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Reference Cavity

  • The dimensions of the first copper prototype were known to be incorrect. Predicting a frequency drop of ~330 MHz and a loaded Q drop of 160.
  • The BPM was clamped together and the feedthroughs also clamped without the vacuum seals so the separation was increased by 0.45 mm.
  • The VNA was connected to the two ports of the reference cavity where S parameters could be measured.

Monopole Mode


S Parameter S12 S21
Frequency /GHz 14.699 14.698
Loaded Q 235 235

  • The measured resonant frequency is 300 MHz below the desired 15 GHz.
  • The feedthrough antennae were not well concentrically aligned and frequency was seen to vary on repeated repositioning of the feedthrough.
  • Simulations need to be performed to see if this effect is reproduced.
  • The Q values can be determined by plotting a smith chart.


S Parameter Frequency /GHz Unloaded Q External Q Loaded Q Loaded Q check
S11 14.698 382 588 231 231
S22 14.699 426 530 238 236

  • The unloaded Q factor is much smaller than expected, bringing the loaded Q down.
  • This could be caused by uneven clamping or not clamping strong enough or by excessive surface roughness.
  • The frequency and Q factor was seen to vary with antenna offset, so the effect of this offset needs to be investigated.

Feedthrough Offset Variation

  • Simulations were performed with the feedthrough offset 1 degrees the along the beam pipe direction.
  • Corresponding to a antenna tip movement range of 0.26 mm within the cavity.
  • Positive values correspond to offsets towards the flat surface of the cavity, negative values correspond to offsets towards the reentrant part of the cavity.

Offset Angle /degrees Frequency /GHz Q_0 Q_ext Q_L
-1.5 14.736 2739 203 189
-1.0 14.743 2732 194 181
-0.5 14.751 2730 193 180
0.0 14.760 2727 197 184
0.5 14.771 2724 210 195
1.0 14.780 2722 230 212
1.5 14.794 2715 260 237


  • The frequency change depending on the antenna offset is ~700MHz/mm.
  • The Q factor change is not enough to explain the low loaded Q factor of the position cavity.

Dipole Mode


S Parameter Frequency /GHz Loaded Q
S11 19.090 98

Position Cavity

  • The BPM was clamped as above and all four ports connected to the VNA.
  • A drop in frequency from 15 MHz was predicted from simulation.


S Parameter Frequency /GHz Loaded Q
S13 14.988 1434
S24 14.991 1545

  • The frequencies are very close to the expected value of 14.985 GHz, the Q factors are three times larger.
  • The coupling here is too weak to rely on the Q factors calculated from the Smith chart but it does offer clues to the source of the high loaded Q.


S Parameter Frequency /GHz Unloaded Q External Q Loaded Q Loaded Q check
S11 14.988 1733 7888 1421 1420
S22 14.991 2026 6246 1537 1530
S33 14.988 1713 8327 1427 1421
S44 14.991 1647 13628 1462 1469

  • The external Q is much higher than the design value of ~600, possibly caused by the incorrect coupler.

Position Cavity with Reference Cavity Antenna

  • The position cavity was simulated with the feedthrough antenna from the reference cavity.

Frequency /GHz Unloaded Q External Q Loaded Q
14.983 3309 3192 1625

  • In the simulation the external Q is much larger than expected, driving the loaded Q up.
  • The value of the loaded Q is closer to that measured from the transmission curves.
  • This is encouraging evidence that using the correct feedthroughs and vacuum seal will results in Q's close to the design values.

Cross Coupling


S Parameter S21 S32 S43 S14
Maximum Signal /dB -30.9 -31.0 -39.8 -36.8

  • All maximum cross couplings are below -30 dB.

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Topic revision: r9 - 21 Aug 2014 - JackTowler

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