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Mark West <MarkW@instantdocuments.com>,  
Subject: Re: [amsat-bb] hardline
References: <199909082146.QAA09713@mut1.muscanet.com>
From: Tovar <tvr@value-nojunk.net>
Date: 12 Sep 1999 10:01:52 -0700
In-Reply-To: "Dave Metz"'s message of "Wed, 8 Sep 1999 16:54:15 -0600"
Message-ID: <87d7vokqrj.fsf@mongrel.kd6pag.ampr.org>
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    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.

Most, but not all by my experience.  I'm running a dual-band 50 ohm direct
feed LPVA into an Alinco DJ-5G.  In my local area, if i run 5W and my SWR
at 2 meters is 1:1.5, then i'm not going to hear my own downlink at 70cm.
I'm not completely sure why and it might have to do with non-linearities
in the antenna from things like imperfect solder joints, but the closer
i come to 1:1, the better i hear my own downlink.  (Now, this may not be
an issue outside of an urbanized area, by the way and this may have to do 
with local condition).  The general rule of thumb is that pattern is much 
more important than SWR, but there are exceptions.

    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.

I'm NOT an engineer (at least, not in the hardware realm).  But if the 
reflection at 2 meters is causing de-sense at 70cm, that matters to me.  
A better diplexer will probably deal with that, but that's not much help 
to me if it's inside the radio.

    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!

I think the more important point is that the 50/75 ohm loss to due mismatch
is *much* smaller than the difference between a 100' of 50 ohm feedline of
any type that i can afford, and 100' of 75 ohm hardline that might become
available to me.

What i don't understand is why people are designing antennas and perhaps
also PA's, to be 75 ohms?  At least then, the mismatch isn't on both ends
of the feedline.  Maybe the engineers around here can explain that one to
me??
                            -- KD6PAG  (Networking Old-Timer, "RF newbie")
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