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From: BOBW@cc.usu.edu (BOB WOOD WA7MXZ)
Newsgroups: comp.dcom.lans
Subject: Re: 50-75 ohm impedance matching
Message-ID: <28221@cc.usu.edu>
Date: 15 Jul 90 20:57:50 GMT
References: <1990Jul13.210239.17750@ux1.cso.uiuc.edu> <8601@uhccux.uhcc.Hawaii.Edu>
Lines: 36
X-Local-Date: 15 Jul 90 13:57:50 PDT

In article <8601@uhccux.uhcc.Hawaii.Edu>, querubin@uhccux.uhcc.Hawaii.Edu (Antonio Querubin) writes:
> Thanks to all those who responded to my base note.  Almost everyone pointed
> out a problem which I hadn't considered - the DC collision detection process
> would be affected by a normal transformer.  A normal transformer would pass
> the AC signal but not the DC.  After pondering this new twist for a few minutes
> I realized that an 'auto-transformer' would pass the DC as well as AC.

Look at your autotransformer design again. Not only does an autotransformer
pass DC but it also shorts it out, directly to ground. The larger the wire
used in the transformer to improve the Q the lower the D.C. resistence to
ground. In short, the matching transformer plan isn't going to work.
> 
> But someone also suggested using the 75 ohm cable directly but with 75 ohm
> terminators instead.  This makes more sense since all that's needed is a 
> point-to-point link.  Connection to a 50 ohm segment would require the 
> impedance matcher above (or a repeater) between the two cable types.

This is going to be your only game plan. Use only 2 nodes on the cable and 
use good 75 ohm 1% terminating resistors. The cable length will have to be
no more than 1200 feet, (1640 is the maximum THICKnet length) IF the video
cable is RG-11 (1/2 inch diameter). If the cable is the smaller 1/4 inch RG59
stuff then the maximum distance will need to be 500 feet or less. These
figures do not guarantee that your idea to use 75 ohm cable will work.
The Ethernet spec is timed for 50 ohm cable and timing will be different
on the 75 ohm cable. If you want to use additional onde on either end you
will need bridges and/or repeaters to keep the number of nodes of the cable
down to 2, one at each building.
-- 
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Bob Wood    WA7MXZ                      bobw@cc.usu.edu
Utah State University                   bobw@usu.bitnet
Chemistry and Biochemistry              tel. (801) 750-1614
UMC 0300
Logan, Utah  84322-0300

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