Broadband nested antenna

Fu, W 2011, Broadband nested antenna, Doctor of Philosophy (PhD), Electrical and Computer Engineering, RMIT University.


Document type: Thesis
Collection: Theses

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Title Broadband nested antenna
Author(s) Fu, W
Year 2011
Abstract This thesis investigates a novel dual frequency range low profile antenna system containing two nested spiral antennas operating over 2 – 18 GHz and 30 – 40 GHz respectively. The exploration and development of a broadband microstrip spiral without cavity backing has been implemented in the range of 2 - 18 GHz. The relationship between the structural parameters of equiangular spirals and their performance is investigated by simulating a variety of spiral structures. The input impedance, gain, axial ratio and radiation patterns of spiral antennas on a grounded dielectric substrate are compared with that of a spiral in free space to demonstrate the shortcomings of such a structure (i.e. narrow impedance bandwidth and poor gain).

An effective way to achieve a broad bandwidth is proposed by introducing an impedance profile to remove the residual current along the spiral arms. Initially a Thin Film Resistive Layer (TFRL) was incorporated into a microstrip equiangular spiral antenna to absorb the residual current along the arms. Four spiral antennas with TFRLs of different thickness were developed to explore the TFRL application technique. Measured results and the difficulties in both simulation and fabrication experienced are demonstrated, analysed and addressed.

An alternative to the use of a TFRL as an impedance profile on microstrip spirals, an exploration of spirals with embedded chip resistors had been conducted. The radiation physics of microstrip equiangular spirals are examined in the time domain using the XFDTD simulation package. XFDTD is used to analyse the current density distribution on the spiral arms under the excitation of pulse and harmonic waves. An analysis of the current density distribution at steady state provides guidelines for arranging the chip resistors efficiently. Chip resistor loading rules have been developed from these outcomes. A two dual arm equiangular spiral antennas with embedded chip resistors has been simulated, fabricated and measured. The antenna has a compact tapered balun which is horizontally placed inside the spiral antenna substrate to reduce the volume of the completed spiral/feed structure.
The spiral with embedded chip resistors and a compact balun is selected as the low frequency element (2 – 18 GHz) for a nested antenna system. A dual arm Archimedean spiral antenna with cavity backing (operating over 30 – 40 GHz) is developed to be nested within the aperture of the low frequency spiral. The high frequency antenna was integrated into the substrate of the low frequency element, in the opening between the spiral arms. An Electronic Band Gap (EBG) architecture was applied to the rim of the nested antenna substrate to mitigate the consequences of surface waves. The entire nested antenna system was fabricated and measured, and the results are presented and discussed.
Degree Doctor of Philosophy (PhD)
Institution RMIT University
School, Department or Centre Electrical and Computer Engineering
Keyword(s) Microstrip
broadband
nested
spiral
antenna
chip resistor
EBG
current distribution
military
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Created: Wed, 27 Jun 2012, 14:02:51 EST by Guy Aron
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