[time-nuts] 75Z vs 50Z for GPS receivers

[Dave Brown]

----- Original Message ----- 
From: "Bill Hawkins" <bill at iaxs.net>
To: "'Discussion of precise time and frequency measurement'" 
<time-nuts at febo.com>
Sent: Tuesday, January 30, 2007 9:34 AM
Subject: Re: [time-nuts] 75Z vs 50Z for GPS receivers


> Dave Brown said, in part,
>
> "The characteristic impedance of a transmission
> line, in purely general terms, is given by the square root of R plus
> jw L divided by G plus jw C, with the usual meaning for symbols 
> used."
>
> Well, I know most of those symbols, but what's gravity (G) doing in
> that equation? By dimensional analysis, G must be conductance, so we
> have Ohms^2 under the root, and the answer is in Ohms.
>
> So, what is R the resistance of, and what has conductance G?
>
> Bill Hawkins



Bill

R, L, G and C are the 'per section' basic transmission line constants. 
You will find these symbols used in many of the texts on transmission 
line theory. The whole idea of the model is to have series and shunt 
reactive and loss (i.e. resistive) components. So R is the series 
resistance, L is the series inductance, C is the shunt capacitance and 
G is the 'shunt' leakage conductance.  G is just the reciprocal of the 
effective ac shunt (leakage) resistance per basic line section and 
accounts for dielectric loss.  It 'may' also have a DC component - if 
the transmission line has poor insulation between conductors. As you 
would expect,  R also is frequency dependent, although in a simple 
model, such as this, it is assumed constant.

Regards,
DaveB, NZ