# Design of a Novel RFID Reader for Oilwell Downhole Applications
## 3 Design of Downhole RFID Sparse Solenoid Antenna
### Parameter Impact Analysis of the Sparse Solenoid Antenna  
According to (1), MATLAB can be used to analyze the effect of the magnetic field strength in various directions of the sparse solenoid, as well as the impact of the solenoid's total length and the spacing between adjacent windings on the magnetic field strength. It is assumed that the solenoid is in a vacuum medium, with a current of 30 mA flowing through the solenoid antenna, and the radius \(a\) of the solenoid antenna is 5 cm. When the spacing between adjacent windings is 0.5 cm and the total length of the solenoid antenna is 1 m, four points are taken within the solenoid for comparative analysis, and the calculated results are as shown in Figure 7.  
Figure 7: Magnetic field strength in various directions of the sparse solenoid antenna. (**a**) Hx, (**b**) Hy, (**c**) Hz, (**d**) Total magnetic field strength H.  
It can be seen that the magnetic field strength in various directions of the solenoid antenna is different. The magnetic field strength in the x and y directions is close to 0, with only a slight increase at the ends of the solenoid, but this is also only at positions further away from the center of the xy plane of the solenoid. The magnetic field strength in the z direction is significant, especially in the middle part of the solenoid, where the z direction magnetic field reaches its maximum value. The total magnetic field strength curve and the magnetic field strength curve in the z direction basically coincide.  
Based on the survey and analysis of electronic tags on the market, it can be found that the magnetic field strength of the currently available electronic tags on the market generally requires 5 A/m to fully return the data. Therefore, the analysis of the solenoid is conducted on the premise of setting the minimum magnetic field strength at 5 A/m. When analyzing the impact of the variation in the length of the solenoid on the magnetic field strength, the current flowing through the solenoid antenna is set to 30 mA, the radius \(a\) of the solenoid antenna is 5 cm, and the spacing between adjacent windings is 0.5 cm. Four points are taken in the solenoid for comparative analysis, and the calculation results are as shown in Figure 8. It can be observed that as the length of the solenoid increases, the total magnetic field strength of the solenoid also increases, but the change is relatively small. Considering various factors, this design adopts a sparse solenoid length of 0.8 m.  
When analyzing the impact of changes in the spacing between adjacent windings on the total magnetic field strength, the current flowing through the solenoid antenna is set to 30 mA, the radius a of the solenoid antenna is 5 cm, and the total length of the solenoid is fixed at 0.8 m. Four points are taken within the solenoid for comparative analysis, and the calculation results are as shown in Figure 9. The variation from 0.25 cm to 1 cm in the spacing between adjacent coils shows that the larger the spacing, the smaller the total magnetic field strength. Especially when the spacing increases from 0.25 cm to 0.5 cm, the magnetic field strength is halved. When the spacing between adjacent coils is greater than 0.75 cm, the total magnetic field strength will fall below 4 A/m. Therefore, in order to  
Figure 8: The effect of different antenna length on magnetic field strength. (**a**) \(l\) = 0.6 m, (**b**) \(l\) = 0.8 m, (**c**) \(l\) = 1.0 m, (**d**) \(h\) = 1.2 m.  
activate the electronic tag by generating a total magnetic field intensity of more than 5 A/m in the solenoid antenna, the spacing between adjacent windings is selected as 0.5 cm.