Extending the Range of eZ430-TMS37157 PaLFI RFID Applications TI Proprietary Information

Extended Range Applications The eZ430-TMS37157 kit range is limited to about 3cm as shipped. This is acceptable for development purposes of many applications, but some applications require significantly longer range. Several factors affect the range that can be achieved. The tag/reader antenna size, quality factor (Q), and base station output power are proportional to the operational range. Antenna design and actual measurements will be examined in this application note.

Using External Power Supply to Increase Output Power

Reader Antenna Design Custom Antenna Design There are many reasons why integrators may wish to make their own antennas: The application needs special sized antennas The antennas must be built into structures/equipment The field needs to be more localized Larger loops are needed to cover a wide area (road loops) Another reason may be to achieve a greater reading distance but integrators are advised that reader antenna size is only one factor. Factors that influence read range include: The size and shape of the tag’s antenna The size and shape of the reader’s antenna The transmitter power (limited by legislation) Environmental noise

Antenna Magnetic Field Patterns TRP.1 TRP.1 TRP. 2 TRP. 2

Achieving Resonant Frequency Resonant Frequency is determined by the inductance (wire coil) and matching capacitance. In TMS3705A reader designs, like the eZ430-TMS37157 base station, the antenna coil inductance should be ~440uH. This inductance is optimal due to the FSK modulation used that internally changes capacitance to shift frequency. The closest standard value matching capacitor is 3.3nF. INDUCTANCE CAPACITANCE

Quality Factor The Quality (Q) Factor is a measure of the effectiveness of an antenna. A high Q antenna will output a higher field strength than a low Q antenna for the same input power. A high Q antenna is also a filter and will reject signals outside the bandwidth. Unfortunately, the higher the Q, the more easily the antenna is de-tuned by the presence of metal. The eZ430-TMS37157 is intended for lower Q antennas (~10).

Quality Factor Calculation The Quality (Q) Factor of an antenna is given by Where p = 3.142 ƒ = 134200 Hz (134.2 kHz) L = Self inductance (henries) R = Resistance @ 134.2 kHz Although the resistance should be measured at 134.2 kHz because of the increasing skin effect with frequency - this formula will give an indication of the Q when measured as a DC resistance

Calculating Inductance of Coil ADU.exe can be used to estimate coil inductance at 134.2kHz.

LCR Meter LCR Meter can be used to accurately measure Inductance Choosing LCR Meter Frequency of operation not critical Must read in the µH range (resolution 0.1 µH) Must be portable (battery operated) Must be robust

eZ430-PaLFI Base Station 35mm Antenna The inductance of the coil is 440μH at 134.2kHz with a Q of 30. With a parallel resistor (Rdamp) of 15k the resonance Q is in a range of 10. This antenna is typical for vehicle immobilizer applications.

35mm Antenna Coil Performance Results Tag Antenna Size Range with 35mm Ring Antenna & USB Power Neosid 11mm Memory Access: 6cm MSP Access: 4cm 10cm x 5.5cm Memory Access:16.5cm MSP Access: 11.5cm 50mm Ferrite Memory Access: 13cm MSP Access: 8.5cm 2.5cm diameter Memory Access: 10cm MSP Access: 7.5cm

15cm Diameter Circular Coil Example Calculation taken from: http://emclab.mst.edu/inductance/circular.html Tag Antenna Size Range with 15cm diameter Antenna & 12V Supply Neosid 11mm Memory Access: 15.5cm MSP Access: 10cm 10cm x 5.5cm Memory Access:46cm MSP Access: 35cm 50mm Ferrite Memory Access: 37.5cm MSP Access: 27.5cm 2.5cm diameter Memory Access: 26cm MSP Access: 19cm

22cm x 22cm Antenna Coil Example Tag Antenna Size Range with 22cm x 22cm Antenna & 12V Supply Neosid 11mm Memory Access: 19cm MSP Access: 13cm 10cm x 5.5cm Memory Access:55cm MSP Access: 43cm 50mm Ferrite Memory Access: 43cm MSP Access: 33cm 2.5cm diameter Memory Access: 31cm MSP Access: 23cm

50cm x 25cm Antenna Coil Example Insert ADU Tag Antenna Size Range with 50cm x 25cm Antenna & 12V Supply Neosid 11mm Memory Access:14cm MSP Access: 4cm 10cm x 5.5cm Memory Access:59cm MSP Access: 46cm 50mm Ferrite Memory Access: 44cm MSP Access: 32cm 2.5cm diameter Memory Access: 30cm MSP Access: 20cm

Results Matrix Memory Access: 6cm MSP Access: 4cm Tag Antenna Size Range with 35mm Ring Antenna & USB Power Range with 15cm diameter Antenna & 12V Supply Range with 22cm x 22cm Antenna & 12V Supply Range with 50cm x 25cm Antenna & 12V Supply Memory Access: 6cm MSP Access: 4cm Memory Access: 15.5cm MSP Access: 10cm Memory Access: 19cm MSP Access: 13cm Memory Access:14cm MSP Access: 4cm Neosid 11mm Memory Access:16.5cm MSP Access: 11.5cm Memory Access:46cm MSP Access: 35cm Memory Access:55cm MSP Access: 43cm Memory Access:59cm MSP Access: 46cm 10cm x 5.5cm Memory Access: 13cm MSP Access: 8.5cm Memory Access: 37.5cm MSP Access: 27.5cm Memory Access: 43cm MSP Access: 33cm Memory Access: 44cm MSP Access: 32cm 50mm Ferrite 2.5cm diameter Memory Access: 26cm MSP Access: 19cm Memory Access: 31cm MSP Access: 23cm Memory Access: 30cm MSP Access: 20cm Memory Access: 10cm MSP Access: 7.5cm

Conclusions Range can be increased significantly by using larger antennas. Higher output power also affects the range. LF Antenna design is simple. (No impedance matching involved like in HF(13.56MHz) In the example of the 50cm x 25cm antenna, it is important to note that read range for smaller tag antennas is actually less than the 22cm x 22cm. This is because the eZ430-TMS37157 base station does not provide enough output power to properly drive the antenna coil.