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Re: [TowerTalk] tuners and power rating

To: towertalk <>
Subject: Re: [TowerTalk] tuners and power rating
From: Dan Zimmerman N3OX <>
Date: Wed, 1 Dec 2010 07:13:58 -0500
List-post: <">>
>  What's a mystery is why the loss
> attributed to SWR under that condition shows as a negative value in TLD,
> albeit a small amount.  The inverse of loss is...well, gain!

Sure, why not?   Short lengths of coax with the right mismatch can show what
you might call mismatch gain...  the mismatched line has less loss than the
matched line.    I think calling this situation "mismatch gain" would be
confusing as heck to some people, but if we insist on calculating the
"additional loss due to line mismatch," this is exactly what happens.   In
some situations, a fraction of a decibel more power makes it through when
the line is mismatched.  So the "extra loss due to mismatch" is negative...
in the context of the "mismatch loss" concept, we must express lessened loss
as a gain.

I discovered a funny example of this effect recently while analyzing a
system I actually use.  If you build a 50 ohm to 75 ohm twelfth-wave
series-section transformer with RG-6 and RG-58 type coax, the extra loss in
the RG-6 portion due to the fact that it's mismatched with 50 ohms is pretty
well balanced by the lessened loss the RG-58 portion of the transformer
....  this is mostly just an amusing factoid given that the total
transformer loss is a fraction of a dB to start with (and the changes then
are correspondingly small)... but I thought it was interesting.


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