G-Zero - First Cavity Results

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Once the probes were properly attached, we got the results below. The graph, figure 1, on the left is expected results and the graph in the middle, figure 2, is measured results.

The results, at first, look very promising with an insertion loss of just 0.6dB and a narrow deep spike at 2.45Ghz. The green line on the "expected" graph matches the yellow line on the measured and red matches blue, respectively.

A quick analysis:

  • The range on the measured results is from 2.4Ghz to 2.5Ghz, the range on the simulation is 2.4274Ghz to 2.4632Ghz
  • Simulated insertion loss is 0.77dB (S2,1), actual is 0.63dB!
  • The measured results show a double resonance (the two blue spikes) as does the simulation (two red spikes)!
  • The maximum S1,1 simulated was -14dB, but actual is -28dB and you can tell the spike goes much much lower where, although there is no picture, it measured -48dB!

However, the width of the spike at the top in the simulation is maybe 2 or 3 Mhz wide (take 2.455Ghz minus 2.445Ghz which is 10Mhz and the width looks to be about a fifth that wide), but the measured width looks to be twice that (take 2.46Ghz minus 2.44Ghz equals 20Mhz and the width is roughly fifth of that or, 5 or 6Mhz wide). The problem is that the Q factor is highly dependent on the width of the spike and the insertion loss. Once calculated, we only got a Q of 350 but were expecting a result in the tens of thousands!

The next week, we hooked the cavity up to a better network analyzer:

The results show an insertion loss of 1.6dB and after playing with the tuning, it is apparent that the cavity is sensitive to flexing. By putting pressure on the cavity, for some reason the insertion loss would drop to 1dB and the resonance got much better, as deep as 40dB. The calculated Q for this test was a paltry 500 which is tens of thousands lower then expected.

Analysis

Unfortunately, this first cavity has a significantly lower Q then expected, making it unusable. From Shawyer's first test cavity, the Q was reported at 5900 and his "Demonstration engine", which we are trying to reproduce, had a reported Q of 50,000. In our case, the likely cause is fabrication tolerances which means a second more accurate method of fabrication resulting in a stiffer cavity may be necessary.


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