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Replies: 10 - Pages: 1 - Jul 25, 2012 1:16 PM Last Post By: AVCM
 Posts: 7 Registered: 05/22/12
Posted: May 22, 2012 2:09 PM
 Posts: 2,675 Registered: 12/01/05
Posted: May 23, 2012 1:39 PM   in response to: AVCM
 I think you are on the right track, but in making the load comparison, the worst case variation represents the sum of the load error and the directivity error. If the same load is used for calibration as measurement, you can infer that the load is 6 dB better than the measured differences.
 Posts: 11 Registered: 05/02/12
Posted: May 23, 2012 3:04 PM   in response to: AVCM
 Sliding Load Calc Example.xls (16.5 KB)
 I wasn't sure exactly what you were looking for but I attached a spreadsheet with an example of how to electrically verify a sliding load's effective return loss. This calculation is for one test point. The six measurements are from the VNA's Smith Chart format and were taken at the six positions of the 911E sliding load. I hope it helps.
 Posts: 7 Registered: 05/22/12
Posted: May 25, 2012 9:50 AM   in response to: Dr_joel
 Posts: 7 Registered: 05/22/12
Posted: May 25, 2012 10:08 AM   in response to: Metatron
 Metatron (interetsing handle...) I posted an expanded description of what I am attempting to do in my response to Dr_Joel's reply. That should flesh out the general idea. I'm going to have to plead ignorance regarding th spreadsheet. What quantities are being used? (Looks like Xi is return Loss, but for Yi I'm clueless). What math operation are you performing on the data? What do you compare the result to to determine 'pass/fail'?
 Posts: 2,675 Registered: 12/01/05
Posted: May 27, 2012 8:38 PM   in response to: AVCM
 What I described early would be load stability (using the same sliding load to measure itself. The key aspect of a sliding load is that the center of the circle that is the locus of the slide points is the impedanc of the load. You can do a TRL calibration and the find the circles and then compute the center and from that compute the impedance of the line portion of the load and that will represent the effective directivity. The stability usually doesn't have a spec but it must be better than the impedance spec of the load or you cannot get to the impedance spec. The load's manufacturer should give a spec on the impedanc of the airline, or the residual directivity of the load.
 Posts: 7 Registered: 05/22/12
Posted: Jun 1, 2012 2:06 PM   in response to: Dr_joel
 Posts: 7 Registered: 05/22/12
Posted: Jul 12, 2012 12:28 PM   in response to: Metatron
 I've had some time to research this and it looks as though your spreadsheet calculates the circumcenter of the 6-point polygons formed by the points on the Smith Chart representing the six slide positions for each frequency. Is this correct? If so, I would be interested in knowing where you found the equations (or, perhaps, you developed them yourself) so I can get a better idea of exactly what is going on. I'd like to be able to understand exactly what each piece of the overall algorithm is doing. The spreadsheet appears to work quite well and seems to give the results I am looking for. Thanks.
 Posts: 7 Registered: 05/22/12
Posted: Jul 12, 2012 12:38 PM   in response to: Dr_joel
 Thanks Dr_joel. Earlier in this thread 'metatron' supplied a spreadsheet that appears to do the 'circumcenter' calculations for the Smith Chart circles at each frequency. It yields an impedance value at the circumcenter of each circle. Given the spreadsheet calculations and your description of deriving the Effective Directivity, I assume then that all I need to do is calculate the Effective Directivity using the expression: (Calculated Impedance-50)/(Calculated Impedance+50) and convert to dB?
 Posts: 2,675 Registered: 12/01/05
Posted: Jul 12, 2012 2:12 PM   in response to: AVCM
 Yup. But this will give the directivity relative to the impedance of the airline. So the uncertainty is the uncertainty of the airline.
 Posts: 7 Registered: 05/22/12