Well... if it is just cable loss and the cable is uniform the R should
approach Z0 as the length increases no matter what the load is... the less
loss the slower the approach to Z0. If the cable is terminated in a pure
resistance equal to Z0 then it should measure Z0 anywhere along the cable.
Now, assuming you have measured your dummy load and it reads r=50 x=0
connected directly to the 259b, there are 2 reasons why you might read
something other than r=50 x=0 on the line.
1. the line Z0 is not really 50 ohms. In which case the measured impedance
will be frequency and line length dependent, repeating the 50 ohms every 1/2
wave along the line or at each frequency when the line is a multiple of a
half wave long.
2. there is a flaw in the line. Water in it, a crimped spot, or something
else that changes the impedance at some point in the line will cause effects
similar to the above problem but could be spotted with a TDR or a frequency
sweep analysis. In this case the impedance will likely not repeat the 50
ohm load every 1/2 wave or at exact harmonics.
David Robbins K1TTT
e-mail: mailto:k1ttt@arrl.net
web: http://wiki.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://k1ttt.net
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of Pete
Smith N4ZR
Sent: Friday, April 04, 2014 13:54
To: TOWERTALK@contesting.com
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
Check out the Reverse Beacon Network at
http://reversebeacon.net,
blog at reversebeacon.blogspot.com.
For spots, please go to your favorite
ARC V6 or VE7CC DX cluster node.
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