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[TowerTalk] Measuring Coax Loss

To: "Tower Talk List" <>
Subject: [TowerTalk] Measuring Coax Loss
From: "Jim Brown" <>
Reply-to: "Tower and HF antenna construction topics." <>
Date: Thu, 11 Feb 2010 11:21:34 -0800
List-post: <">>
N6RK suggested that the MFJ259B would be a good way to measure coax loss with 
a reflection measurement at one end only. I responded I was looking for more 
precision than that. Several years ago, I carefully calibrated my 259B using 
W8JI's procedure. Today, I compared loss measurements of a 208 ft length of 
Commscope 2427K (the plenum version of 3227) made both with my 259B and with 
my HP generator/spectrum analyzer setup. 

The HP data has one digit better precision -- the spectrum analyzer readout 
is 0.01dB, while the 259B reads 0.1 dB. Within those limits, the MFJ yielded 
results that were usually within rounding error of the HP setup between 10MHz 
and 150 MHz, which is where I stopped measuring (because that's the highest 
frequency I cared about for my applications). For example, at 50, 100, and 
150 MHz the HP numbers were 2.07 dB, 2.84 dB, and 3.37 dB; the MFJ yielded 
2.1 dB, 2.8 dB, and 3.3 dB. At 3 MHz (the lowest frequency I could measure 
with the MFJ), the numbers were 0.62 dB and 0.6 dB. 

For obvious reasons, you really need the greater precision of the HP setup to 
get good data for frequencies and lengths where the loss is relatively small.


Jim K9YC


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