[time-nuts] GPS-Attached Equipment Lightning Protection

Didier Juges didier at cox.net
Sat Mar 1 10:43:54 EST 2008


 

> -----Original Message-----
> From: time-nuts-bounces at febo.com 
> [mailto:time-nuts-bounces at febo.com] On Behalf Of SAIDJACK at aol.com
> Sent: Saturday, March 01, 2008 2:25 AM
> To: time-nuts at febo.com
> Subject: Re: [time-nuts] GPS-Attached Equipment Lightning Protection
> 
> 
> Yes, this is the case, but it's easy to induce a couple 100 
> Volts into  the center conductor through the exposed elements 
> of the antenna. 
>  
...
>  
> bye,
> Said
> 

This is due to a property known as "transfer impedance". Transfer impedance
is the ratio of voltage induced (diffferentially) on the center conductor of
a coax cable versus the current flowing through the braid (externally).

Transfer impedance is solely driven by shield/braid and termination
(connector) quality and has nothing to do with the characteristic impedance
of the cable. 

The effects of poor transfer impedance are similar to poor shielding and
usually related. Signals get in the cable from the outside. The remedies
against that are the same: better shield and better connectors.

It is seldom, if ever, specified when you buy a piece of cable. Cables with
solid outer conductor, such as semi-rigid cable, have much better (lower)
transfer impedance than woven braid cable for instance.

Didier KO4BB

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