Antenna isolation for wireless communications systems

(1)

Ansoft, San Diego 10/12/01

(2)

Company History:

- Created in May 2000

- Funded by Sevin Rosen and US trust

Targeted markets and applications:

- Embedded antennas for Cell Phones - Laptops and all wireless applications

between 800 MHz and ISM Located:

San Diego Tech Center

9605 Scranton Road, suite 850

San Diego, CA 92121

(3)

for wireless communications systems

Gregory Poilasne, S. Rowson, L. Desclos

(4)

1 - Antenna and Surrounding Interaction a) Antenna placement

b) Radiation pattern - Efficiency evaluation c) Bit rate Measurements

3 - Internal antenna isolation

a) Experimental and numerical results 2 - Isolation Measurements

a) Interaction with the case and the components b) Interaction with the user

CONCLUSION

(5)

a) Antenna placement

Home-RF solution

(6)

a) Antenna placement

Proposed solution

Antenna position

(7)

b) Radiation pattern - Efficiency evaluation

Efficiency = power at the antenna input radiated power

Takes into account:

input mismatch

loss into the antenna

loss into the surroundings

Measured by comparing:

free space dipole to

the antenna inside the system

(8)

b) Radiation pattern - Efficiency evaluation

Antenna X Ripples due to the

interactions Isolated Antenna

Smooth radiation pattern

4dB Efficiency difference

(9)

c) Bit rate Measurements Practical efficiency measurement:

Antenna

Laptop Communication

channel

Bit rate measurement versus

the distance between the two laptops

Antenna

(10)

c) Bit rate Measurements

Distance (feet)

0 20 40 60 80 100 120 140

Bit rate (bit/s)

0 100 200 300 400 500

Proxim facing transmitter Photonic RF facing transmitter Proxim looking away from transmitter Photonic RF looking away from transmitter

Parking Lot Test - Minimum Possible Multipath (Office test is Isotropic)

ET Antenna is vertical

ET antenna ET antenna

AUT AUT

(11)

c) Bit rate Measurements Practical efficiency measurement:

Antenna Attenuator

Laptop Communication

channel

Bit rate measurement versus

the attenuation

(12)

c) Bit rate Measurements

Ref : Ideal dipole away from any absorber.

ET ant: Isolated antenna

Antenna X: antenna sold with the Home RF card.

0 50 100 150 200 250 300 350 400 450 500

0 1 2 3 4 5 6 7 8 9 10 11 12

Attenuation (dB)

Bit Rate (KB/s)

Dipole PRFC ant Symphony

ET ant.

Ant. X

Approximately 4.5dB of efficiency difference

Efficiency difference

(13)

Interaction between the antenna and the case: Reduction of efficiency

Need for a tool to measure the interaction: Antenna Isolation

(14)

a) Interaction with the case and the components

Shields tend to short

circuit antennas

(15)

Example of a PIFA

E field

Metallic shield

(16)

d

Point A

Decay associated Point A

p

Antenna

Shield

(17)

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 42 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28

30-35 25-30 20-25 15-20 10-15 5-10 0-5

antenna

Shift in MHz

(18)

y = 0.0245Ln(x) + 0.048

y = 0.0109Ln(x) + 0.0195

-0.01 0 0.01 0.02 0.03 0.04 0.05 0.06

0 0.2 0.4 0.6 0.8 1 1.2

Freq (GHz)

measurements

fitting

Distance in cm

(19)

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 42 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28

19-20 18-19 17-18 16-17 15-16 14-15 13-14 12-13 11-12 10-11 9-10 8-9 7-8 6-7 5-6 4-5 3-4 2-3

antenna

Shift in MHz

C B

(20)

b) Interaction with the user

3D Radiation pattern

Loss into the user body

Need to shield the user - Performance

- Health

Frequency shift/matching

Need for isolated antennas

- Performance

(21)

Frequency: 1.575GHz Bandwidth: 8MHz Critical case

Antenna

(22)

Frequency shift: 2.5MHz

Blue curve: free space

Antenna

(23)

b) Interaction with the user Hand over the side of the antenna

Blue curve: free space

Frequency shift: 1MHz

Antenna

(24)

Current distribution:

It has to be confined nearby the antenna.

Nearfield distribution:

The nearfield has to be shaped away from parasitic elements

Confined the field as much as possible Nearfield probing: long and no so accurate

Simulation tools: Faster and faster, easy to visualize

(25)

Experimental and numerical results

PIFA:

Currents are leaking

on the sides.

(26)

Isolated Antenna:

Low currents outside

the antenna.

(27)

PIFA with shield:

Coupling with the shield.

Frequency shift:

9%

(28)

Isolated antenna

with shield:

Low coupling with the shield.

Frequency shift:

3%

(29)

Isolation means:

- Low interaction between an antenna and its surroundings Inside the enclosure: components, case

Outside the enclosure: user body, table,...

- Better efficiency in real world applications - Semi-standard products

drop-in component

fast turn-around design and optimization