# SWR, reflection coefficient and attenuation calculations

Question asked by JepJep on May 27, 2006
Latest reply on May 29, 2006 by JepJep
Hi!

I`m trying to measure frequency response of DUT with NA and power meter, using NA as source. Lets say that I have measured source power at one frequency with power meter (-0.60 dBm). This signal goes to DUT. I measured SWR on DUT at the same same frequency, that would be 1.3, so the Reflection coefficient would be (SWR-1)/(SWR+1)=0.1304. Mismatch loss would then be -10*log10(1-0.01304^2)=0.0745 dBm. So due this, the actual power what goes into DUT is -0.6dBm-0.0745dBm=-0.6745dBm. Then I measure power output from DUT with the same power meter and the result would be -0.95dBm. So is the total attenuation due the SWR -0.95dBm-(-0.6745dBm)=-0.2755dBm. Am I using right calculations? I tried to do this procedure, and i still get about 0.1dBm error at those frequencys that have high SWR, so what goes wrong? Is the mismatch loss calculated right, and is there yet something else that I should measure/calculate? Im measuring with NA and power meter because I have measured S21 of the DUT with 2-3 different analyzers but somehow everyone gives me a slightly different response, so I can`t trust this S21 measurement.  This project that I`m working on is actually a power measurement, but I added a switch into transmission line. So if I do the same measurement first with correct cables, then using these cables and switch, the measurement error can be calculated. This is how I got that 0.1dBm error. This error is on these frequencys that have high SWR (up to 1.3). I think in this measurement the desirable error would be something like 0.02dBm.

EDIT:

I askes actually same question a while ago, and I got advice to use NA source & receiver Power meter calibration. I have Rhode & Schwarz NRVS power meter. Can I do this calibration with this meter, or does it have to be HP meter?

Thanks!