hf antenna design and use

HF Antenna Design and UseTomás Grajales, VP of R&D

Texas Instruments RFID Authorized Distributorfor the United States

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

Design considerationsSize of antennaTest equipmentRange of the antennaTag speed and separationGain or Q factor and VSWRPower levelDesign ExampleAntenna Patterns and tag orientationCertification

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.

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 smaller

tags. Most times holes are related to high Q and they are more common with10% modulation.

Test equipment VSWR meter LCR meter Antenna analyzer Oscilloscope Different tags

VSWR Meter

Scope and antenna loops 50 MHz dual channel scope

Antenna Analyzer

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.

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.

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 be adjusted.

Tags will block the Magnetic field from reaching other tags behind it.

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.

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.

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 restart the unit..

Power Levels Consider Power and Voltage rating of

components used to build your antenna.

Range Relative to Output Power curve,

Design Example 1 Capacitor matched antenna design is

the most common due to its low cost of manufacturing and components. The following 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.

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 we have C1 = 118 pF. To form a balanced circuit use 2 capacitors of twice the value.

Antenna Patterns 1 Antennas in phase Recommend a 50 ohm power splitter

or build a splitter using 75 ohm cables.

Antenna Patterns 2 Out of Phase antennas

(180deg.)

Antenna Patterns 3 Rotating Field, (90 deg.)

Antenna Patterns 4 Reading Horizontal tags

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 been certified in given specific systems with operating power of 1.5 W in the US and 4 W in Europe.

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

TI’s web site. Users look for help from a company with

experience in RFID antennas.