Characteristic Mode Analysis of Smart Phone Antenna using … · 2018-12-24 · Characteristic Mode...

Characteristic Mode Analysis of Smart Phone Antenna using HWFEKO

Ridho Chayono, Peter Futter, and Jordi Soler CastanyAltair Engineering 26 October 2016

© 2016 Altair Engineering, Inc. Proprietary and Confidential. All rights reserved.

Outline

• Introduction to CMA and Solver Features

• Typical CMA Workflows

• Antenna Design Example

• Conclusions

• New CMA Features


Background : Characteristic Mode Analysis (CMA)

• Characteristic modes are defined as the orthogonal current modes that are supported on a conducting surface

• Eigenvalue equation is derived from MoM impedance matrix:

𝐗𝐗 𝑱𝑱𝒏𝒏 = 𝝀𝝀𝒏𝒏𝑹𝑹 𝑱𝑱𝒏𝒏

• They represent possible currents / fields naturally supported by the structure

λ/2 λ 2λ


CMA Parameters and Modal Resonance

• CMA parameters include• Modal current distribution, modal fields and radiation

patterns• Eigenvalue (λ), modal significance (MS), characteristic

angle (CA)

• A mode is resonant when • λ = 0, MS = 1, CA = 180°• A resonant mode is likely to be excited at that frequency

• When an excitation is included, additional parameters are available:

• Modal excitation coefficient MEC (how easily the excitation can excite a mode)

• Modal weighting coefficient MWC (overall modal presence)


CMA Feature Sheet

• Features• User can choose number of modes to calculate• Modes are automatically tracked over frequency • Metals, dielectrics (SEP/VEP), planar multilayer substrates (GF), coatings/TDS

• Parameters in POSTFEKO• Modal current distribution, modal near- and far-field patterns• λ, MS, CA, MEC, MWC• Macros available for advanced plotting and modal synthesis

• Limitations• MLFMM, FEM, FDTD and hybridization with asymptotic solvers are not possible

Typical CMA Workflows

Exci

te th

e co

rrect

m

odes

Verif

y th

e de

sign

pa

ram

eter

sU

nder

stan

d th

e st

ruct

ure

CMA of fundamental structure

Is the correct mode(s) excited

Modify excitation(s) or antenna(s):type/amplitude/relative phase/position

Design specs met?

yesno

no

yes

check modal resonances,

currents

select desired mode(s)

Verification of full design

CMA including excitations



• Initial CMA investigation of the structure:• Simple but representative structure

• No excitations are needed initially • Not necessarily include antenna geometry • Where possible model metal as surfaces• Where possible remove dielectrics

• Which modes are resonant and at which frequencies (λ = 0, MS = 1, CA = 180°)

• Is there a specific modal current distribution/field distributions/radiation pattern that is desired?

• If so, can this be achieved with a single mode or is it necessary to combine modes

𝑱𝑱𝟏𝟏

𝑱𝑱𝟐𝟐

𝑱𝑱𝟑𝟑

Modes on the PCB

Simulation Approach for MIMO Antenna Diversity Strategies, P. Futter et al., EDICON 2015

Understand the structure



• Detailed CMA investigation of the model:• Choose antenna(s) types appropriately• Place antennas to couple to desired modes• Feed arrangements (amplitude, phase) to couple to desired

modes

• How well am I able to excite the desired modes and do I excite any unwanted modes (Modal excitation and weighting coefficient)

• For multi-mode scenarios combine modal current/field/radiation patterns (Lua script is available)

𝑱𝑱𝟏𝟏 𝑱𝑱𝟐𝟐

antenna 1 active(antenna 2 passive)

antenna 2 active(antenna 1 passive)

Excite the correct modes




• Verification of CMA design• Final modifications of structure, feed location(s), amplitude(s)

and phase(s) finalized• Change to realistic model, e.g. include dielectrics, metal

thicknesses that may have been removed• Standard simulation to verify the design requirements were met

CMA design

Verification



Design Example – Smart Phone Antenna Design


Prelude – (In the beginning …)

