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Re: [TowerTalk] Understanding Coax Loss Measurements

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Understanding Coax Loss Measurements
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Mon, 30 Nov 2020 14:00:16 -0800
List-post: <mailto:towertalk@contesting.com>
On 11/30/2020 12:44 PM, Kirk Kleinschmidt via TowerTalk wrote:
I may have improperly used my power ratio to decibels calculator.:)
With exactly 10 W going into the cables, I measured
8 W out (RG-11); 7.2 W (RG-213); and 7 W (RG-6) at 30 MHz.
Upon reflection (pun intended), my 2.8 dB figure can't be correct...

We can't use a 50 ohm wattmeter to measure loss in 75 ohm cable, thanks to the standing waves resulting from the mismatch. A study of how transmission lines work is advised. See the ARRL Handbook and Antenna Book.

What DOES work is the method I outlined earlier with AC6LA's spreadsheet and a properly calibrated Vector Analyzer.

73, Jim K9YC
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