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Date: Mon, 11 Sep 1995 11:47:56 -0400
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Subject: Coaxial Runs
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In a message dated 95-09-11 06:02:37 EDT, you write:

>Does anyone know what the maximum distance is for a Thinnet (50 ohm
>coaxial cables) network?  I want to set up some machines in my BIG studio
>and the building is at least 150 feet away from my house...would need to
>put the cables into some sort of PVC piping and run it underground deep
>enough to avoid problems with the killer New England winters.

Here at the house, my roommate pipes frames to my PAR board through three
different BNC cables hooked together with adapters, that add up to 50 feet.
In other
words we are sloppy and still get away with 50 feet.

Stay away from other cable runs if possible. AC power cords radiate a
powerfull field.
When you run a circular saw on a long extension cord, the lights brown-out
not just
because of the saw's draw, but more from the extra large magnetic field that
must initially be created the instant the trigger makes contact.

Make sure your 2 and only termination points are right at the EtherNet
boards. Coaxial
transmission is extremely sensitive to signal reflections. A partial
reflection will occur
any time there is a change in the impedance ( resistive, capacitive or
inductive ).
Using the adapters like we do is a bad idea over long distances as each one
could
possibly introduce a signal reflection. Even a tiny cut in the shielding can
disturb the
inductive impedance causing a reflection.

When using 50 ohm terminators, use ONLY 50 ohm cable. Maximum power transfer
will
occur when your cable and loads are matched. I think most coaxial sold
through places
like Radio Shack is 75 ohm cable balanced for cable TV transmission. Good
coaxial
cable is almost lossless. So, any problems encountered are usually due to
external
signals and hums, or some kind of signal reflection internally.

I suggest you lay the cable out above ground and try it before going to the
trouble
of burying it. :-)

Dan Esmond ( BS in EE )
Animagic
Austin, TX
512-258-4120

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