Design of Platform-Mounted HF Antennas with Enhanced Bandwidths

Using the Characteristic Mode Configuration in FEKO

Ting-Yen Shih and Nader Behdad*Department of Electrical and Computer Engineering

University of Wisconsin-Madison, USA

Altair FEKO WebinarMarch. 30, 2016

This material is based upon work supported by the Office of Naval Research under ONR Awards No. N00014‐11‐1‐0618, N00014‐15‐1‐2207, and N00014‐16‐1‐2098.

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Greetings from UW-Madison

We are here

Aerial view of the engineering campus Behdad research group

Aerial view of the campus

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Our Group’s Research Interests

Small Antennas• Broadband HF, VHF, UHF antennas• Small, low-profile UWB antennas• Small-aperture direction finding• Biomimetic antenna arrays

EM Periodic Structures• Affordable phased-array antennas• Frequency selective surfaces• Broadband, true-time-delay

microwave lenses and reflectarrays

Biomedical Applications• Microwave ablation for treatment of

cancer• EPR and microwave imaging

systems

MMW and Infrared Systems• Multi-model antenna-based IR

detectors• IR filters and FSSs• MMW phased-arrays

Wireless Comm. Applications• Antennas for new comm. 

techniques that enhance efficiency of spectrum usage. 

High-Power Microwaves• Vacuum Electronics Devices (VED)

• Metamaterial-enhancedresistive wall amplifier

• New cathodes

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Our Group’s Research Interests

MMW/IR Systems

High‐Power Microwaves EM Periodic Structures

Biomedical Applications

Small Antennas

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Moving on to the main subject …

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Motivation

HF frequency band:• 3-30 MHz range • λ=10-100 m

Applications: • Military communication

systems• Over the horizon radar systems

HF Antennas:• Whip antennas, often 4 to 11 m

Challenge:• Antenna size↓ Bandwidth↓

and Radiation Efficiency↓

HF Whip Antenna

Goal: Enhance bandwidth of platform-mounted HF antennas.

Ref.:“Marines Face Steep Cuts to Expeditionary Vehicle.” [Online]. Available: http://www.nationaldefensemagazine.org/archive/2006/September/Pages/Marinesfacesteep2870.aspx.

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HF Antennas

Ref.: S. R. Best, “On the use of scale brass models in HF shipboard communication antenna design,” IEEE Antennas Propag. Mag., vol. 44, no. 2, pp. 12–23, Apr. 2002.

Ref.: G. Marrocco and L. Mattioni, “Naval structural antenna systems for broadband HF communications,” IEEE Trans. Antennas Propag., vol. 54, no. 4, pp. 1065–1073, Apr. 2006.

By combining properly shaped and loaded wire radiators together with the mast of a ship, it is possible to achieve a broadband HF compact radiating system.

Twin whip antennas are used to increase the effective diameter of the radiator, thus improving bandwidth performance over that of a single-whip radiator.

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Approach: Use the platform as the main radiator and the mounted antennas as a coupling mechanism to excite the desired platform mode(s).

Technical Approach

Canonical Rectangular Platform Simplified EFV Platform

Ref.: T.-Y. Shih and N. Behdad, “Bandwidth Enhancement of Platform-Mounted HF Antennas Using theCharacteristic Mode Theory,” IEEE Trans. Antennas Propag., in press.

Platform models:

Problem: Many antennas working at the high frequency (HF) band tend to have significantly smaller dimensions than the wavelength at which they operate and thus suffer from narrow bandwidths.

Expeditionary Fighting Vehicle

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Modal Significance(Potential contribution)

Orthogonal set of eigencurrents Jn nnn RJXJ

njMS

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R: Real part of the impedance matrix ofthe electric field integral equation.X: Imaginary part of the impedance matrixof the electric field integral equation.λn: Eigenvalue associated with theeigencurrent.

