[time-nuts] Data collection cycle -- multiple sources to one counter?

David Kirkby david.kirkby at onetel.net
Sun Jun 19 09:37:21 EDT 2005

Brooke Clarke wrote:

> When working with the HP 4380S Balanced Network Analyzer system I found 
> that using flexible cables resulted in an unstable system.  My first 
> improvement was to use 0.141" semi rigid coax (it's much more on the 
> rigid side) and got a mandrel to bend it.  Later I discovered you can 
> get flexible 0.141" coax that's not "rigid" and it worked great, no 
> mandrel needed.  The point is that you probably will need to have good 
> control on all the coax lines.  The lines will change length with 
> temperature, so the longer they are the more temperature variation there 
> will be.

If its not moved, is semi-rigid any more stable than other coaxes, if 
its not moved?

I don't know all the mechanisms for delays in coax, but the temperature 
coefficient of the dielectric constant of the dielectric must be one of 
them, and quite possibly the most significant. I would have thought that 
dominated any change due to the expansion of the metal. This would 
suggest you need to find the optimal dielectric.

I've got a bit of cable here with what is basically an air dielectric, 
with the inner conductor supported on a spiral of some sort, which means 
99% of the dielectric is air. However, that cable is not practical at 
small sizes.

PTFE might for example be a lot better/worst than polyethylene in the 
variation of dielectric constant with temperature.

I'm not sure that RG405 semi-rigid (which is what I guess you mean) is 
necessarily the best choice.

I used semi rigid a lot, but prefer the bigger one, since its a lot 
easier to solder, as my eyses can see it!!

David Kirkby,

Please check out http://www.g8wrb.org/
of if you live in Essex http://www.southminster-branch-line.org.uk/

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