𝑱𝑱𝟏𝟏

𝑱𝑱𝟏𝟏

𝑱𝑱𝟒𝟒


Design Example : Smart Phone Antenna, GSM 1800 MHz Band

• Previously, designs where demonstrated in the low-band (900 MHz), where less modes are resonant

• This example focusses on the 1.8 GHz band (1.71-1.88 GHz), more resonant modes are present

• Incorporate the antenna design into phone frame

• Challenge is to excite specific modes, or combination of specific modes to achieve desired performance

• Selective mode excitation concepts [1]• Shaping and tuning modes using grounding points,

slots and gaps

𝑱𝑱𝟏𝟏 𝑱𝑱𝟑𝟑𝑱𝑱𝟐𝟐 𝑱𝑱𝟓𝟓𝑱𝑱𝟒𝟒

[1] Inductive and Capacitive Excitation of the Characteristic Modes of Small Terminals; R. Martens, et al., Loughborough Antennas & Propagation Conference 2011


Design 1 : Quarter Wavelength Resonator

𝑱𝑱𝟕𝟕

• λ/4 resonator integratedinto the frame

• Capacitive elementexcites dominant mode

Only dominant modes shown


Design 2 : Half Wavelength Resonator (Iteration 1)

𝑱𝑱𝟒𝟒 𝑱𝑱𝟐𝟐

• λ/2 resonator integratedinto the frame

• Capacitive element toexcite dominant mode

• Good low-band (900 MHz)performance




𝑱𝑱𝟒𝟒 𝑱𝑱𝟏𝟏

• Introduce grounding pins to suppress anti-resonance in mode 1

• Pin on lower edge introduces λ/4 passive resonator




𝑱𝑱𝟔𝟔 𝑱𝑱𝟏𝟏

• Introduce slots in frame tosuppress mode 4resonance

• Achieved 2 dominant,wideband modes withgood patterns



Design 3 : Alternative Narrowband Design

𝑱𝑱𝟐𝟐 𝑱𝑱𝟏𝟏

• λ/4 resonator integrated into the frame

• λ/4 passive resonator etched into PCB



Design Comparison : S11 and Total Radiated Power (TRP)

• Optenni Lab is used to design matching circuits for each of the 5 designs covering the GSM1800 band

• Design 3 does not cover the full band

Minimum Matching Network Efficiency (dB)

Design 1 -0.1

Design 2 -1

Design 2+ -0.4

Design 2++ -0.3

Design 3 -0.8 (band limited)


Design Comparison : Gain at 1.795 GHz

Design 1 Design 2 Design 3


Conclusions• Modes can be controlled (tuned, shaped) using correctly placed excitation elements [1], as well

as by introducing fixed points, e.g. grounding points, slots, resonant lengths etc.

• Initial CMA designs without excitations:• Can address impedance matching in a second step, either through rigorous optimization or including a

matching circuit

• Bandwidth of the resonant modes gives a good indication of the actual antenna bandwidth

• CMA gives a broader understanding of how the antenna operates• Modal currents, modal radiation patterns • Can be applied to make more informed design decisions

• At higher frequencies where multiple modes are resonant it is more challenging to isolate single modes, especially if a only a single excitation is available


New CMA Features


New Mode Tracking Algorithm

• Mode tracking:• A logical mode will be tracked over the entire simulated frequency range • Indexing is assigned based on the MS at the lowest frequency

• Challenging task, especially when many modes are supported, or highly resonant structures• Decaying modes, fictitious modes, cross avoidance, etc.

• Symptoms • Modes not tracked correctly over intersections

• New mode tracking algorithm released in FEKO 14.0.410 mode tracking • Much more robust and reliable


Updated Macros

• CMA plotter script updated (improved handling and performance)www.feko.info/support/lua-scripts/cma-plotter

• CMA synthesis tool • Combine results from different modes e.g. modal currents, near fields & far fields• Especially useful for structures with multiple resonant modes

http://www.feko.info/support/lua-scripts/cma-plotter


Thank you!

www.altairhyperworks.com/feko

Characteristic Mode Analysis of Smart Phone Antenna using … · 2018-12-24 · Characteristic Mode...

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