> On Jan 24, 2018, at 4:05 PM, Brent Hilpert via cctalk <cctalk@classiccmp.org>
> wrote:
>
> On 2018-Jan-23, at 12:27 PM, Paul Koning via cctalk wrote:
>> The Ethernet spec says that the cable OD is in the range .365 to .415 inch,
>> which is 9.27 to 10.54 mm. The nominal OD of RG-8/U is .405 inches, or
>> 10.28 mm, which is within spec for Ethernet cable.
>>
>> One place where the two cable specs differ is in the velocity factor, 0.66
>> for RG-8/U and 0.77 for Ethernet cable. That relates to the dielectric --
>> solid polyethylene for RG-8/U and foamed material (unspecified) for
>> Ethernet. Also, Ethernet requires a solid inner conductor (for the tap)
>> while RG-8/U may come stranded. (Maybe only in some variants, I'm not
>> sure.) And there are the stripes, of course, but those have no electrical
>> significance. You can use a tape measure if you don't have the stripes.
>
> I was attempting some calculations to see if I could derive the 2.5M
> transceiver spacing and was wondering what the velocity factor for the cable
> was, as it should affect the transceiver spacing in theory.
The velocity factor is specified as 0.77. The Manchester encoding of 10 Mb
Ethernet means the dominant frequency is 10 MHz, which in the coax would have a
wavelength of 23.1 meters (0.77 * c / 10e6). So the 2.5 meter spacing is
0.1082 wavelengths, i.e., a number chosen NOT be be a round value. If you look
at the integer multiples of that value you'll find hardly any that are close to
an integer; the first one I see is 37x which is 4.004. This ensures that there
is very little in the way of coincident reflections.
There are lots of other spacings that would work, for example 2.0 meters is
also a decent choice -- all that is required is "pick a spacing such that
nearly all the tap points are not round multiples of 1/2 wavelength of 10 MHz
apart". 2.5 seems a fine choice given the max cable length (500 meters) and
station count (100); there is no real benefit in picking a smaller value.
If you're using actual RG-8/U (VF = 0.66) then the answers come out different,
of course.
paul
