Research On Miniaturization Of VHF Antenna

VHF communication has become more and more widely used in maritime and airborne communication and TV broadcasting.With the development of mobile communication,for backpack-type or handheld terminal devices such as tablet computers,maritime positioning search and rescue devices,the requirements for the miniaturization of their communication systems are becoming higher and higher.Antennas that can send and receive electromagnetic waves play an important role in communication systems.Its compact design is critical.Meanwhile,the length of VHF wave in free space varies 1m to10m,so the VHF antenna need a wide ground provided by carriers in general.It's too hard for potable equipment.This contradiction has also become an urgent problem to be solved.Aiming at the miniaturization of VHF antenna,this thesis proposes three solutions.The first one:because of the space filling characteristics of Hilbert fractal curve,it is applied to the miniaturized design of monopole antenna.The second one:according to the theory of zeroth order resonance,the resonance frequency of the antenna is determined only by the reactance value in its equivalent circuit in the zeroth-order-resonance state,regardless of its physical size.By introducing reactance into the antenna of the coplanar waveguide structure,it can work in the zeroth-order-resonance state.The third one:use the slow wave structure,meander line and interdigitated structure that is suitable for the planar antenna to load it on the back of the antenna of the coplanar waveguide structure,thereby achieving miniaturization of antennas.Details are as follows:Firstly,the fractal theory is introduced.A planar monopole antenna based on Hilbert fractal geometry is designed.The antenna has three resonance points in the VHF band.They are 88MHz,200MHz and290MHz.Therefore,three pass bands is obtained.Their relative bandwidths are 6.4%,20.4%and 1.5%.The resulting antenna size is only 268*140*1.5mm~3(L?W?H).Compared with the general 1/4wavelength monopole antenna,the height is reduced by about 67%.Secondly,the zeroth-order resonance theory is analyzed that is absence of some structures.Under the condition of the default parallel part structure,take advantage of the flexibility of the coplanar waveguide structure and adopt the asymmetric structure on the left and right.The interdigitated structure,variable capacitance and meander line are added to the antenna.Changing the variable capacitance value is the main method to realize the zeroth order resonance.The designed antenna has a size of 80*68*1mm~3(L?W?H).Use variable capacitance to achieve tuning and the tuning range is 256?290MHz,which can adapt to the communication requirements in VHF high frequency band.Based on the above-mentioned antenna,change the structure of the grounding band on the right to introduce two slot capacitances for the antenna to reduce the resonance frequency of the antenna.The resulting antenna size is 84*88*1mm~3(L?W?H)and the tuning range is 200?215MHz.Thirdly,two kinds of plane slow wave structures,meander lines and interdigitated structures,are proposed.Combine them with a fractal monopole and zeroth-order resonant antenna.Load it on the back of the antenna based on the original size of the original antenna unchanged.The resonant frequency of the 2nd-order Hilbert fractal monopole is reduced by 34MHz after the meander line is loaded and the reduction ratio reaches 32.7%.Its gain is only 2.1d B lower than the original antenna and the shape of the radiation pattern is unchanged.The resonant frequency of the 2nd-order Hilbert fractal monopole loaded with interdigital structure has also been reduced.The first resonance point has dropped to 72MHz,and five resonance points and four pass bands have been obtained in VHF band.The gain at the first resonance point only drops by 2.3d B.After the zero-order resonant antenna is loaded with the meander line and the interdigitated structure,the resonance frequency is reduced and the effect of the latter is more obvious.Compared with the original antenna,the resonant frequency of the former antenna is reduced by 47MHz,and the rate of decrease is 22.7%.The latter is reduced by 77MHz and the rate of decrease is 37.2%.After the two slow-wave structures are loaded,the zeroth-order resonant antenna has a narrow working bandwidth and has little effect on the radiation pattern of the zeroth-order resonant antenna,but the loss is high.