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using an HP 8405A to measure SWR ?
On Sun, 01 Jan 2006 16:51:37 -0800, dansawyeror
wrote: Owen, Yes, you are right. The single coupler doesn't isolate the channels. Putting a 10 dbm attenuator between the tee and the coupler changes the 50 Ohm reading. I think a dual directional coupler is required. I will have to put this on hold until that problem is solved. Dan, it seems to me that you should be able to make measurements with a single directional coupler (DC) You could connect your signal generator to the directional coupler via a 40dB attenuator, and put the chan A probe T on the sig gen end of the attenuator. This sample should be fairly independent of the reflection from the unknown load (to the extent of the 40dB attenuator), and so approximately proportional to the incident wave alone. The sample from the DC "reflected" port (properly terminated) is fairly independent of the incident wave (depending on the F/B ratio of the coupler) and so is approximately proportional to the reflected wave alone. Calibration of the B channel magnitude with a s/c and o/c taken as rho=1 provides the basis for measurement of Gamma. The angle of Gamma should be calibrated to 180 and o deg respectively. BTW, the angle of Gamma for a 50 ohm termination is unimportant if rho is very small. The angle of Gamma is real important for s/c and o/c and ought be almost exactly 180 deg difference (if not, you have a instrument problem). Following this procedure, if the magnitude of the B channel on the unknown load measures for example 9.5dB below the B chan magnitude on a s/c, then the return loss is 9.5dB and the VSWR is 2:1. rho (the magnitude of Gamma) is 0.333 and you could measure the phase offset from the o/c angle to determine the angle of Gamma. Why won't this work? Owen -- |
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