# Design of a Novel RFID Reader for Oilwell Downhole Applications
## 4 Experimental Testing  
To verify the proposed design, the experimental test system is implemented, as shown in Figure 10a, in which there is a simple downhole RFID system, including a designed HDX reader circuit, a sparse solenoid antenna, the commercial HDX microtags of RI-TRP-WR3P, and the battery. The length and the spacing between adjacent windings of the antenna is set to 0.8 m and 0.5 cm, respectively, based on the previous analysis. An oscilloscope of Tektronix DPO4104B is employed to measure the waveforms of the designed HDX reader circuit and the sparse solenoid antenna. As the signal modulation and demodulation module is a crucial part of the reader hardware circuit, the modulation signal generated by the microcontroller at 134.2 kHz is tested first. This signal, after being emitted from the microcontroller, is amplified by the MIC4452 VM driver chip, and then applied to the reader antenna terminals. The waveform measured using an oscilloscope is shown in Figure 10b. It can be seen that one period is approximately 7.48 us, with a frequency of 133.7 kHz and an error of less than 1%. Then, the signal demodulation testing is conducted. When the signal returned by the antenna enters the XR2211 demodulation chip, adjusting the center frequency of the XR2211 peripheral circuit allows for the output of a demodulated signal from Pin 7. This signal is in non-return-to-zero code. Measuring Pin 7 of the XR2211 with an oscilloscope yields the waveform depicted in Figure 10b. It can be observed that the data starts with a continuous high level of approximately 1000 us, and there is also a continuous high level of approximately 1000 us at the end, along with a consecutive 15-bit high-low level transition. According to the RI-TRP-WR3P electronic tag's chip manual, it can be inferred that the data are returned with full completeness and exhibit an accuracy rate of 100%. Therefore, the designed HDX RFID reader can communicate with tags normally. Figure 9: The effect of different adjacent winding spacing \(h\) on magnetic field strength. (**a**) \(h\) = 0.25 cm, (**b**) \(h\) = 0.5 cm, (**c**) \(h\) = 0.75 cm, (**d**) \(h\) = 1.0 cm. Then, the performance tests of the sparse solenoid antenna are carried out; here, the traditional tightly wound solenoid antenna, which is often adopted as the reader antenna in the commercial downhole RFID system of Weatherford or the research of various downhole RFID systems, is employed for performance comparison. To avoid the influence of tag orientation on the reading times, the tag entered along the axial direction of the antenna. Considering the rapid movement of electronic tags with downhole fluids, it is evident that the longer the antenna length, the better in order to allow the electronic tag to stay in the reader antenna area for sufficient time to complete the data exchange. In other words, longer antennas can provide a greater number of tag reading times. The tag reading cycle is measured as 54 ms, as shown in Figure 11.  
For traditional tightly wound solenoid antennas, excessive antenna length will result in larger antenna inductance values and increase the difficulty of antenna impedance matching. Fortunately, the proposed sparse antenna with larger lengths do not have the  
Figure 11: Measurement waveform of tag reading cycle. Figure 10: (**a**) Experimental test system. (**b**) 134.2 kHz modulated signal. (**c**) Demodulated signal. problem of large inductance. Using an impedance analyzer, the inductance of the designed sparse solenoid antenna with a length of 0.8 m was measured to be only 309 \(\mu\)H. The inductance value of a tightly wound antenna of the same length is 1.24 mH, which is nearly four times that of a sparse antenna. When the inductance value is the same, the length of the traditional tightly wound solenoid antenna is significantly reduced to 21 cm, which is much smaller than the length of a sparse solenoid antenna of 80 cm.