[time-nuts] Non-impedance matched antenna cables

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Jun 13 21:20:32 EDT 2008

Mark Sims wrote:
> A while back I did a project that used a 100 foot piece of (supposedly) fairly good quality RG58.  I checked it with my TDR and then installed it in an outdoor application where it ran for a couple of weeks in good ole' Texas weather.  The outdoor connection was covered by a glass jar so water infiltration of the connector was not an issue.  Also the cable was not hanging, so stretching under its own weight was probably not an issue.  
> When the project was then over I happened to check the cable with the same TDR.  Its effective electrical length had changed by around a foot!   A half-wave of GPS is around 4 inches.  I think that most commercial coax is way too unstable to maintain that kind of precision in a real-world installation.  A .2% shift in velocity factor or cable length in a typical 100 foot run would be 1/4 wave of GPS signal.
This demonstrates that achieving nanosecond propagation delay stability 
is unlikely when using long runs of inexpensive cable exposed to diurnal 
temperature cycling.

Thermal cycling of plastics like PE does little for their mechanical 
The effect is probably even worse for gas injected foam dielectric 
cables unless a solid outer conductor is use to constrain the dielectric.
A better quality cable with greater phase stability over time and 
temperature cycling is required to minimise this effect.
Mineral filled cables should have much greater stability when 
temperature cycled.
Buried cable runs and/or temperature controlled high stability cable 
runs are often used to improve stability.

Maybe this is one reason why Trimble feel that using RG59 is OK, as in 
long runs the 2 way attenuation is sufficient to minimise the effect of 
reflections at the receiver and antenna.
The 2 way attenuation of 100 ft of RG59 is around 30dB at 1575.42MHz 
reducing the effect of mismatches at either end by a factor of 30.

For short cable runs it is well worthwhile trimming the cable length 
particularly if the cable temperature excursions are small (except for 
PTFE cable at 15C).


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