In article <1992Jul20.140143.19341@yang.earlham.edu>, jeremiah@yang.earlham.edu (Jeremiah Wilton) writes:
> I am wondering what kind of coax is used in Ethernet thin and thick wire
> connections. I am unconvinced that some really expensive special stuff
> is necessary. Might not RG-58 or 59 work for thinwire? How about
> composite video cable (the thin cheap stuff)? The real question is: What
> are the limiting factors in coax for Ethernet? Capacitance, inductance,
> impedance, resistance? Sheilding? Wire gauge?
Well, all of them in a way. Ethernet is designed for 50 ohm, so you have to use
that. For very short runs, thats about it. Longer runs you need to worry about
cable losses, and the shielding matters if you want to avoid causing
interference to radio/TV, or picking up interference from heavy machinery,
motors etc.
>
> Why is RG-58 used in low-power FM applications (like receiving) while
> RG-59 is used for TV? I know about 50 ohms vs. 75 ohms, but what does > that mean, and why are they different sizes?
If you have a signal source, like a transmitter and you want to get
maximum energy from it into something else, like an antenna, then you need to
make sure that the impedance of the source matches the impedance of the
target. If it doesn't, not all the energy will be transferred, some will
'bounce' off the connection. At best this wastes the signal, at worst enough
power can be reflected to damage the transmitter. I have accidentally wired
a 5metre length of 93 ohm cable into a 50ohm thinwire ethernet network. The
whole network went to pieces, 10-second file transfers took 15 minutes!
All cause by packets 'bouncing' off the cable mismatch & corrupting other
data on the wire.
The actual choice of impedance (50ohm, 75ohm, 93ohm etc) doesn't matter as long
is the whole system (transmitter/cable/antenna or antenna/cable/reciever) is
the same. It just so happens that some simple antennae have an impedance of
about 50ohms at their tuned frequency, so 50ohms is an obvious choice for the whole system. When you start adding extra elements to an antenna, the impedance
drops but folding the a dipole into a loop increases it again. Somewhere along
the way, 75ohms came out as the impedance of a 'standard' TV antenna, and
hence of TV cable. If I remember correctly a folded dipole has 300ohm
impedance, and led to the use of 300ohm balanced feeder in some places.
As to the sizes, the impedance of a co-ax cable depends on the ratio of the
diameters of the inner & outer conductors, so different impedances will have
different sizes. Larger diameter cables also have less loss, so they can
carry more power from a transmitter, or keep a weak signal 'alive'
