaps. The result is a frequency reduction of about 7%. Interestingly for this coil, the measured value is already higher than the model predicts, ie sk16b55: bare d=0.161m h/d=8.71 sr=0.90 b/h=0.39 turns=1976 f1 161.4kHz 156.4kHz -3.1% which means that if we add in the additional C, we could be as much as 10% low in the frequency prediction. Hmm, might have to take another look at this coil. > Soo..., what will be the least errorprone way, to perform those > measurements in the future, presupposed, we want yet being able, > to connect a topload to the coil, meaning wirelength=0 "not > feasible"? Would a relatively short end of wire (i.e. 5cm, in the > sk5b185 case), pointing radially outwards, be acceptable? Well when the topload is actually in place, the additional C of the lead wire surely is negligible, it only really shows up in the bare coil. As to how to account in the model for the top lead wires and terminals on bare coils, I'm just not sure yet - needs more thought but I'm pleased that it offers corrections of about the right magnitude to fix some, at least, of the small radius/high elevation difficulties. > I finally got all components for building Terry's low-Z > amplifier, and hope to do it within the next days. This will > shurely better my future results. I'm sure that will improve matters, and you might be able to find the point of resonance by monitoring the DC input current to the amplifier - tuning for maximum. Cheers, -- Paul Nicholson, Manchester, UK. --
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.