Hf Antenna Design and Use779

7/28/2019 Hf Antenna Design and Use779

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HF Antenna Design and Use

Toms Grajales, VP of R&D

Texas Instruments RFID Authorized Distributorfor the United States

Down load this presentation at www.rfidusa.com/TIBootCamp.html


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Design considerations

Size of antenna

Test equipment

Range of the antenna

Tag speed and separation

Gain or Q factor and VSWR

Power level

Design Example

Antenna Patterns and tag orientation

Certification


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Antenna Size 1

Normally the larger the antenna, the greater is the

reading range. Rule of thumb 1.2 X R or smallest

side.

As the antenna size increases:

The matching to the reader is more difficult, as

inductance gets bigger. Example - 60 X 20

has an inductance of approx. 3 uH. Requires a

resonance capacitance of approx 70 pF. Use

resonance chart , or use formula w2=1/LC.

Field strength is reduced closer to the antenna

compared to smaller antennas.


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Antenna Size 2

The received Noise is normally

increased. Consider a site survey. Smaller antennas:

Harder to measure inductance.

Shielding may be required when high

gain (Q) or power is used.

Holes may develop with the smallertags. Most times holes are related to

high Q and they are more common

with10% modulation.


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Test equipment

VSWR meter

LCR meter Antenna analyzer

Oscilloscope

Different tags


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VSWR Meter


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Scope and antenna loops

50 MHz dual channel scope


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Antenna Analyzer


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Antenna Range 1

Tag size is very important. One

antenna(20 X 20), 1 watt, 10% mod.,parallel to antenna.

Large ISO 19 in.

Access ISO card 16 in.

Square ISO 15 in.

Small ISO 7 in. One large gate ant. will cover 22 in.

with large foil.

Gate antennas(2) cover 37 in. with

large foils.


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Antenna Range 2

Reflective antennas can be used to

enhance the range. Higher Q reflector

antenna is normally used to energize

the tag.

A 30(small) to 40(large) degree tag

rotation from optimal position will still

read.

Antenna signal balance helps with

canceling common mode noise,

recommend using a Balun and ferrite

cores.


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Antenna Range 3

Cable quality, length and position to

the antenna affect the range. Metals will absorb energy, distort the

field pattern and cause de-tuning.

Some of the de-tuning caused by the

mutual inductance of eddy currents

and change in inductance can beadjusted.

Tags will block the Magnetic field from

reaching other tags behind it.


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Tag Detection Speed and Separation

ISO tags can have a single block read

60 times per second. Inventory and

other commands will slow this down.

At 38.4 KBPS I have tested detection

of 100 tags between 3 and 4 seconds.

Detection of large number of tags

requires a low Q antenna. A 20 X 20 antenna should read tags

going by at a speed of 6 ft./sec.


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Antenna Gain and VSWR 1

Antenna gains of 20 or less are

recommended for ISO tags.

Verify the gain that can be supported

by the reader. For example the demo

unit can drive an antenna with a Q of

25 but with 125 mw of power.

Q = f0 / (fh-fl) where fc = center freq.,fh = 3 dB point(.707 Vp)above center

freq. , fl = 3 dB point below center

freq. Measure with antenna loop ,

scope, and analyzer driving the

antenna.


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Antenna Gain and VSWR 2

Most transmitters designed today use

a 50 ohm source.

A VSWR of 1 and not higher than 1.3

indicates the antenna is receiving over

98% of the available power.

When using the S-6550 and your own

antenna remove jumper 2 and restartthe unit..


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Power Levels

Consider Power and Voltage rating of

components used to build your

antenna.

Range Relative to Output Power curve,


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Design Example 1 Capacitor matched antenna design is

the most common due to its low cost of

manufacturing and components. Thefollowing steps are used to have a

starting point. It will require time to

adjust correctly.

Measure inductance of your

antenna using the LCR meter.Measured 1.36 uH.

Calculate the resonant capacitance

using the formula C= 1/w2L where

w= 85197480 or use chart.

Calculated 101 pF.


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Design Example 2

Now calculate the equivalent

resistance for your desired Q using

the formula R = wL X Q. If the Q = 20,

R = 2317 ohms.

Calculate C2 = C X (R/50)1/2 , so we

calculate C2 = 687 pF.

Calculate C1 = 1/ (1/C - 1/C2), so wehave C1 = 118 pF. To form a balanced

circuit use 2 capacitors of twice the

value.


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Antenna Patterns 1 Antennas in phase

Recommend a 50 ohmpower splitter

or build a splitter using 75 ohm cables.


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Antenna Patterns 2

Out of Phase antennas

(180deg.)


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Antenna Patterns 3

Rotating Field, (90 deg.)


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Antenna Patterns 4

Reading Horizontal tags


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Certification

It is the responsibility of the system

integrator to obtain approvals before

operating and selling RFID systems.

In the US the FCC Part 15 and in

Europe the EN 300 330 regulate the

use of RF transmission devices.

The RI-ANT-T01 antenna has beencertified in given specific systems with

operating power of 1.5 W in the US

and 4 W in Europe.


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Conclusion

There is more to RFID Antennas than

some copper pipes from the hardware

store.

Integrators be prepared to spend time

adjusting and testing your antenna,

and you will need the basic tools

described to test. Use the HF Antenna Design Notes

from TIs web site.

Users look for help from a company

with experience in RFID antennas.

Hf Antenna Design and Use779

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