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VSWR - Into 50 ohm load or port other port ? - RF Cafe Forums
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| TMC
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Post subject: VSWR - Into 50 ohm load or port other port ?
Posted: Fri Aug 28, 2009 9:28 pm
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Joined: Wed May 28, 2008
2:20 pm
Posts: 3
Location: CT
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Using a 20GHZ VNA to test a coax cable. Why would
I get a better VSWR reading if I connect the cable
to port 1 with the other end into a broadband load
versus into port 2 ? I would think during calibration,
that port 2 is now null at the reference plane ?
I would think both readings should be identical.
Any help is appreciated !!!
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Ming |
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Post subject: Re: VSWR - Into 50 ohm load or port other port ?
Posted: Fri Aug 28, 2009 9:39 pm
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| Captain |
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Joined: Tue Jan 13, 2004
8:42 am
Posts: 10 |
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Hi TMC,
How much better? Are we talking -42
dB vs. -40 dB, which is basically the same for all
practical purposes, or -22 dB vs. -20 dB, where
the 2 dB difference is more meaningful? The 40 dB
RL is hard to measure with certainty so a couple
dB is not too surprising.
One explanation
could be that the length of the test cable is such
that the actual VSWR of the VNA Port 2 compared
to the WB load is different enough that the reflected
signal is significantly different.
If you
have one available, take a Smith Chart program and
model your system, with cable length, assuming even
a 50+j0 ohm cable and then let the load be something
other than 50+j0 ohms and you can see how the measured
VSWR can be appreciaby different, especially if
you're measuring in the -30 or -40 dB realm.
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SunshineDesign |
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Post subject: Re: VSWR - Into 50 ohm load or port other port ?
Posted: Wed Sep 09, 2009 12:02 am
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Joined: Sat Nov 08, 2008
11:35 pm
Posts: 30
Location: Ramona, CA
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Ming good answer. At <-35dB Return Loss the delta
difference in reflected power is very small. -10
to -20 range it is much wider a delta for the same
dB change in Return Loss. The difference in return
loss is more due to characteristics of the calibration
methods (i.e. 1/2 port) and how it uses the 2 port
params to correct the 1 port derivations. So in
essence the VNA over corrects in the 2 port case
with a load vs measuring the thru S22 of the VNA
Port 2. With the VNA you must define the reference
plane and keep the calibration at or between those
points of reference.
_________________
Sunshine Design Engineering
Services
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Posted 11/12/2012
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