Ref.: M. Cabedo-Fabres, E. Antonino-Daviu, A. Valero-Nogueira, and M. F. Bataller, “The theory of characteristic modes revisited: A contribution to the design of antennas for modern applications,” IEEE Antennas Propag. Mag., vol. 49, no. 5, pp. 52–68, Oct. 2007.

Characteristic ModesTheory

Characteristic modes of an object are a complete orthogonal set of currents.

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Ref.: Z. Miers, H. Li, and B. K. Lau, “Design of bandwidth-enhanced and multiband MIMO antennas usingcharacteristic modes,” IEEE Antennas Wireless Propag.Lett., vol. 12, pp. 1696–1699, 2013.

Ref.: K. K. Kishor and S. V. Hum, “A two-port chassis-modeMIMO antenna,” IEEE Antennas Wireless Propag. Lett., vol.12, pp. 690–693, 2013.

Ref.: Y. Chen and C.-F. Wang, “HFband shipboard antenna design usingcharacteristic modes,” IEEE Trans.Antennas Propag., vol. 63, no. 3, pp.1004–1013, Mar. 2015.

Ref.: Y. Chen and C.-F. Wang,“Electrically small UAV antenna designusing characteristic modes,” IEEETrans. Antennas Propag., vol. 62, no. 2,pp. 535–545, Feb. 2014.

Characteristic ModesApplicationsHandset Devices

Military Applications

Ref.: T.-Y. Shih and N. Behdad,“Bandwidth Enhancement of Platform-Mounted HF Antennas Using theCharacteristic Mode Theory,” IEEETrans. Antennas Propag., in press.

Our Work

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Technical ApproachDetails

This strategy allows us to enhance bandwidth because the maximum volume available is utilized.

Our strategy is to use the platform as the main radiator. A desired platform mode is excited by utilizing the mounted antennas as coupling mechanisms

between the external circuit and the platform.

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Canonical Rectangular PlatformStep 1: Characteristic Modes of the Platform

Modal Significance(Potential contribtion)

MS ≥ 0.707

njMS

11 • Resonant: MS attains a value of 1

• Significant: MS ≥ 0.707

Characteristic Mode Analysis (FEKO)

Canonical Rectangular Platform

Dominant mode in the HF band: mode 1Significant above 9 MHzResonant above 11 MHz

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Canonical Rectangular PlatformCurrent Distribution of the Characteristic Modes

Mode 1 Mode 2

NVIS

Ref.: United States and Marine Corps, Antennahandbook. Washington, DC: Headquarters, U.S.Marine Corps, 1999.

Normalized Total E-FieldMagnitude [dBV]

0.0

-30.0

-15.0

Normalized SurfaceCurrent [dBA/m]

0.0

-30.0

-15.0

Mode 1 and mode 2 indicate that their maximum radiations are toward zenith (suitable for the near vertical incidence skywave (NVIS) propagation mode).

The current distribution and the radiation patterns of the first four platform modes were examined with FEKO.

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Ref.: B. A. Austin and K. P. Murray, “The applicationof characteristic-mode techniques to vehicle-mounted NVIS antennas,” IEEE Antennas andPropagation Magazine, vol. 40, no. 1, pp. 7–21, 30,Feb. 1998.

Canonical Rectangular PlatformCurrent Distribution of the Characteristic Modes

Mode 3 Mode 4

Normalized Total E-FieldMagnitude [dBV]

0.0

-30.0

-15.0

Normalized SurfaceCurrent [dBA/m]

0.0

-30.0

-15.0

Groundwaves and Skywaves

The current distribution and the radiation patterns of the first four platform modes were examined with FEKO.

Modes 3 and mode 4 generate radiated fields that are useful for ground-wave or conventional skywave communications.

