[time-nuts] Non-impedance matched antenna cables
holrum at hotmail.com
Fri Jun 13 20:37:35 EDT 2008
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.
Depends on the cable length and the temperature excursions it experiences.
For short runs its well worthwhile.
For longer runs the cable attenuation will help reduce the effect of
mismatches at either end.
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