February 1, 2017E2-105 EITC2:30 PM

Orientation-Dependent Adaptive Antenna for Low Earth Orbit Satellite Communications

Dept. of Elec. & Comp. Eng.MSc Thesis Defence

Bo Woods

Examining CommitteeDr. Greg Bridges, Dept. of Elec. & Comp. Eng. (advisor)

Dr. Puyan Mojabi, Dept. of Elec. & Comp. Eng. Dr. Madjid Birouk, Dept. of Mech. Eng.

Presentation Overview

1. Motivation

2. Specific Absorption Rate (SAR) Levels

3. Desired Radiation Properties for Satellite Communications

4. Techniques for Reducing SAR Levels

5. Back-to-Back Patch Antenna

6. Impedance Monitoring System

7. Conclusion

8. Future Work

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Motivation• Remote areas satellite communications only viable option

• High transmit power required for satellite communications

• Leads to unsafe human exposure levels

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• Require a method of sensing the head and shaping radiation away from the user to safely utilize the full transmit power

• No compact methods currently available for commercial use

SAR Levels• Specific absorption rate (SAR) levels are a measure of how much RF energy is

absorbed by a lossy dielectric

• IEEE P1528 method of calculating SAR in pt. form. Averaged over 1 g or 10 g cubes of tissue:

SAR =σ E

2

2ρ

W

kg(1.1)

ConditionUncontrolledEnvironment

ControlledEnvironment

Peak averaged SAR for the head, neck and trunk averaged over 1 g of tissue

1.6W

kg8W

kg

Peak averaged SAR in the limbs average over any 10 g of tissue

4W

kg20

W

kg

SAR restrictions for 100 kHz – 6 GHz [1]

[1] Limits of Human Exposure to Radiofrequency Electromagnetic Energy in the Frequency Range from 3 kHzto 300 GHz, Industry Canada Safety Code 6, 2015.

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SAR Levels

• Power is proportional to E2 determine SAR for different input powers using:

SARx =Px

PmeasSARmeas

W

kg(1.2)

• Max transceiver power for a handheld satellite device is typically 2 W [2]

[2] Solara Remote Data Delivery Incorporated, “Field Tracker 2100,” user’s manual, 2010.

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Commercial SAR Levels• Most commonly used handheld satellite antenna quadrifilar helix

• 1 < G < 6 dB, robust, wide beamwidth, compact, insensitive to nearby objects

• From a SAR certification report for a quadrifilar helix [3]

SARmeas = 1.547W

kg; Pmeas = 370 mW

• From Eq. 1.2: When Px = 2W SARx = 8.36W

kg

[3] Certificate of Compliance R&D SAR Evaulation, “SHOUT nano,” RF Exposure Lab, San Marcos, CA, USA, Test report no. SAR.20120617, June 2012.

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Quadrifilar helix

Desired Radiation Properties for Sat Comms• Iridium satellite network was chosen since it allows for lightweight handheld devices to be

used on the ground

• Iridium has 66 satellites that traverse a polar orbit 780 km above Earth (low earth orbit)

• Satellite coverage drops < 20% as the elevation angle > 40∘ [4]

• Use omnidirectional pattern for optimal coverage

Optimal satellite coverage with an omnidirectional antenna pattern

[4] F. Vatalaro et al., “Analysis of LEO, MEO, and GEO global mobile satellite systems in the presence of interference and fading,” IEEE Journal on Selected Areas in Communications, vol. 13, no. 2, pp. 291–300, Feb 1995.

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Desired Radiation Properties for Sat Comms

• Circular polarization required due to:

• Random orientation of the user with the satellites

• Faraday rotation in the ionosphere due to Earth’s magnetic field

• G > −3.5 dB [5]

• Axial Ratio < 4 dB, where AR =Eθ

Eϕ

[5]

• Frequency band 1616 − 1626.5 MHz

• SAR< 1.6W

kgwhen antenna is near the user’s head [1]

[5] ICAO Technical Manual for Iridium Aeronautical Mobile Satellite Service, v1.1, International Civil Aviation Organization, Montreal, QC, Canada, 2006, p. 41.

