# RFID Technology as a Low-Cost and Passive Way to Digitize Industrial Analog Indicators
## 3 Materials and Methods
### Application Requirements and Restrictions  
RFID technology covers a wide bandwidth from 125 kHz to 5.4 GHz. In the low-frequency band, the antenna size increases due to the increase in wavelength, and the data transmission speed and reading range decrease. Despite the advantages of the high-frequency band, such as reducing the antenna size and increasing the speed and reading range, it is not an appropriate option for industrial environments due to reducing the penetration of EM waves in objects. Therefore, we have selected the 866 MHz (mid-frequency band), the most common frequency band in industry applications.  
Antenna Pattern and Dimension Limitation  
Bench test outputs showed that their RSSI chart pattern changed in two ways whenever the cover rotated and the tags exited the EM shield. Rectangular tags create an M-shaped pattern with two peaks, and square and circular tags create an A-shaped pattern with one maximum point. Therefore, due to the manometer shapes, the best option for our application was a circular dipole RFID tag. In terms of dimensions, manometers are designed and manufactured in less than one centimeter to several tens of centimeters. We chose the WIKA pressure gauge PGS21.100 as a case study for digitization. Detailed specification material and dimensions of the manometer are given in [30]. Due to the manometer dial plate limitation (94 mm diameter), we had to keep the RFID tag dimension under about 20 mm. The green and red area tags were located at a distance of \(\hat{\measuredangle}/(8)\) from each other.  
Antenna Material and Substrates  
Choosing the suitable material and substrate is one of the most important parts of designing an RFID tag. As shown in Figure 4, the entire body of the manometer, except for the central part, is made of stainless steel. The RFID tags are all implanted on a standard single-layer FR4 PCB substrate with 1.6 mm of thickness coated with a 35 \(\upmu\)m thick copper layer as the RFID tag antenna. This layer is located directly on the dial plate.  
RFID Chip Impedance Matching and Resistant  
Numerous chips are designed and manufactured in the RFID frequency bands, but they are mostly not solderable and not available for commercial use in mass-production assembly lines. NXP UCODE G2 iM is a series of passive identification transponder chips in the 900 Mhz frequency band. UCODE G2 iM SL3S1003 is a six-pin chip in the solderable SOT886 package [31], which provides \(-17.5\) dBm read sensitivity. As shown in Figure 5, pins 1 and 3 are the RFP and RFN pins connected to the tag antenna, and pins 4 and 6 are the chip external battery pins (active mode configuration).  
To transfer the maximum power absorbed by the tag antenna to the chip, the internal impedance of the designed antenna must be precisely equal to the conjugate input impedance of the chip. The higher the sensitivity and operating frequency of the tag, the  
Figure 4: Digitized manometer components and materials.  
Figure 5: RFID tag Antenna and chip Geometry.  
more it allows the designer to design tags in smaller sizes. The internal impedance of the chips has a complex parameter that can be expressed as follows:  
\[\mathrm{Z_{c}=R_{c}+jX_{c}\cdot Z_{a}=R_{a}+jX_{a}} \tag{1}\]  
The chip impedance (\(\mathrm{Z_{c}}\)) and antenna impedance (\(\mathrm{Z_{a}}\)) are both frequency-dependent and contain a real impedance part (R) and complex impedance part (X). The internal resistance table of the SL351003 chip in the frequency band is given in Table 2[31]. Due to the European frequency band standard, the impedance of 866 MHz in designing and tuning the tag antenna has been considered.