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Re: [TowerTalk] Stability of velocity factor in coax cables

Subject: Re: [TowerTalk] Stability of velocity factor in coax cables
From: Jim Lux <>
Date: Sun, 01 Jan 2012 09:08:32 -0800
List-post: <">>
On 1/1/12 8:38 AM, Rob Martin wrote:
> I am making delay lines using coaxial cables.
> How does the velocity factor of a cable change with temperature and over time?
> Does anybody have information about this?

For run of the mill coax the effect is small (tens of ppm per degree C).

You've got several factors working together:
1) The CTE of the coax longitudinally-> it gets longer as the 
temperature goes up

2) The CTE of the dielectric -> it gets bigger in diameter, which 
decreases the epsilon and changes the ratio of inner/outer diameter, 
changing the capacitance per unit length, changing the propagation 
speed.  But.. again a few ppm/degree effect.

For some coax, there are charts on the mfr websites.

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