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RE: hardline



 > From:          Mark West <MarkW@instantdocuments.com>
 > To:            amsat-bb@AMSAT.Org
 > Subject:       RE: [amsat-bb] hardline
 > Date:          Wed, 8 Sep 1999 07:56:05 -0500 

 > > Don't worry about any matching for 75  to 50 
 > > ohm. It makes no difference at all. 
 > 
 > Not! This is nonsense, do not believe this. Impedance
 > matching is very important, especially in transmission.
 > Mismatched impedances cause reflections that set up 
 > standing waves in the wire, and just like sound waves, 
 > these waves create interferance patterns, which consume
 > power and degrade the signal.
 > 
 > - Mark West
 > 

Dear Mark,

You are only slightly right on theroretical grounds.  You will find 
that in practice the mismatch between a 50 ohm and 75 ohm system is 
trivial at 2M and 70cm.  Check the math on a Smith Chart.  The mismatch 
gives you a SWR of 1:1.5 !  Pretty low I would say in any real world application.

Now I invite the better engineers in the group to quote the actual 
loss in signal in db, say that our 1:1.5 mismatch will cause in 100' of
5/8" 70 ohm CATV coax at 70cm.  I bet it is pretty small.

Next point:  What about using baluns to match the impeadances?
Again not worth doing.  In the real world, you will find that the losses
in the baluns and connectors exceed the losses caused by the 
mismatch.

Bottom line, does 70 ohm CATV line work?  Darn right it does.
I have a 70cm repeater using 150' of it with no problems for the
last ten years.  CATV coax is widely used on FM repeater systems
due to its low cost (as in often free) and low loss.  Next time your
offered a 200' roll end, don't pass it up!

--73-- David WA0AUQ

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