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Preliminary Simulation

  • The geometry below was simulated with CST, where the return loss was monitored and the level and the resonant frequency of the signal was recorded.

wggeom.png

  • The return loss is very sensitive to the separation between the waveguide wall and the antenna, the level being below -30 dB at 15 GHz over a 29 micron separation range, below -40 dB this range is only 9 microns.

Originals11.png originalbw.png

Design Investigation

  • Two modified feedthrough designs were investigated: an antenna with a bead on the tip ,left, and a "stepped" antenna of two different radii, right.

beadgeom.png longgeom.png

Design -30 dB Range /um -40 dB Range /um Optimum Separation /mm
Original 29 9 0.169
Bead 58 19 0.4
Stepped 54 18 0.6

  • The bead design was chosen to be investigated further as it had a slightly larger acceptable separation range. The stepped design also seemed to be quite sensitive to the step parameters.

Bead investigation

  • The radius and length of the bead were altered and the separation ranges were measured.

r20.png r30.png

  • Bead Length was 1 mm.
  • For the -20 dB threshold, a bead radius of 1.65 mm gives the largest separation, for -30 dB, 1.55 mm gives the largest range.
  • 1.55 mm bead radius was chosen because 1.65 mm is too close to the outer coaxial radius of 1.75 mm.

l20.png l30.png

  • Bead radius was 1.55 mm.
  • The separation range increases with decreasing bead length.
  • Due to an error a bead length of 0.5 mm was chosen to be studied further, even though 0.3 mm gives a better separation range.
  • However, it was later discovered in ACE3P that decreasing the bead length increased the simulated loaded Q. 0.5 mm turned out to be a fair compromise between Q factor and separation range: ClicRedesignSim.

  • A bead of radius 1.55 mm and length 0.5 mm at -20, -30 and -40 dB thresholds gives separation ranges of 204, 63 and 20 um respectively.

Coaxial Tolerance

  • The offset and angle of the inner coaxial component relative to the outer were altered to investigate the tolerances on the coaxial.

Offset /um -20 dB Range /um -30 dB Range /um -40 dB Range /um Resonant Frequency /GHz
0 204 63 20 15.018
10 205 61 21 15.076
30 205 63 18 15.138
50 204 62 10 15.236

CoaxOffset_05.png off05.png

  • Increasing the offset of the inner coaxial from the center of the outer coaxial does not have an appreciable change on separation ranges at -20 and -30 dB threshold levels.
  • There is a large change in the peak frequency, >200 MHz, which accounts for the change in the -40 dB separation range. At larger offsets this frequency change will be great enough to affect the other threshold values as well.

Z Tilt /degrees -20 dB Range /um -30 dB Range /um -40 dB Range /um Resonant Frequency /GHz
-0.5 191 64 - 14.584
0 204 63 20 15.018
0.25 201 66 17 15.169
0.5 197 65 - 15.312
1 191 56 - 15.572

Y Tilt /degrees -20 dB Range /um -30 dB Range /um -40 dB Range /um Resonant Frequency /GHz
-0.5 205 49 - 15.638
0 204 63 20 15.018
0.5 200 59 - 15.302

Tilt05Z.png

  • In the Z direction (the long dimension of the waveguide) a negative tilt (towards the cavity) has a greater affect on the frequency and therefor the separation ranges.
  • However, over a 0.5 degree range, the change in the allowed range is still small.
  • In the Y direction (dimension parallel with the beam pipe) the negative tilt (towards the waveguide center) again affects the frequency more.
  • The ranges are affected less by tilts in this axis.

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Topic revision: r4 - 25 Jul 2013 - JackTowler

 
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