Omnidirectional WLAN Antennas With Common-Mode Current Suppression

Antenna is a device for transmitting and receiving electromagnetic waves,which is critical in wireless communication.In recent years,the development of antenna technology is rapid,and many antennas for different scenarios have been proposed.In practical applications,the antenna is usually fed by a coaxial line.The coaxial lines have balanced structure.The electromagnetic waves are propagated between the outer surface of the inner conductor and the inner surface of the outer conductor.The currents on the surface out of phase with equal amplitude to form a differential feeding structure.The outer surface of the outer conductor of the coaxial lines is theoretically grounded,and its voltage and current are zero.However,at the feeding port,the voltage of the coaxial line is unbalanced.Part of the currents on the inner surface of the outer conductor of the coaxial line passes through the outer surface of the conductor to flow back to the source,forming a common-mode current.The existence of common-mode current on the feeding coaxial line will cause the antenna radiation pattern to be distorted,and the radiation efficiency will be reduced.Therefore,how to suppress the common-mode current on the feeding coaxial line has great research value.Based on the above problems,this thesis conducted relevant research:1.A broadband omnidirectional antenna with common-mode current suppression is presented.Firstly,a loop-slot antenna is designed,and the influence of various parameters of the loop-slot antenna on the antenna performance is analyzed,which will pave the way for subsequent chapters.Then,1/4 L₀ and 3/4 L₀ of the loop-slot antenna are bent inward by90°,and the two loop-slot antennas after the bending are spliced together to form a three-dimensional structure to achieve omnidirectional radiation.Finally,a power-division balanced balun is designed to realize the impedance matching of the loop-slot antenna array while providing balanced feed.The simulation and test results of this antenna show that the antenna's impedance bandwidth is 1.75-2.73 GHz(|S₁₁|<-10 d B),and the roundness at 2.2GHz,2.4 GHz and 2.6 GHz are 1.4 d B,1.6 d B and 1.5 d B,respectively.More importantly,the antenna can suppress the common-mode current on the outer surface of the feeding coaxial line.2.An omnidirectional WLAN antenna with common-mode current suppression is proposed.Firstly,it is analyzed that the ring structure has a voltage zero point,at which point the feeding coaxial line can balance the common-mode current of the feeding coaxial line.The principle is applied to the loop-slot antenna.Three half-cut loop-slot antennas are spliced together at 120°intervals along the cut to achieve omnidirectional radiation.Coaxial lines are drawn at the zero point of the loop-slot antenna to achieve a common-mode current suppression.Secondly,a metal branch is introduced at the current zero position of the loop-slot antenna,which changes the high-frequency and low-frequency current directions,reducing the antenna size and increasing the roundness of the antenna.Finally,the edge of the loop-slot antenna is cut at an angle to change the phase of the induced current at the edge of the high-frequency current,so that the roundness of the antenna is further improved.The simulation and test results of the antenna show that the antenna's impedance bandwidth is2.21-2.59 GHz and 5.11-6.20 GHz(|S₁₁|<-10 d B),and the roundness at 2.4 GHz,5.2 GHz and 5.8 GHz are 0.95 d B,1.84 d B and 2.43 d B,respectively.Also,the antenna can suppress the common mode current over all the operating bandwidth on coaxial line and effectively reduce the antenna pattern distortion.3.An omnidirectional dipole WLAN antenna with common-mode current suppression is designed.Firstly,a dipole antenna is designed to work in the WLAN frequency band.Then,the principle proposed in Chapter 4 that the feed coaxial line drawn at the zero point of the ring structure voltage can balance the common mode current of the feed line is applied to the dipole antenna.Secondly,the structure is improved to a floor structure more suitable for a dipole antenna.Finally,the artificial magnetic conductor(AMC)structure was introduced to further suppress the common mode currents in the 2.4 GHz,5.2 GHz and 5.8 GHz,respectively.The simulation and test results of the antenna show that the antenna's impedance bandwidth is 2.27-2.58 GHz and 4.16-8.48 GHz(|S₁₁|<-10 d B),and the roundness at 2.4 GHz,5.2 GHz and 5.8 GHz correspond to 1.9 d B,3.9 d B and 4.1 d B respectively.Furthermore,the antenna can suppress the common mode current over all the operating bandwidth on coaxial line and effectively reduce the antenna pattern distortion.