Tom Duckworth wrote:
Magnus,
We've made this measurement using a 20 ps time interval counter and a
GPS disciplined Rubidium frequency standard as the time base; making
many concurrent measurements with no dead time between. The resultant
measurement was very close to the 1 ns/ft benchmark with RG-59 (BNC
connectors), 10 MHz source. So we felt ok with using the 1 ns/ft estimate.
Well, I think you should reconsider. Theory says that
v = 1/sqrt(my*epsilon) = 1/sqrt(my_r * epsilon_r) * c
For a coaxial cable, we have the magnetic properties about the same as
vacuum, so my_r = 1 is a fair approximation. RG-58 and RG-59 use solid
polyethylene having epsilon_r = 2,25 and that cranks out as v = 0,66*c,
in agreement with tabulated values:
http://hyperphysics.phy-astr.gsu.edu/Hbase/tables/diel.html
http://www.epanorama.net/documents/wiring/coaxcable.html
Notice how foam-PE rates at 0,78*c rather than 0,66*c. This is due to
the lower dielectric constant (about 1,64) of foam PE.
I think it this relationship is wellfounded. I use 2 dm/ns for coax and
fiber myself for my reality check calculations and found good
correlation with reality whenever I tried it.
Oh, did you use sine as waveform? A few different frequencies and/or
amplitude would be good to ensure that biases could be canceled out. I
do that for reality check myself. Swapping the cables is another.
You have to excuse me, but the value you have measured does not really
correlate well with my experience or view of theory.
Cheers,
Magnus
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