MSc Defence PPT

February 1, 2017
E2-105 EITC
2:30 PM
Orientation-Dependent Adaptive Antenna for
Low Earth Orbit Satellite Communications
Dept. of Elec. & Comp. Eng.
MSc Thesis Defence
Bo Woods
Examining Committee
Dr. 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
1/29

Motivation
• Remote areas  satellite communications only viable option
• High transmit power required for satellite communications
• Leads to unsafe human exposure levels
2/29
• 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)
Condition
Uncontrolled
Environment
Controlled
Environment
Peak averaged SAR for the head, neck
and trunk averaged over 1 g of tissue
1.6
W
kg
8
W
kg
Peak averaged SAR in the limbs
average over any 10 g of tissue
4
W
kg
20
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 kHz
to 300 GHz, Industry Canada Safety Code 6, 2015.
3/29

SAR Levels
• Power is proportional to E
2
 determine SAR for different input powers using:
SARx =
Px
Pmeas
SARmeas
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.
4/29

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 = 1.547
W
kg
; Pmeas = 370 mW
• From Eq. 1.2: When Px = 2 W  SARx = 8.36
W
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.
5/29
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.
6/29

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< 1.6
W
kg
when 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.
7/29

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)
8/29

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
4
separation, which did not fit the thin form factor
requirement for modern commercial phones
9/29

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
11/29

Theory of a Single Patch Antenna
12/29

Making a Patch Circularly Polarized
13/29
• 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
14/29
• 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
15/29

Radiation Pattern (Both Patches On)
Back-to-back patch antenna (both patches on) radiation pattern with respect to a RHCP
receiver. Simulation shown on the left and measured on the right. Max gain +4.5 dB.
16/29

Radiation Pattern (One Patch On)
Back-to-back patch antenna (one patch on) radiation pattern with respect to a
RHCP receiver. Simulation shown on the left and measured on the right.
17/29

Specifications (Both Patches On)
18/29

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
kg
SAR
restriction for distances
of ≥ 3 cm
19/29
[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
20/29
• Possible applications:
• Locking into satellites directly overhead
• Reducing radiation exposure to the human user
Simulation in HFSS

Impedance Monitoring System
21/29
• 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
22/29
• 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
23/29
Measured reflection coefficient
1. No phantom present
2. Phantom present on the off patch side

Isolation Test With Only One Patch On
24/29
• Also verified the radiation pattern was
not influenced by the phantom when it
was on the off patch side

Assumptions
25/29
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
26/29
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 ≤ 1.6
W
kg
for a 2 W
transceiver power output
27/29

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
28/29

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
29/29

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