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Re: [TowerTalk] PL259 Insertion Loss?

Subject: Re: [TowerTalk] PL259 Insertion Loss?
From: Jim Brown <>
Date: Mon, 16 Jan 2012 08:03:26 -0800
List-post: <">>
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 

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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