Performance evaluation of wearable antennas using flexible substrates.
Rizwan, Muhammad (2015)
2015
Master's Degree Programme in Electrical Engineering
Tieto- ja sähkötekniikan tiedekunta - Faculty of Computing and Electrical Engineering
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Hyväksymispäivämäärä
2015-08-12
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tty-201507281438
https://urn.fi/URN:NBN:fi:tty-201507281438
Tiivistelmä
Body Area Network (BAN) is an expansion of Personal Area Networks (PANs) which enabled different devices to communicate with each other by placing them on human body. The applications of BAN include measuring run time physiological changes, foul playing detection in sports and navigation. We can also continuously keep the record of patient’s health by monitoring the changes in human body. The frequency bands used in such systems are Industrial, Scientific and Medical (ISM) band (2.40 GHz to 2.50 GHz) and Wireless Body Area Network application band at 2.45 GHz. Wearable antennas are used as transceiver nodes in WBAN systems for sending and receiving the data or information. These antennas are kept flexible so that they do not hinder the movements of the human body. To make sure these antennas are flexible, different flexible materials are now-a-days used as substrates for the designing of the wearable antennas.
Due to the constant motion of human body, it is difficult to get the proper polarization alignment of the transceiver nodes for better power reception. Circular polarization (CP) operation eliminates the need to continuously align two nodes for receiving maximum power. Previously reported wearable antennas are mostly non-flexible, linearly polarized, large in size or have thick substrate which makes them difficult to be used in wearable applications.
In this thesis, two different flexible substrates have been used to design circularly polarized wearable antennas and their performance is analyzed near human body. The selected antenna type is microstrip patch antenna with circular patch configuration operating for ISM band and WBAN applications at 2.45 GHz. To have a better idea of performance of flexible substrates, two substrates i.e. Denim and Ethylene Propylene Diene Monomer (EPDM) foam are selected having 1 mm and 3 mm thickness respectively. Copper tape with thickness 0.25 mm is used as conductive part for both the antennas. To achieve circular polarization a rectangular slot along the diagonal axis is inserted at the center of the circular patch. Performance of both antennas is analyzed in free space and in human body vicinity.
The antenna showed good agreement between simulated and measured free space results, however due to fabrication inaccuracies, some shifting of operating frequency is observed. EPDM antenna shows better results in terms of return loss, bandwidth and Axial Ratio (AR) compared with Denim antenna in free space. Free space bending is analyzed in two planes i.e. xz and yz, with two different bending radii (50 mm and 75 mm). Bending analysis showed that the performance of the antenna is affected more when the antenna is bent along the direction which determines its resonance length. Impedance matching is improved when the antenna is bent in xz-plane. Beam width increases in the plane of bending which results in decreased antenna gain.
Human body is made up of 65-75 % water. The dielectric constant of water is very high (above 75 at 20°C) and at higher frequencies it absorbs power and can reduce the efficiency of any antenna placed nearby. The near body performance of the designed antennas is analyzed by varying the distance between the antenna and the human body using a polyethylene foam sheet of different thicknesses. Three different distances were selected i.e. 0 mm (directly on skin), 2 mm and 5 mm to have a better idea of the effects with respect to varying distance between the human body and the wearable antenna. The performance of the antenna in terms of the input matching and the impedance bandwidth is analyzed on two different body parts i.e. arm and leg, with bending in both xz-plane and yz-plane. The results show a decrease in return loss due to lossy nature of the human body, and increase in bandwidth due to the lowering of the Q factor of the antenna. The antenna gain is increased due to low penetration depth and reflections from the human body in simulation and real on-body measurements at high frequencies. The percentage increase in gain near human body for EPDM and Denim is around 19.56% and 10.66% respectively.
The summary of results shows that the designed antennas operate for desired frequency bands with good efficiency in all simulated and measured scenarios; however EPDM antenna is better in terms of antenna impedance and radiation characteristics, weight, wearing comfort, and can naturally and forcibly retract to its original dimensions after deformation. The copper tape used in fabricating the antenna peels off with time and makes the antenna less reliable. In future, the conductive part can be printed using Inkjet or Micro Dispense 3D Printer for a more reliable and accurate design of the wearable antenna.
Due to the constant motion of human body, it is difficult to get the proper polarization alignment of the transceiver nodes for better power reception. Circular polarization (CP) operation eliminates the need to continuously align two nodes for receiving maximum power. Previously reported wearable antennas are mostly non-flexible, linearly polarized, large in size or have thick substrate which makes them difficult to be used in wearable applications.
In this thesis, two different flexible substrates have been used to design circularly polarized wearable antennas and their performance is analyzed near human body. The selected antenna type is microstrip patch antenna with circular patch configuration operating for ISM band and WBAN applications at 2.45 GHz. To have a better idea of performance of flexible substrates, two substrates i.e. Denim and Ethylene Propylene Diene Monomer (EPDM) foam are selected having 1 mm and 3 mm thickness respectively. Copper tape with thickness 0.25 mm is used as conductive part for both the antennas. To achieve circular polarization a rectangular slot along the diagonal axis is inserted at the center of the circular patch. Performance of both antennas is analyzed in free space and in human body vicinity.
The antenna showed good agreement between simulated and measured free space results, however due to fabrication inaccuracies, some shifting of operating frequency is observed. EPDM antenna shows better results in terms of return loss, bandwidth and Axial Ratio (AR) compared with Denim antenna in free space. Free space bending is analyzed in two planes i.e. xz and yz, with two different bending radii (50 mm and 75 mm). Bending analysis showed that the performance of the antenna is affected more when the antenna is bent along the direction which determines its resonance length. Impedance matching is improved when the antenna is bent in xz-plane. Beam width increases in the plane of bending which results in decreased antenna gain.
Human body is made up of 65-75 % water. The dielectric constant of water is very high (above 75 at 20°C) and at higher frequencies it absorbs power and can reduce the efficiency of any antenna placed nearby. The near body performance of the designed antennas is analyzed by varying the distance between the antenna and the human body using a polyethylene foam sheet of different thicknesses. Three different distances were selected i.e. 0 mm (directly on skin), 2 mm and 5 mm to have a better idea of the effects with respect to varying distance between the human body and the wearable antenna. The performance of the antenna in terms of the input matching and the impedance bandwidth is analyzed on two different body parts i.e. arm and leg, with bending in both xz-plane and yz-plane. The results show a decrease in return loss due to lossy nature of the human body, and increase in bandwidth due to the lowering of the Q factor of the antenna. The antenna gain is increased due to low penetration depth and reflections from the human body in simulation and real on-body measurements at high frequencies. The percentage increase in gain near human body for EPDM and Denim is around 19.56% and 10.66% respectively.
The summary of results shows that the designed antennas operate for desired frequency bands with good efficiency in all simulated and measured scenarios; however EPDM antenna is better in terms of antenna impedance and radiation characteristics, weight, wearing comfort, and can naturally and forcibly retract to its original dimensions after deformation. The copper tape used in fabricating the antenna peels off with time and makes the antenna less reliable. In future, the conductive part can be printed using Inkjet or Micro Dispense 3D Printer for a more reliable and accurate design of the wearable antenna.