From: "Malcolm Watts"
Date: Fri, 22 Dec 2000 09:13:19 +1300
Subject: Re: [TSSP] Coils at high elevation
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. 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).
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.
> Ultimately I won't mind too much if we cannot get exact Fres at
> high elevations, so long as we can prove that this is just inability
> to quantify external capacitance well enough. The main worry is option
> (d) which might indicate a fault in determination or employment of
> Cint. Cint seems to produce some interesting phenomena in the coil,
> features of the resonance which are a surprise to me, eg elevated
> current maximum and in some cases a forward transfer impedance higher
> than that of a 'lumped' inductor. For this reason eliminating
> possibility (d) is most important I think.
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).
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).
Regards,
malcolm
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.