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

2. 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.

3. Using External Power Supply to Increase Output Power

4. 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

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

6. 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

7. 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).

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

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

10. 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

11. 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.

12. 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

13. 15cm Diameter Circular Coil Example
Calculation taken from:
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

14. 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

15. 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

16. 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

17. 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.