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VSWR - Into 50 ohm load or port other port ? - RF Cafe Forums

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TMC
 Post subject: VSWR - Into 50 ohm load or port other port ?
Posted: Fri Aug 28, 2009 9:28 pm 
 
Lieutenant

Joined: Wed May 28, 2008 2:20 pm

Posts: 3

Location: CT

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 !!!


 
   
 
Ming
 Post subject: Re: VSWR - Into 50 ohm load or port other port ?
Posted: Fri Aug 28, 2009 9:39 pm 
 
Captain
User avatar

Joined: Tue Jan 13, 2004 8:42 am

Posts: 10

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.


 
   
 
SunshineDesign
 Post subject: Re: VSWR - Into 50 ohm load or port other port ?
Posted: Wed Sep 09, 2009 12:02 am 
 
Colonel

Joined: Sat Nov 08, 2008 11:35 pm

Posts: 30

Location: Ramona, CA

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.

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