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[TowerTalk] Best vs. Maxwell -- Again??( LONG !!)

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Subject: [TowerTalk] Best vs. Maxwell -- Again??( LONG !!)
From: (Tom Rauch)
Date: Thu, 18 Mar 1999 09:36:00 -0500
First let me touch on a suggestion. Someone suggested extreme 
examples are "good".

OK, here's one. Lowest loss in RG-8 cable occurs when the cable 
is terminated with 340 ohms (7:1 VSWR). 

Oh, did I forget to mention this is at 160 meters with a 25 foot long 
Radio Shack cable? Sorry, I thought everyone was interested in 
that case!

What do we learn from extreme examples? Not much, unless we 
already almost completely already understand what is going on. 

> My questions is, are the graphs that you see in the ARRL handbook
> and ON4UN's Low Band DXing book, which relate mismatch loss to 
> matched loss, correct generically or do they contain an implicit
> assumption about the loss mechanism (dielectric loss due to 
> displacement current in the dielectric versus I^2R heating losses in the
> conductors)? 

I can't speak for ON4UN's book (which is mostly a collection of 
data from other sources), but the ARRL Handbooks I've looked at 
are right on target for LONG cables. For short cables, the problem 
is one of voltages and currents and reflection rules don't apply at all.

As a matter of fact, efficiency increases with a line Zo mismatch in 
an electrically short cable. If the cable is long part of a wavelength 
long, the Handbook charts are correct.

In other words, is the math kind to us in such a way that you
> can accurately predict the mismatch loss armed only with a "one size fits
> all" graph and a measurement of the matched cable loss, or do I need the
> additional knowledge of what is going on inside the cable to make an
> accurate prediction of the mismatched loss?

By the time loss is meaningful in our applications, the standard 
equations pretty much fall into line.

When designing system with mismatches on electrically short 
lines, the equations do not work. But in almost all of our 
applications, we wouldn't care about the error.

Don't feed an eight foot whip on 80 meters through 20 feet of cable, 
and expect the formulas (or the antenna) to work properly. The 
Handbook solution will work perfectly for our 100 foot feedlines on 
40 meters, no matter what the load. 

Maybe Ian or someone can put the exact breakover point into a 
simple rule of thumb.It would take me half a day of effort to do that.

73, Tom W8JI

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