Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!cs.utexas.edu!samsung!munnari.oz.au!uhccux!querubin
From: querubin@uhccux.uhcc.Hawaii.Edu (Antonio Querubin)
Newsgroups: comp.dcom.lans
Subject: 50-75 ohm impedance matching
Message-ID: <8592@uhccux.uhcc.Hawaii.Edu>
Date: 13 Jul 90 13:05:29 GMT
Reply-To: querubin@uhccux.UUCP (Antonio Querubin)
Organization: University of Hawaii
Lines: 27

Not sure if this is the appropriate news group, but if it's not, let me
know and I'll re-post:

Don't know if anyone has run into this situation before but we have a
requirement to setup a point-to-point link between two buildings using a
previously installed 75-ohm video cable.  For various reasons, we can't 
just replace the cable with a 50-ohm thin-net cable to run an ethernet
link.  So the simplest and cheapest way to get the link is to use the
existing cable.  Now it seems that one should be able to solve the impedance
mismatch and associated reflection problems by using a 75-to-50 ohm impedance-
matching transformer at each end of the video cable so that the ethernet
transceivers on either side effectively sees a 50-ohm cable.

I'm considering building two of these transformers as a small afternoon
project and giving it a try.  Question:  has anyone done this before and what
kind of problems did you run into?  I know the turns ratio has to be the
square root of 1.5 but I haven't the foggiest idea of how many turns to wind
onto a toroid.  Too many turns and I might have a saturation problem, too
little and I might get a lot of signal loss.  Anyone got a good recommendation
here?

Does anyone sell an inexpensive ready-made transformer for this kind of
situation?

Antonio Querubin, Jr.
Internet:  antonio_querubin-manoa@uhplato.uhcc.hawaii.edu
BITnet:  antonio_querubin-manoa@uhplato