Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!samsung!munnari.oz.au!uhccux!querubin
From: querubin@uhccux.uhcc.Hawaii.Edu (Antonio Querubin)
Newsgroups: comp.dcom.lans
Subject: Re: 50-75 ohm impedance matching
Message-ID: <8611@uhccux.uhcc.Hawaii.Edu>
Date: 16 Jul 90 08:05:06 GMT
References: <1990Jul13.210239.17750@ux1.cso.uiuc.edu> <8601@uhccux.uhcc.Hawaii.Edu> <28221@cc.usu.edu>
Reply-To: querubin@uhccux.UUCP (Antonio Querubin)
Organization: University of Hawaii
Lines: 29

In article <28221@cc.usu.edu> BOBW@cc.usu.edu (BOB WOOD WA7MXZ) writes:

>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.

Yes, you're quite right.  But considering that the DC signal is a relatively
short pulse on the order of milliseconds at the most the autotransformer may
work anyway.  The trick is to make the thing have wide enough bandpass to
accept both the low frequency pulse and the high frequency component.  I have
a few ideas on how to do this.  My curiosity about the thing is high enough
for me to pursue the matter time permitting.  However, as you said:

>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.

I think we'll play with that idea first.  And it is RG-59.  Thanks for the
response!

Antonio Querubin, Jr.
querubin@uhccux.uhcc.hawaii.edu