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Modified Reference Cavity Measurements at CERN

The signal was excited using a weakly coupled probe and measured from the output feedthroughs. We only had one spacer and we put this on feedthrough 2. The spacings between the end of the feedthrough and the wall of the reentrant part of the cavity were therefore:

FeedthroughSorted ascending Spacing/mm
1 0.0035
2 0.0080 (nominal)

Port 1 of the VNA was connected to the antenna while port 2 was connected to feedthrough 1. Feedthrough 2 was terminated with a matched load (left) and short circuited (right).

port1wload_a.png port_1wshort.png

Port 2 was connected to feedthrough 2. Feedthrough 1 was terminated (left) and short circuited (right).

port2wload.png port2wshort.png

Using the trace data, the measured quality factors were:

Feedthrough QL QL+short f0/GHz f0,short/GHz
1 76 116 14.951 15.107
2 73 214 14.963 15.119

The loaded quality factor is very low, ~75. This can be attributed to the lack of a spacer for feedthrough 1. In simulation, the loaded quality factor is 150 with this spacer and 77 without and so and increase by a factor of ~2 in the loaded quality factor is expected with the feedthrough gaskets in place. An increase in the resonant frequency is also expected. In simulation, this increase is 33MHz for the loaded cavity. There is also another mode present when feedthrough 2 was tested with feedthrough 1 short-circuited. This would've affected the measurements. If possible, these measurements will be repeated at the beginning of May with more spacers.

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Topic revision: r3 - 17 Apr 2012 - FrankieCullinan

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