[time-nuts] Non-impedance matched antenna cables
holrum at hotmail.com
Sat Jun 14 00:02:09 EDT 2008
I suspect that there is an initial rapid aging process (break-in period) that occurs followed by longer term aging effects (much like a crystal oscillator). The particular cable that I used was not new, but had only been used indoors before. I suspect that each temperature cycle a cable goes through would tend to stretch the conductor more than it contacts (but then most plastics seem to shrink over time). Moisture infiltration into the cable would most likely accumulate and lower the VP which would increase its effective length.
I do have a VERY expensive controlled cable that I use to calibrate TDRs. It is sealed in a can with hermetic connections to the outside world. Originally cost like $20,000. Over 20+ years I have never been able to detect any change in its properties. VP was specified to 4 digits. Unfortunately I didn't get the full cal book with it. I only have the one VP:temp value. Still, it is way more accurate than any TDR that I have (and I have LOTS of them). A man with one TDR calibration cable is blissfully ignorant... a man with two TDR calibration cables never knows what VP it is...
Does the electrical length continue to change with time or does it
stabilise after a number of diurnal temperature cycles?
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