[TowerTalk] Fwd: Elementary feedline loss question
hanslg at aol.com
Fri Apr 4 10:27:55 EDT 2014
You are not able to measure the losses in a coax when you terminate it with a matched load (such as a dummy). The 56 ohms you are reading is the total error in the meter, the coax and the dummy load.
If you disconnect the dummy load and set the "259" in "loss" mode you are able to measure the loss. If you set the 259 in "loss" mode and connect the dummy in the other end you will get a very discouraging result as the meter will indicate a very high loss in the cable.
A little warning: The MFJ-259B and all the other MFJ's SWR meters are very useful instruments but are not any "Gold" standards. You always have to take the results as an "indication" as the errors sometimes can be very large, 10 - 20 % are not unusual.
I use the MFJ-259B every time I set up antennas and if the adjustment withe MFJ-259 looks good my transmitter usually likes it. ;-)
Best 73 and good luck with you loss measurement,
Hans - N2JFS
From: Pete Smith N4ZR <n4zr at contesting.com>
To: TOWERTALK <TOWERTALK at contesting.com>
Sent: Fri, Apr 4, 2014 9:56 am
Subject: [TowerTalk] Elementary feedline loss question
If I terminate a long 50-ohm coaxial cable with a 50-ohm dummy load, and
put an MFJ-259B on the other end, and it reads R=56, X=0 at a given
frequency, what is the mathematical relationship between the measured R
(leaving calibration out of it, for now) and the loss in dB?
73, Pete N4ZR
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