Body Worn/Wearable AntennasPrecision Indoor Personnel Location and Tracking for Emergency Responders

Technology WorkshopWorcester Polytechnic Institute

August 7, 2007

MegaWave Corporation200 Shrewsbury Street, PO Box 614

Boylston, MA 01505Presented: Marshall W. Cross P.E., VP R&D

mcross@megawave.com

2

OverviewDefinition and Performance Metrics• Key design considerations

Brief history of Body Worn/Wearable Antenna (BWA) designTypical body worn antenna performance• Computer modeling, performance measurements

Design examplesSummary

3

Definitions and MetricsBody wearable: part of clothing1

Body worn: not part of clothingVHF/UHF: 30-3,000 MHz (λ=10 to 0.1m)Metrics:• Frequency coverage/instantaneous bandwidth • VSWR (50 Ω)• Power gain radiation pattern vs. orientation, position, equipment,

nearby environment• Polarization• Physical survivability• Physical integration – user comfort

1 Hall and Hao, “Antennas and Propagation for Body-Centric Wireless Communications,” Artech House, 2006

4

Body Worn Antenna (BWA) Brief Historyand General Considerations

Prior to advent of sophisticated codes, analytical models were developed• Andersen, J., et al, (IEEE Trans. Veh. Tech., Vol. VT-26, #4, Nov.’77) developed

analytical model for VHF radio mounted antennas near body• Krupka, Z., (IEEE T-AP, Vol. AP-16, #2, Mar.’68) developed the notion of body

resonance in the VHF bandMeasured data was obtained

• King et al, (IEEE T-AP, May 1977, pp. 376-379) collected measured data of handset antenna at various locations (head, shoulder and torso) and distances from body

– Gain decreases rapidly with decreased separation from body– Body shields antenna so azimuth patterns are not omnidirectional (this effect is seen at

frequencies as low as 200MHz although F/B ratio is lower at 200MHz)

Computation of on-body performance is complex• Wave impedance that body presents varies with frequency

– At 900MHz, η = 40 + j 6 ohms, lower at lower frequencies– Tends to “short out” E-field antennas located close to the body– Efficiency is reduced

5

REMCOM detailed body model

Computer codes•Finite Difference Time Domain •Transmission Line Matrix Method

Human body model options•Detailed body models

–Include all organ and tissue parameters–Complicated, run time and resource

intensive–Torso non-uniform so difficult to model

conformal antenna (problems found in past with elements intersecting the body tissue)

•Simplified human body–Lossy cylinder with dielectric constant

and conductivity–Based on “SALTY” by Siwiak2

–Rectangular model of body found to give excellent agreement with measurements

Typical Body Worn Antenna Design Approach

body antenna elements

counterpoise

Simplified Body Model

2 Siwiak, “Radiowave Propagation and Antennas for Personal Communications”, Artech House

6

MeasurementsFabricate preliminary lab prototypeTest on liquid filled phantom (saltwater or combination of additives3)Measure VSWR, azimuth radiation patterns, gainFabricate and test final antennas on phantom

No head/arms and full body salt water

phantoms

3 K. Ito, “Numerical and Experimental Human Body Phantoms”, IET Seminar, Body Centric Wireless Communications, April 2007.

7

Data ComparisonRadiation Patterns

Computed Measured

1250-2500MHz Wearable Antenna on PhantomMeasured Azimuth Patterns

-30-25-20-15-10

-505

100

10 2030

4050

60

70

80

90

100

110

120

130140

150160170

180190200

210220

230

240

250

260

270

280

290

300

310320

330340 350 1250

15001750200022502500

1250-2500MHz Wearable AntennaXFDTD Simulated Azimuth Patterns

-30

-25

-20

-15

-10

-5

0

5

100

10 2030

40

50

60

70

80

90

100

110

120

130

140150

160170180

190200210

220

230

240

250

260

270

280

290

300

310

320330

340 350125015001750200022502500

8

Data Comparison -VSWRComputed

Measured

VSWR

Insertion Loss (dB)

9

Data ComparisonAverage Azimuth Gain

1250-2500MHz BWA on Phantom

-10

-7.5

-5

-2.5

0

2.5

5

1250 1500 1750 2000 2250 2500

Frequency (MHz)

Ave

rage

Azi

mut

h G

ain

(dB

i)

Computed

Measured (Cables & Combiner Loss Included)

10

Wearable Antennas – Civilian GarmentExisting frequency bands:

• 136-174, 400-450, 800-900, 1250-2500MHz

• Wideband elements could also be integrated

Casual outer jacket selected for integration

• Other garments possibleAntennas fabricated using conducting fabric on black cotton carrier insert that lies between jacket shell and liningVSWR<2.5:1 in all bands

antenna feed point detail

selected host garment

-30-25-20-15-10

-50

0

30

60

90

120

150

180

210

240

270

300

330

400

425

450

-30-25-20-15-10

-505

010 20 30

4050

60708090100

110120

130140

150160170180

190200210220

230240

250260270280290

300310

320330340350 800

900

1250-2500MHz Wearable Antenna on PhantomMeasured Azimuth Patterns

-30-25-20-15-10-505

100

10 20 3040

5060

708090100

110120

130140

150160170180

190200210220

230240

250260270280290

300310

320330340350

125015001750200022502500

Measured Azimuth Radiation Patterns – 400-450, 800-900 and 1250-2500MHz

11

Tactical Vest Wearable AntennaExisting designs for 136-174MHz and 400-450MHz antenna systemsVest is adjustable by means of side tabs that velcro to back of vest

• Challenge integrating antenna into vest and retaining adjustable features

Appliqué pouch antenna overcomes this• Matching nylon loop pile fabric on exterior of

antenna pouch retains same functionality as vest• Coaxial cable contained in pouch can be laid out to

fit wearer and retained with velcro tabs

-30-25-20-15-10

-50

0

2 2 . 5

4 5

6 7 . 5

9 0

112 . 5

13 5

15 7 . 5

18 0

2 0 2 . 5

2 2 5

2 4 7 . 5

2 7 0

2 9 2 . 5

3 15

3 3 7 . 5 400425450

12

Future Force Warrior BWA

Plate antenna couples to ballistic armor platesOnly over-the-shoulder wire visible on ensemble when installed

surrogate radio mounted whip

BWA over shoulder

wire

13

Measured Signal StrengthFFW BWA vs Body Mounted Whip

STANDING FARM 06/08 232MHz 1WATT XTMR

-90-85-80-75-70

-65-60-55-50

100 150 200 250 300 350 400 450 500

Distance (m)

Rec

eive

d Po

wer

(dB

m) WHIP

MEGAWAVE

THEORY

PRONE FARM 06/08 232MHz 1WATT XTMR

-110

-100

-90

-80

-70

-60

-50

100 150 200 250 300 350 400 450 500

Distance (m)

Rec

eive

d Po

wer

(dB

m) WHIP

MEGAWAVE

Standing performance within 2-3dB of whipProne performance significantly better than whip

14

SummaryPerformance of BWAs limited by body, equipment and nearby environmentAntennas need to operate in all body positions from standing to proneExcitation of surface/creeping waves (h < λ)Modeling vs. experiment • Fair to good agreement depending on complexity of antenna

configurationChamber/range measurements on EM phantoms required for repeatabilityDon’t forget EMC: digital to RF and RF to digital!