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Canonical Rectangular PlatformStep 2: The Desired Mode, Mode 1

rad

me

rad

meMode W

WWP

WWQ ),max(4),max(21_

F0 Q 10-dB BWQChu 10 MHz 1.3 49.8%

QMode1 10 MHz 8.7 7.7%

E‐field [dBV]

Mode 1

Since mode 1 is the dominant mode of the platform in the HF band and it is suitable for NVIS applications, it was selected in this work. However, the presented concepts are equally applicable to the excitation of the other modes.

A more realistic Q calculation:: Stored electric energy : Stored magnetic energy: Radiated power: Radiated energy.

eWmW

radPradW

Examine the bandwidth limit of mode 1 by calculating the lower bound of the radiation Q.

• QChu is too optimistic.

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Canonical Rectangular PlatformDetails of Realistic Q Calculation

Sto

red

Ene

rgy

Tota

lEne

rgy

Rad

iate

d E

nerg

y

The E and H fields used for the calculations were acquired from FEKO.

Radiation Q is the ratio of stored and radiated energies. The stored energy was calculated by subtracting the radiated energy from the total energy.

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Canonical Rectangular PlatformStep 3: Excitation of Mode 1

Exci

ting

Mod

e 1

E‐field [dBV]

Mode 1

The current distribution of mode 1 indicates that the entire platform acts as a fat dipole with an electric current oriented along the y direction.

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To minimize the internal reactance, a capacitor is placed in series with the excitation source and the half loop.

Current distribution: A single half loop does not effectively synthesize the required magnetic

current distribution on the periphery of the platform.

0.33%(1x)

0.64%(2x)

1 half loop

Mode 11 half loop

Canonical Rectangular PlatformOne Half-Loop Scenario

Full loop in free space

Bandwidth enhancement: 2x of the stand-alone full loop antenna

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• All half loops fed with the same magnitudes and phases using power splitters

2 half loops 4 half loops

Canonical Rectangular PlatformIncreasing the Excitation EfficiencyThe electric current density of mode 1 is the strongest at the edges of the structure ICEs should preferably be placed at the edges.

E‐field [dBV]

Mode 1

The number of coupling loops (ICEs) can also be increased to provide a more uniform excitation of the mode.

20Bandwidth is 2x, 7x, and 10x of the full loop in free space

2.20%0.33% 

3.27% 0.64%

Input reflection coefficients

Canonical Rectangular PlatformSimulated S11

Scenario f0(MHz)

BW* ηtot

QChu 10 49.79%QMode1 10 7.69%1 full-loop in free space

12 0.33% (1x)

97.1%

1 half-loop 12.32 0.64% (2x)

99.1%

2 half-loops 10.76 2.20% (7x)

99.7%

4 half-loops 10.16 3.27% (10x)

99.7%

Loop number ↑ Bandwidth ↑

*Ideal power splitters and capacitors were used.

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Radiating components are distributed over a larger volume• Available volume is used more efficiently

E‐field Directivity Directivity Directivity

1 half loop 2 half loops 4 half loopsMode 1

Canonical Rectangular PlatformRadiation Patterns and Current Distributions

Loop number ↑ Current distribution & rad. pattern approach mode 1

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Scenario BW1-full loop in free space 0.33%

4-full loops in free space (Out of phase) 0.22%

4-full loops in free space (In phase) 0.87%

4-half loops on the platform 3.27%

Canonical Rectangular PlatformContribution of the Platform

Verify that solely increasing the number of loops in free space has limited effect on the system’s bandwidth.

The presence of the platform is critical in achieving the enhanced bandwidth.

