On 1/15/2012 8:26 AM, Steve Hunt wrote:
> Consider a perfect 50 Ohm load connected to a source through three 6ft
> lengths of 50 Ohm lossless coax with Vf=0.66.
Good analysis -- it's always important to put real numbers to any
problem to understand what matters and what doesn't. BUT -- real coax
is NOT lossless. If we're going to get nitpicky about connectors we
should be equally realistic about the coax itself. One of the real world
results of loss is that the mismatch becomes less as we move down the
line from the antenna to the transmitter For example, a 3:1 mismatch at
the antenna will typically be seen as only 2:1 at the end of few hundred
feet of coax. This does not reduce the loss in the line, but it DOES
determine the load (and tend to reduce the mismatch) seen by the
transmitter.
And since we're talking numbers, consider this one -- the WORST CASE
additional loss due to a 1.5:1 mismatch (measured at the antenna) is
0.18dB, no matter how long the line or how much the matched loss, and
for lines with low loss the additional loss is even smaller. Using that
previously cited family of plotted curves in the ARRL Handbook for your
28 MHz example, a 2 dB matched loss (about of 300 ft of LMR400 at 28
MHz) would be increased by only 0.1dB by a 1.5:1 mismatch.
This family of curves has been in every edition of he ARRL Handbook I've
owned, going as far back as the '50s, and it's one of the most important
things in the Handbook from a practical point of view. If you take one
thing from this thread, go find your Handbook and study that family of
curves. It will cause you to stop obsessing about SWR, and start
thinking instead about better antennas and lower loss feedlines.
73, Jim K9YC
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