Re: surge impedance ?

From: 	Greg Leyh[SMTP:lod-at-pacbell-dot-net]
Sent: 	Monday, August 25, 1997 3:31 AM
To: 	Tesla List
Subject: 	Re: surge impedance ?

Tesla List wrote:
> From:   Psychology div.[SMTP:Jason.judd-at-anu.edu.au]

> can someone please explain surge impedance and its importance. I have been
> heaing a lot about it and it seems quite an important thing to take into
> account.

This isn't an eval for your graduate thesis, is it? 8)

Surge impedance is actually a term borrowed from the power industry,
and refers to the effective line impedance that a power xmsn line has
to ground.  Over a long distance, a power xmsn line behaves much like
a 300 ohm twin-lead cable -- in fact a typical 'surge impedance' for
a cross-country power line is about 400 ohms.  This 400 ohm value is
useful for determining what the surge current will be at one point on
the line if there is a fault at another point.

In radio frequency work, the equivalent value for a circuit is called
the 'characteristic impedance' (Zchar).  For a tank circuit, Zchar is
simply the sqroot of (L/C), independent of frequency.  This is useful
in determining the Q of a tank circuit, as Q = Zchar/Rloss.  Since Q
directly determines the losses in a TC, it is something that all TC'ers
should keep an eye on when designing the primary circuit.

The optimum Zchar is often a topic of debate, since there is an
unpleasant tradeoff:

A Zchar that is too low reduces the coil efficiency
A Zchar that is too high reduces the output voltage

You will find that everyone has their own idea of the best Zchar, also
referred to as 'surge impedance', or less directly as a special number 
of turns (at a given frequency of course -- Zchar = 2*PI*Fres*Lpri).