[TowerTalk] Understanding Coax Loss Measurements

[Kirk Kleinschmidt]

Jim and the group,
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...

Thanks,
Kirk, NT0Z

My book, "Stealth Amateur Radio," is now available from www.stealthamateur.com and on the Amazon Kindle (soon) 

    On Sunday, November 29, 2020, 11:59:15 AM CST, Jim Thomson <jim.thom at telus.net> wrote:  
 
 Date: Sat, 28 Nov 2020 11:43:28 +0000 (UTC)
From: Kirk Kleinschmidt <sohosources at yahoo.com>
To: Towertalk Reflector <towertalk at contesting.com>
Subject: [TowerTalk] Understanding Coax Loss Measurements

<Method 2: RF Power meter and TX. Calibrate: Connect TX output to RF power meter, then through the test length of coaxial cable, then to dummy load. Carefully set TX output to to 10 W. Swap test cable and power meter, so TX output now goes to test <length of cable, then to power meter, then to dummy load. Record power levels of between 7 and 9.1 W with TX power set to same level as "calibration " run, but with meter now placed after test cable length.

<Testing 100-foot lengths of RG-6, RG-11 and RG-213.
<With the power meter, loss values are RG-6, 3 dB; RG-11, 1.9 dB; RG-213, 2.8 dB
<As you can see, these methods don't line up well.
<Help?
<--Kirk, NT0Z

##  How  did  you manage to arrive at a whopping  2.8 db  loss ,  at  30 mhz,  with  100  feet of  RG-213 U  ? 
The bird wattmeter  method at each end of the coax has worked  flawlessly for years.  DL  at far end of feedline, or ant with 1:1  swr. 

##  Try it again,  but use exactly 100 watts  at the near end.  You  should measure aprx 77  watts  at the far end... at 30 mhz. 

Jim  VE7RF



 
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