From: Paul
Date: Fri, 22 Dec 2000 09:32:52 +0000
Subject: Re: [TSSP] Coils at high elevation
Malcolm Watts wrote: > > Hi all, > I have my new signal generator up and running now. It has a > source impedance of 0.5 Ohms resistive at its terminals compliant to > about +-0.5A at 5Vpp. Indicators on the front panel show if current > limiting (and hence internal impedance rising) is occurring. I did a > couple of quick and dirty tests on a couple of smallish resonators > and found no current limiting at resonance at full generator output > voltage. Not too surprising if you consider that Zin = Zo^2/Zout and > Zout is Q limited. Yes, I don't think any TC base will have Z low enough to active your current limiting! With such low output Z you will not need to pad the base connection. > It would be interesting to know what forcing the > base to that kind of impedance does to the shunt impedance. Does it > make Zout reach for the stars? (rhetorical). The impedance transformation will apply as you say. The output impedance will be described in a later section (still to come) of pn2511. > On 21 Dec 00, at 9:47, Paul wrote: > > > Yes, lets forget Zin for now. Maybe a very thin wire feed direct to > > base would be less invasive than a coupling winding from the point of > > view of capacitance? Tune for minimum Z magnitude or max current on > > the feed rather than introduce a probe? > > I was thinking about this when I wrote the piece above. Reading some > of the posts about proposed setups makes me think that the ideal > indoors situation would be anechoic at RF so reflections were > eliminated. However, care would have to be taken to minimize and > quantify proximity effects contributing to capacitance. Yes, we have one of these chambers at work for RF testing. I think it would approximate a very lossy cavity at the low frequencies used by a TC, since in normal use (at VHF, UHF) there are several wavelengths between test article and walls and the walls act as a matched termination for the far field. At low frequency I think the walls would provide a high R return path for displacement currents. Maybe I'll have to book the chamber for a night - its 30 miles away but I can drive the chamber and instruments remotely from here. > Anyway I am poised to do a bit of measuring in the new year and it > could be done outdoors. Unfortunately the ground is very dry now and > unlikely to improve in the near future (sad times ahead for the > garden). I don't think high ground resistance will detract from what we want to determine here. Dry ground - I've forgotten what thats like :) > A coil which should be measured is my h/d=1 litz secondary (6.6" > dia) which should be about the best quality you can get in a coil > that size. I will discuss exact measurement conditions when I am > ready to forge ahead. From memory, total inductance is around 350uH > (again, to be measured accurately - to be followed up as Tesla would > say). You'll only need to be a couple of feet off the ground with such a small coil. The h/d=1 will emphasise Cint which is desirable for this test. Regards, -- Paul Nicholson, Manchester, UK. --
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.