In article <[EMAIL PROTECTED]>, "Richard B. Gilbert" <[EMAIL PROTECTED]> wrote:
> Unruh wrote: > > David Woolley <[EMAIL PROTECTED]> writes: > > > >> Unruh wrote: > > > >>> With your 100m setup you really want a buffer amp on the line. At 100m, > >>> the > >>> one way trip is .3ms, with reflections every .6ms which might make the > >>> system a bit weird. To get rid of the reflections you need a 100ohm > >>> termination if you use cat5e cable to lengthen the 5m wire on the > >>> receiver. > > > >> Round trip time for 100m of velocity factor 2/3 cable is approximately: > > > >> 3 * 2 * 100 / (2 * 3E8) > > > >> = 3E2/3E8 > >> = 1 microsecond. > > > > I was taking the speed to be c. And it is only a one way trip-- from the > > GPS to the computer, not round trip. So I will buy your .5us (having no > > idea what the velocity factor is of cat 5e cable) but not your > > round trip ( which is twice that but is irellevant on a properly terminated > > cable) > > > > "c" is not correct. "c" is the speed of light in vacuum! Electricity > in copper is somewhat slower. 1 nanosecond per foot is a useful rule of > thumb! It's an approximation but a useful one. If you need something > more exact, you might consult the manufacturer. Cat 5 cable consists of > four twisted pairs with two turns to the inch. The physical and > electrical lengths of the cable are slightly different! Here are the standard rules-of-thumb: For typical solid dielectrics, propagation is 66% of C. This also applies to light in silica optical fibers. For typical foam dielectrics, propagation is 75% to 85% of C. Joe Gwinn _______________________________________________ questions mailing list [email protected] https://lists.ntp.org/mailman/listinfo/questions
