| Wireless capsule endoscopy(WCE)can not only help doctors monitor the patient's digestive tract in real time,provide more accurate information for the diagnosis and treatment of the disease,but also avoid the discomfort to the patients caused by traditional capsule endoscopy.The antenna is the key device for the communication of WCE.It transfers the physiological and medical data that collected by medical sensors in the form of electromagnetic wave to the receiving device in vitro.Because of the particularity of the working environment of WCE,the flexible antenna must have excellent performance and meet the requirements so that it can improve the security of the WCE system and the reliability.In this paper,based on the premise of the absorption and radiation characteristics of the antenna in the lossy medium,flexible antennas for WCE system are designed and analyzed.The main contents are as follows:1.An ultra-wideband flexible loop antenna using flexible material(Polyimide)as a substrate is designed to work in the medical implantable communication service(MICS)band(402MHz-405MHz).The antenna is bent on the outer wall of the capsule.Compared with air,the dielectric constant of human tissue is larger,which increases the electrical size and realizes the broadband characteristics of the antenna.After parameter optimization,the dimension of the antenna is ??mm15.0mm13mm27.Impedance bandwidth of the antenna reaches 250MHz-600MHz(87.5%),covering the required MICS frequency band.Broadband characteristic guarantees that the antenna can still work properly in the presence of parameters' variations such as substrate thickness,implantation position,and the presence of a battery.The far field gain of the antenna is-32.3dBi,which ensures the reliability of the antenna.At the same time,the reliable transmission distance of the antenna communication link under different transmission rates is calculated.2.A miniaturized helical dipole antenna HDA1(Helical dipole antenna 1)with low specific absorption ratio operating in the wireless medical telemetry service(WMTS)band(1.4 GHz)is designed.In order to realize the miniaturization of this antenna,the currentvector alignment(CVA)is adopted to reduce the physical size of the antenna.And it is compared with the other two antennas.The patch length of HDA2(Helical dipole antenna 2)is the same as HDA1,but its resonant frequency is 1.58 GHz,which shifted 12.9% to the high frequency.The resonant frequency of MDA1(Meander dipole antenna)is 1.404 GHz,but the patch length of MDA1 is 53.7% longer than that of HDA1;The 1-g SAR of the antenna in the small intestine is 132.28 W / kg,which means that the battery can provide more energy for WCE while ensuring human safety.Finally,an external receiving antenna is designed to study the influence of the distance and angle between the transmitting and receiving antennas on the energy coupling.The measured results show the consistency with the simulation results.3.Using a flexible material polyimide as a substrate,an offset dipole antenna operating at ISM band and WMTS band is designed.In order to realize the dual frequency characteristic of the antenna,the L shaped metal branch is loaded on the original antenna structure.When no metal branch is loaded,the antenna operates at 1.4GHz.After the L-type metal branch is loaded,a new resonant frequency point is introduced at 433 MHz to form a dual band antenna with bandwidths of 350MHz(250-600MHz)and 200MHz(1.3-1.5GHz).The gain of the antenna in the muscle at 433 MHz and 1.4 GHz are-35.57 dBi and-22.86 dBi,respectively.Finally,a dipole antenna with operating frequency of1.4GHz is fabricated as the receiving antenna,and the near-field communication link of the antenna is tested.The actual measurement results are in good agreement with the numerical simulation results,which verifies the correctness of the theoretical design. |
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