TSSP: List Archives

From: Finn Hammer
Date: Wed, 06 Jun 2001 21:39:42 +0200
Subject: Re: [TSSP] Interesting article on Medhurst, Wheeler, modeling,etc.

Poul, all!

The wire diametre is 0.25mm, and with 20m insulation, it ends up
occupying 0.29mm per turn. The coil is closewound.

The generator is 50 ohms. impedance.

I had no groundplane under the coil, but have PCB sheets 1 by 1 metre
square, which I can use. Is that big enough?.

>From your other post today, I got an idea of how to measure the coil,
but one thing is uncertain still:

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?

My instrumentation includes:

function generators:

Nordmende SRG 389 (2mHz)
Grundig TG 11 (300kHz)

Meters:
Fluke 87
Iso-tech 9053 handheld LCR meter

scopes:

Gould 10000 (20mHz)
Tektronix 485 (350 mHz)

It bothers me some that I don`t have a counter that goes high enough to
follow the function generator home on the higher modes, but got to think
of a method described in:
Crawhurst: Audio Measurements.
By creating lissajous figures on the scope, it is possible to determine
the relationship btwn 2 frequences. So by doing that, I could get a
frequency count on a lower frequency than the one exciting the coil,
(and within the range of the fluke) and then multiply with the
appropriate factor. I always wanted to try that, but never had a reason
before now, so this evening I spent getting the hang of that, which
included having to pull the nordmende apart and lubricare the idler
wheels of the string that mooves the dial. It is an accurate method, but
also hard to get the generators into sync.

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.

Cheers, Finn Hammer

Paul wrote:
> 
> Finn Hammer wrote:
> 
> > I took this to mean that too few measure, and that there
> > is a lack of measurements on short coils, so I wound a 500
> > turns coil, 160 mm in diametre and 146 mm long. It measures
> > 29.4mH on my handheld LCR meter.
> 
> Thanks Finn. The h/d=0.91 is just a very nice size for this
> kind of test.  Well into the region where Cint begins to be
> very prominent,  and therefore just the right choice!  I'd
> like to take a real close look at this coil.  Do you have
> any sort of ground plane beneath it?  A foil sheet perhaps?
> 
> And can you let me know the wire gauge?
> 
> > Sine generator to base of coil, sitting one metre up from
> > the floor on a 160 mm sewer pipe (pvc), scope probe dangling
> > 1/2 meter from the coil, with a cliplead as an antenna clipped
> > to the tip of the probe. I was close to the coil too, about one
> > meter, but long enough to not make a difference if I moved
> > around.
> 
> OK. What's the output impedance of the generator?  How does
> it connect to the base - coax?  Where the does the generator
> get earthed?  Shouldn't be too critical since we're not
> interested in the Q factor.  We just need to make sure
> that there is a well defined ground surface dominating the
> area around the coil, and that the signal ground of the
> generator connects directly to this through the coax screen.
> 
> > Frequences at which the scope trace roze high:
> > 340 kHz (measured with fluke 87 as counter,
> >          it will not measure beyond 650kHz)
> > 725 kHz
> > 1100 kHz
> > 1450 kHz
> 
> How were the higher frequencies measured, from the generator
> dial?
> 
> > Sorry guys, perhaps there is a description of a good measurement
> > procedure I could follow?
> 
> We'll figure it out as we go along!
> 
> > I`m here to learn, and to help provide some of that experimental
> > evidence.
> 
> Just what we need Finn, and you've come up with just the
> right coil I think.  Looking forward to working with it - its
> going to be an interesting one.  You'd better let us know what
> instruments you have at your end.
> 
> Cheers,
> --
> Paul Nicholson,
> Manchester, UK.
> --


Maintainer Paul Nicholson, paul@abelian.demon.co.uk.