Fabrication and Measurement of Foam Optical Components for Intensity Mapping Experiments

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Abstract

An investigation into the fabrication and measurement of foam optical components is presented. The thesis will concentrate on the solution to some of the practical challenges faced when constructing an instrument for intensity mapping. The work presented is based around a project entitled BINGO. A challenge for this project is the large size and number of feed horns required. As an alternative to conventional fabrication methods, foam and copper tape has been used to build a prototype consisting of 78 foam sheets. To evaluate horn performance polar diagram measurements are needed to ensure a good agreement with theory. As the BINGO receivers require to be fed with circularly polarised outputs, the construction and testing of a foam and copper tape septum polariser is also examined. The polar diagram measurements were made using a turntable within a screened enclosure and carried out at 1440 MHz using a CW signal transmitted using a yagi antenna. Due to the large feed size it is not possible to rotate about the phase centre. Distance measurements from the transmitter to feed and rotational axis to phase centre were therefore taken to allow the geometry of the test range to be analysed. A computer script was written to correct the measurements for geometry. Measurements for E and H plane along with a 45 degree cut all resulted in an excellent agreement with theory down to approximately - 40 dB from the peak over an angular scale of approximately +/- 140 degrees. Although measurements seem to exhibit instability at levels below approximately - 40 dB, with careful angle alignment features of the beam pattern down to approximately - 70 dB can be detected. 45 degree cross-polarisation yielded poor results without the expected zero-axis null. At 90 degrees the null was clearly seen highlighting the need for crucial alignment. Overall the horn performance is exceptional and provides much confidence in the fabrication process. Two identical foam and copper tape septum polarisers were also constructed to a 1mm tolerance to allow back-to-back testing. Signal probes fabricated from a copper taped form gave an S11 to S22 match of approximately - 30 dB over 100 MHz with a return loss of just 0.1 dB in a rectangular foam waveguide. The initial test results for the polarisers were poor and it was discovered that oxidation of the copper tape was the cause. Once rectified results showed that the construction method is feasible. Overall the fabrication method of foam and copper tape appears to be viable, but more detailed testing is required before the fabrication techniques are used in a real experiment.

Layman's Abstract

An investigation into the fabrication and measurement of foam optical components is presented. The thesis will concentrate on the solution to some of the practical challenges faced when constructing an instrument for intensity mapping. The work presented is based around a project entitled BINGO. A challenge for this project is the large size and number of feed horns required. As an alternative to conventional fabrication methods, foam and copper tape has been used to build a prototype consisting of 78 foam sheets. To evaluate horn performance polar diagram measurements are needed to ensure a good agreement with theory. As the BINGO receivers require to be fed with circularly polarised outputs, the construction and testing of a foam and copper tape septum polariser is also examined. The polar diagram measurements were made using a turntable within a screened enclosure and carried out at 1440 MHz using a yagi antenna. Due to the large feed size it is not possible to rotate about the phase centre. Distance measurements from the transmitter to feed and rotational axis to phase centre were therefore taken to allow the geometry of the test range to be analysed. A computer script was written to correct the measurements for geometry. The measurements resulted in an excellent agreement with theory. Overall the horn performance is exceptional and provides much confidence in the fabrication process. Two identical foam and copper tape septum polarisers were also constructed to a 1mm tolerance to allow back-to-back testing. Signal probes were also fabricated from a copper taped form. The initial test results for the polarisers were poor and it was discovered that oxidation of the copper tape was the cause. Once rectified results showed that the construction method is feasible. Overall the fabrication method of foam and copper tape appears to be viable, but more detailed testing is required before the fabrication techniques are used in a real experiment.

Keyword(s)

Foam Optical Components; Intensity Mapping

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