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Measuring a Long Run of Waveguide using TDR

Question asked by red58fish on Jul 3, 2008
Latest reply on Jul 6, 2008 by Dr_joel
I'm using an 8753C to measure WR1500 waveguide. The run of guide is divided into 80 lengths at 136" each. I'm sweeping from 540 to 710MHz with 801 points and using the time domain bandpass mode.

Looking in the time domain I can see the reflections from the flanged connections as expected. However, I also see unexpected "reflections" between some of the flanges, as if there was a dent in the middle of the 136" length causing a reflection on the order of the flange (-40dB) These unexpected reflections occur in groups. In other words, the first 8 lengths of guide show no reflection in the middle and the next 10 do, and then another approximately 17 without, and then about 10 more with the reflection.

If I remove the first 10 lengths of guide and remeasure, the location of these reflections moves out 10 lengths and keeps about the same pattern of recurrence.

I think I must have something wrong with the 8753C setup. I've increased the #of points with no change.

What could be the problem here?