Re: Wire length,resonance, and Q (fwd)
---------- Forwarded message ----------
Date: Wed, 13 May 1998 17:33:24 +0300
From: Harri Suomalainen <haba-at-cc.hut.fi>
Subject: Re: Wire length,resonance, and Q (fwd)
>That's why the so called quarter wave wire length stuff is
>so WORTHLESS. Quarter wave resonance and a physical
>Quarter wave are two different animals.
This 1/4 resonance is often misunderstood matter. Most beginners
think of it as if wire needed to be 1/4 wavelength long. This is
not the case. The coil itself (considered as a transmission line)
needs to be 1/4 wavelenght long!
To know the required *coil* lenght you need to analyze the coil
to know the velocity factor. This is unfortunately dependent on
coil form too (including the lenght). However, once you have
solved that one you can make it resonate at whatever frequency
you want or something else.
The basic difference between these two approaches is huge.
You do not consider electricity flowing in one wire. It does not
do that. It will also travel as electromagnetic waves (in capasitance
of coil and transformer effects). Therefore it is very feasible to
consider the whole thing as a transmission line where waves
travel from bottom to top, reflect back and so on.
Transmission line calculations are described in Vacuum Tube Tesla
Coils (a bad and hard-to-read book, beware!) I've found that this type
of calculation method is close to the truth in practical system too.
(I've measured systems too to compare results. I was able to find
power draw of coil at the predicted frequency etc)
LC-approximations can be valid and usually are fairly valid too. That is
usually enough for most cases.
Whatever you do, forget all about the wire lenght. It is unimportant
in determing the resonant point. Consider the coil as a coil, not a
piece of wire!
Harri Suomalainen mailto:haba-at-cc.hut.fi
We have phone numbers, why'd we need IP-numbers? - a person in a bus