Measurement of circularly polarized antennas
Viuho, Max (2025)
2025
Sähkötekniikan DI-ohjelma - Master's Programme in Electrical Engineering
Informaatioteknologian ja viestinnän tiedekunta - Faculty of Information Technology and Communication Sciences
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Hyväksymispäivämäärä
2025-02-14
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tuni-202502142206
https://urn.fi/URN:NBN:fi:tuni-202502142206
Tiivistelmä
Measurement of antennas is important to verify their performance. Goal of this work is to improve measurements of circularly polarized antennas in Radientum’s antenna measurement system. The system is capable of polarization measurements using amplitude-phase method, which requires magnitude and phase measurements of electric field in two orthogonally polarized components. Measurement results using the method have turned out to be unreliable. It has been suspected that reason for this problem is that phase of the electric field is not measured correctly, which is caused by an inadequate calibration of the system.
In this work measurement capability of the system is improved to measure circularly polarized antennas more reliably. Achieving the goal requires studying antenna characteristics, most common antenna measurement techniques, and antenna measurement ranges. Additionally, it is studied how such a system should be calibrated for gain and polarization measurements, after which the calibration process is carried out. After the calibration process has been established and applied, correctness of polarization measurements is verified with verification measurements, and with comparison to another system. Lastly, measurement uncertainties in the setup are evaluated by measuring and estimating different sources of measurement uncertainty.
The calibration process was accomplished successfully. It was found out that before this calibration process, orthogonally polarized measurement channels of the system were not giving consistent readings, making polarization measurements unreliable. The problem concerned not just phase but also amplitude of the measured electric field. Verification measurements prove that polarization measurements are now more reliable. The channels were giving readings with differences over 2 dB in amplitude and 10˝ in phase of measured electric field before the calibration process. After the calibration process differences are at most about 0.3 dB and 3˝. To further ensure correctness of measurements a circularly polarized test antenna was used to compare Radientum system to measurements in Satimo Starlab system in Tampere University. Before the calibration process there were over 3 dB differences in measured axial ratio between the systems. After the calibration process differences are mostly less than 1 dB in the measured frequency band.
Finally, measurement uncertainties in gain and polarization measurements are evaluated. Uncertainty in gain measurements is estimated to be 2.5 dB in frequency range of 0.4-8.5 GHz. Uncertainty in measured axial ratio of a circularly polarized test antenna is estimated to be 2.2 dB. This study suggests that chamber reflections have the greatest contribution to combined uncertainty.
In this work measurement capability of the system is improved to measure circularly polarized antennas more reliably. Achieving the goal requires studying antenna characteristics, most common antenna measurement techniques, and antenna measurement ranges. Additionally, it is studied how such a system should be calibrated for gain and polarization measurements, after which the calibration process is carried out. After the calibration process has been established and applied, correctness of polarization measurements is verified with verification measurements, and with comparison to another system. Lastly, measurement uncertainties in the setup are evaluated by measuring and estimating different sources of measurement uncertainty.
The calibration process was accomplished successfully. It was found out that before this calibration process, orthogonally polarized measurement channels of the system were not giving consistent readings, making polarization measurements unreliable. The problem concerned not just phase but also amplitude of the measured electric field. Verification measurements prove that polarization measurements are now more reliable. The channels were giving readings with differences over 2 dB in amplitude and 10˝ in phase of measured electric field before the calibration process. After the calibration process differences are at most about 0.3 dB and 3˝. To further ensure correctness of measurements a circularly polarized test antenna was used to compare Radientum system to measurements in Satimo Starlab system in Tampere University. Before the calibration process there were over 3 dB differences in measured axial ratio between the systems. After the calibration process differences are mostly less than 1 dB in the measured frequency band.
Finally, measurement uncertainties in gain and polarization measurements are evaluated. Uncertainty in gain measurements is estimated to be 2.5 dB in frequency range of 0.4-8.5 GHz. Uncertainty in measured axial ratio of a circularly polarized test antenna is estimated to be 2.2 dB. This study suggests that chamber reflections have the greatest contribution to combined uncertainty.