From: John Tserkezis 
Reply-To: Fidonet AVtech Echo 

Jasen Betts wrote:

>  JT>   I only wish I could have a portable version for use at client's
>  JT> offices,

> you could use a pocket sized oscilloscope, but it'd no longer be al-cheapo.

  That and, no-one uses coax ethernet anymore.

> the maximum cable lenght spec used for ertherenet isn't due to signal loss but
> due to having the cable short enough that a collision can be
> detected by all stations.

  Do you mean signal stregth for detection of the clash, or propigation delays 
involved?

  I thought delays on coax were well within spec even with max length cables. 
With twisted pair, cable length limitations are based on signal strength and 
quality by the time it gets to the end of the run.  Propigation delays are only 
a problem if you're stacking hubs, up to four deep is considered as far as you 
would want to go before delays start causing weird problems.

  Though this can be easily worked around by using stackable hubs, where in 
large multi-user (in the hundreds) environments, is the way you're supposed to 
do it anyway.

>  >> I wonder how much loss you get in 100m of that cable at 10Mhz.
> 
>  JT>   Easily calculated.  Cable specs state loss at specified freqency
>  JT> use, you just add it up for your length of cable.
> 
> I don't have any specs handy, do you?

  The following is from the back of an old DSE cattledog, though only part of 
the list.  I've picked the two more common cable types.  Ask if there are other 
cables types you're interested in, there are about 15 different types listed, 
some 50 ohm, some 75 ohm, some 93 ohm.

                     Attenuation in dB/100metres
Type                50MHz  100MHz  200MHz  400MHz     Velocity factor
RG-58C/U  50ohm     10.8   16.1    23.9    37.7       66%
RG-213/U  50ohm      5.2   7.2     10.5    15.4       66%


  Though you're not likely to use RG213 in network applications, it is in 
common use on long runs for radio transceivers.
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