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1 half loop 2 half loops 4 half loopsPrototypes

Feed networks

Experimental Studies1:80 Scaled Prototypes

Power splitters

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Experimental StudiesMeasured S11

Scenario f0(MHz)

BW

1 half-loop on the platform (Center)

707.55 0.91%

2 half-loops on the platform (Edge)

650.00 1.78%

4 half-loops on the platform (Edge)

652.60 5.12%

Input reflection coefficients

Experimental results closely follow the theoretical predictions

Loop number ↑ Bandwidth ↑

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Experimental StudiesMeasured 2-D Radiation Patterns (x-z plane)

1 half loop on the platform

2 half loops on the platform

4 half loops on the platform

E-field

Mode 1

The radiation patterns of the 4 half loop case are highly correlated with mode 1 of the platform

• Omni-directional on the x-z plane• Figure eight shaped on the y-z plane

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Experimental StudiesOther topologies

Scenario f0 (MHz) BW

3 half-loops on the platform (Edge+Center) 634.15 1.97%

4 half-loops on the platform (Edge+Center) 639.60 2.84%

4 half-loops on the platform (Center) 672.65 3.75%

Other topologies Input reflection coefficients

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Scenario CE placement f0(MHz)

BW Max. D (dBi)

ηtot

1 half-loop on the platform Center 707.55 0.91% 4.2 26%2 half-loops on the platform Edge 650.00 1.78% 3.5 37%3 half-loops on the platform Edge + Center 634.15 1.97% 3.8 33%4 half-loops on the platform Edge + Center 639.60 2.84% 2.8 30%4 half-loops on the platform Center 672.65 3.75% 3.1 26%4 half-loops on the platform Edge 652.60 5.12% 3.9 32%

The bandwidth of these antenna systems can be enhanced by distributing the coupling loops over the entire surface of the platform.

Experimental StudiesComparison

The system volume is used more efficiently when the number of our coupling loops increases.

31.5 5.5%

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For the cases that a similar number of coupling loops are used, placing the loops at locations where the current density of the desired mode is the strongest improves the bandwidth of the system.

Mode 1

E-field

Experimental StudiesStrategic Placement

Scenario CE placement f0(MHz)

BW Max. D (dBi)

ηtot

4 half-loops on the platform Edge + Center 639.60 2.84% 2.8 30%4 half-loops on the platform Center 672.65 3.75% 3.1 26%4 half-loops on the platform Edge 652.60 5.12% 3.9 32%

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• Principal factors contributing to the loss: Losses of the power splitters and the feed networks. Losses of the impedance matching networks of each individual loop.

Matched loss magnification [*]:

• The total efficiencies of full scale models Simulation: >99% Measurement: Expected to be 86-88% (based on the losses and

quality factors of the lumped elements available at HF band)

Experimental StudiesTotal Efficiency

VSWR4)1VSWR()1VSWR( 222

L

LLa

VSWR=25 VSWR=100 VSWR=400L= 0.01 dB La= 0.12 dB La= 0.47 dB La= 1.65 dBL= 0.10 dB La= 1.10 dB La= 3.33 dB La= 7.49 dBL= 1.00 dB La= 5.95 dB La= 11.02 dB La= 16.77 dB

[*] R. C. Hansen and R. E. Collin, Small antenna handbook. Hoboken, N.J.: John Wiley & Sons, 2011.

: Magnified loss: Intrinsic loss: Voltage standing wave ratio

aLL

VSWR

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The Cost of Approaching the Limit

The bandwidth enhancement factor will eventually saturate due to the increasing mutual coupling between the loops.

It is anticipated that an optimum number of coupling loops exists that, when arranged properly, can provide the bandwidth closest to the maximum available bandwidth.

0.9% increase in bandwidth, drastic increase of system complexity

8 half loops

4.17%3.27%

2.20%

0.64%

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The Expeditionary Fighting Vehicle (EFV) • Formerly known as the Advanced Amphibious Assault Vehicle• Amphibious assault vehicle developed for the U.S. Marine Corps

Simplified EFV PlatformThe Expeditionary Fighting Vehicle

The EFV program was cut from a 2012 proposed budget by the White House. In this study, it was only used to demonstrate the feasibility of our method on realistic platforms.

Ref.: “Expeditionary fighting vehicle,” Oct. 2014, page Version ID: 621202886. [Online]. Available: http://en.wikipedia.org/wiki/Expeditionary Fighting Vehicle

Ref.: “Expeditionary Fighting Vehicle (EFV),” Army Technology. [Online]. Available: http://www.army-technology.com/projects/efv/.