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Techniques for Reducing SAR Levels• Increasing the space between the antenna and user’s head

• Using shielding techniques such as:

• Ferrite loading

• Artificial magnetic conductor (AMC) based surfaces

• Adaptive beam shaping (active or passive approach)

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Adaptive PEC/PMC Beam Shaping Device• Designed a normal-mode helix with a circularly polarized pattern

• Designed a PEC/PMC metamaterial plane to reflect the circularly polarized pattern

• To work properly it required a 𝜆

4separation, which did not fit the thin form factor

requirement for modern commercial phones

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My Technique

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Back-to-Back Patch Antenna• Theory of a Single Patch Antenna

• Making a Patch Circularly Polarized

• Design of the Back-to-Back Patch

• Radiation Pattern

• SAR Simulation

• Beam Shaping Using Feed Phasing

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Theory of a Single Patch Antenna

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Making a Patch Circularly Polarized

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• Exciting both modes 90∘ out of phase will generate circular polarization

Exploded view of the design

Design of the Back-to-Back Patch

Reflection coefficient simulated in HFSS

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• Both patch elements have identical dimensions and RHCP

• Built on 1.57 mm thick RT/duroid 5880 substrate (εr

′ = 2.2; tanδ = 0.0009)

• 10 dB bandwidth covers the Iridium band of 1616 – 1626.5 MHz

Measurement Setups

Reflection coefficient measurement setup Radiation pattern measurement setup

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Radiation Pattern (Both Patches On)

Back-to-back patch antenna (both patches on) radiation pattern with respect to a RHCPreceiver. Simulation shown on the left and measured on the right. Max gain +4.5 dB.

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Radiation Pattern (One Patch On)

Back-to-back patch antenna (one patch on) radiation pattern with respect to aRHCP receiver. Simulation shown on the left and measured on the right.

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Specifications (Both Patches On)

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SAR Simulation

• For a 2 W transceiver power output

• 0.71 W delivered to each patch element

1.5 dB loss in the feed network

• Within the 1.6 W

kgSAR

restriction for distances of ≥ 3 cm

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[6] Certification of Compliance R&D SAR Evaulation, “Radi-Chip with iPhone 5 & iPhone 5s,” RF Exposure Lab, San Macros, CA, USA, Test report no. R&D.20160703, July 2016.

3D SAR simulation for 0.71 W delivered to each patch element

[6]

Beam Shaping Using Feed Phasing

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• Possible applications:• Locking into satellites directly overhead

• Reducing radiation exposure to the human user

Simulation in HFSS

Impedance Monitoring System

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• Objective automatically monitor presence of the head and turn off patch 1 if the phone is placed near the head

Reflection Coefficient Measurement with a Phantom

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• Impedance change of antenna monitored by measuring the reflection coefficient

• “Blue phantom” mixture of glyercin and water with ෝεr = 42.5 − 𝑗22.5 around 1600 MHz.

Courtesy of the Electromagnetic Imaging Lab (EIL) at the U of M.

Isolation Test With Only One Patch On

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Measured reflection coefficient

1. No phantom present

2. Phantom present on the off patch side

Isolation Test With Only One Patch On

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• Also verified the radiation pattern was

not influenced by the phantom when it

was on the off patch side

Assumptions

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1. Impedance monitoring would only occur before the phone transmits

2. Only patch 1 would be turned on/off

3. Patch 2 would always be on

4. A user about to transmit would have the phone placed up against their head in time for the impedance monitoring system to detect them

5. Antenna would be located at a place where the user’s hand would not cover patch 2An example of a miniaturized version of the back-to-back

patch antenna in a phone with a user making a call

Implementation and Operation

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High-level block diagram of the impedance monitoring system

State diagram showing operation (Tx = transmit)

Conclusion• An adaptive beam shaping device was designed and implemented using a back-to-

back patch antenna controlled by an impedance monitoring system

• This approach also lets us do beam shaping and gain enhancement

• The back-to-back patch antenna met the radiation guidelines for Iridium communications

• The system was shown to reduce SAR levels to safe levels ≤ 1.6W

kgfor a 2 W

transceiver power output

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Future Work• Miniaturizing the back-to-back patch antenna to fit inside a phone

• Study the effects of the user’s hand on the phone near the antenna

• Recently found a patent by Google doing a similar approach using an impedance monitoring system

• Explore alternative methods of controlling the patches

• Intermittently turn off each patch to monitor the patch impedances separately

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Acknowledgements

• Dr. G. Bridges, Dr. P. Mojabi, & Dr. M. Birouk

• Tom Tessier & Michael Kenny

• Robin Raju

• Brad Tabachnick

• Tyler Teed

• Kyle Nemez

• Cory Smit

• Zoran Trajkoski & Sinisa Janjic

• James Dietrich

• Dwayne Chrusch

• Dr L. Shafai

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