Electronics World Cover,TOC,and list of posted Popular Electronics articles QST Radio & TV News Radio-Craft Radio-Electronics Short Wave Craft Wireless World About RF Cafe RF Cafe Homepage RF Cafe in Morse Code Google Search of RF Cafe website Sitemap Electronics Equations Mathematics Equations Equations physics Manufacturers & distributors Engineer Jobs Twitter LinkedIn Crosswords Engineering Humor Kirt's Cogitations Engineering Event Calendar RF Engineering Quizzes AN/MPN-14 Radar 5CCG Notable Quotes App Notes Calculators Education Magazines Software,T-Shirts,Coffee Mugs Articles - submitted by RF Cafe visitors Simulators Technical Writings RF Cafe Archives Test Notes Wireless System Designer RF Stencils for Visio Shapes for Word Search RF Cafe Sitemap Advertising Facebook RF Cafe Forums Thank you for visiting RF Cafe!

VSWR - Into 50 Ohm Load or Port Other Port ? - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement


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.

_________________
Sunshine Design Engineering Services





Posted  11/12/2012

RF Cafe Software

   Wireless System Designer - RF Cafe
Wireless System Designer

RF & EE Symbols Word
RF Stencils for Visio
Calculator Workbook
RF Workbench
Smith Chartâ„¢ for Visio
Smith Chartâ„¢ for Excel

About RF Cafe

Kirt Blattenberger - RF Cafe WebmasterCopyright
1996 - 2022
Webmaster:
Kirt Blattenberger,
 BSEE - KB3UON

RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The Internet was still largely an unknown entity at the time and not much was available in the form of WYSIWYG ...

All trademarks, copyrights, patents, and other rights of ownership to images and text used on the RF Cafe website are hereby acknowledged.

My Hobby Website:
 AirplanesAndRockets.com

Try Using SEARCH
to Find What You Need. 
There are 1,000s of Pages Indexed on RF Cafe !

height-line