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Mode 1

MS ≥ 0.707

Resonant: MS attains a value of 1 Significant: MS ≥ 0.707

Mode 2 Mode 4Mode 3

Mode 1 (dominant)

Simplified EFV PlatformSteps 1-2: Characteristic Modes of the Platform

F0 Q 10-dB BWQMode1 11 MHz 7.33 9.1%

Mode 1: platform acts as a horizontally-polarized dipole antenna.

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All half loops are fed with the same magnitudes and phases.

Simplified EFV PlatformStep 3: Excitation of Mode 1

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Scenario* f0(MHz)

BW

QChu 11 44.41%

QMode1 11 9.10%

1-full loop in free space

14.03 0.43% (1x)

1-half loop 13.53 0.75% (1.7x)

2-half loops 11.94 3.42% (8x)

3-half loops 11.48 3.52% (8.2x)

4-half loops (Top)

11.15 3.36% (7.8x)

4-half loops (Top and Side)

11.12 3.85% (9x)

Input reflection coefficients

Loop number ↑ Bandwidth ↑

The four half loop scenario with two half loops on the sides offers a wider bandwidth than the one where all four half loops are on top of the EFV. * Realistic power splitters were used in these simulations.

Simplified EFV PlatformSimulated S11

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Loop number ↑ + Strategically placed Current distribution approaches Mode 1.

Mode 1

E-field

Simplified EFV PlatformRadiation Patterns and Current Distributions

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Simplified EFV PlatformRealistic ground effect

Realistic ground

Isolated(no ground)

Scenario εr tanδDry Ground 2.55 0.016Wet Ground 20 0.35

Input reflection coefficients

Realistic ground planes were realized using homogeneous half space in region Z < 0 (exact Sommerfeld integrals) in FEKO.

Soil dielectric properties at 10 MHz

Ref.: A. R. Von Hippel, Dielectric materials and applications; papersby twenty‐two contributors. Cambridge; New York: published jointlyby the Technology Press of M.I.T. and Wiley, 1954.

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Summary

Due to the large wavelengths in the HF band, designing a low profile resonant antenna with efficient radiation is not practical.

We successfully developed a method

to systematically and efficiently approach

the bandwidth limitation of a

platform mode, which allows us to achieve the bandwidths that

stand-alone antennas cannot achieve.

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Related Publications[1] T.-Y. Shih and N. Behdad, “Bandwidth enhancement of platform-mounted HF antennas using the

characteristic mode theory,” IEEE Trans. Antennas Propag., in press.[2] T.-Y. Shih and N. Behdad, “Bandwidth enhancement of platform-mounted HF antennas using the theory

of characteristic modes,” in Radio Science Meeting (USNC-URSI NRSM), 2016.[3] T.-Y. Shih and N. Behdad, “Bandwidth enhancement of HF antennas mounted on military platforms using

a characteristic-modes-based design approach,” in International Symposium on Antennas and Propagation (ISAP2015), 2015.

[4] T.-Y. Shih and N. Behdad, “Bandwidth enhancement of expeditionary-fighting-vehicle-mounted antennas using the characteristic mode theory,” in Antenna Applications Symposium, 2015, pp. 563–575.

[5] T.-Y. Shih and N. Behdad, “Bandwidth enhancement of platform-mounted HF antennas using the characteristic modes theory,” in 2015 IEEE Antennas and Propagation Society International Symposium (APSURSI), 2015, pp. 1608–1609.

[6] T.-Y. Shih and N. Behdad, “Design of platform-mounted HF/VHF antennas using the characteristic modes theory,” in 2015 International Workshop on Antenna Technology (iWAT), 2015, pp. 84–86. (Invited Paper)

Contact InformationNader Behdad behdad@wisc.edu Ting-Yen Shih tshih6@wisc.edu