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
w8ji@contesting.com
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