A Study On Single-Sideband Time-Modulated Phased Array And Low-Profile Ultra Wideband Log-Periodic Array Antenna

Low-cost and high-precision phased array antennas and ultra-wideband low-profile conformal antennas are widely employed in the fields of communication and guidance,which have been hot research areas.Single-sideband time-modulated technology is an unconventional phased scanning technology proposed in recent years and it could realize high-precision beam scanning with low cost.However,it has a series of common problems,such as low efficiency and high sideband level.Log-periodic antenna is one of the frequency-independent antenna to realize ultra-wideband bandwidth.Because of the limitation of size,there are many challenges to realize conformal and low-profile antenna with the bandwidth near 1:10.To solve these problems,a series of theoretical and technical researches were carried out in this paper,and the high-efficiency low-sideband time-modulated phased array antenna and the 2-18 GHz conformal low-profile log-periodic array antenna were successfully realized.The main innovations in this dissertation are as follows:1.High-efficiency single-sideband time-modulated phased array antenna:In order to avoid the drawback of high loss in time-modulated array antenna,the theory of loss in single-sideband time-modulated time modulated array antenna is further summarized.On this basis,high-efficiency single-sideband time-modulated array antenna with reconfigurable power divider is proposed.Its frequency spectrum and formula of all kinds of losses are deduced.Several important parameters in the circuit corresponding to frequency spectrum are also analyzed.To experimentally verify the feasibility of the proposed design,the 8-elements time-modulated array is manufactured,whose sideband level is-16.9 d B and allowable signal bandwidth without aliasing effects is eight times of modulating frequency,and its time-modulated pulse,frequency spectrum and radiation pattern of scanning beam are measured,respectively.2.Low-sideband-level single-sideband time-modulated phased array antenna:In traditional single-sideband time-modulated array antennas,their modulated pulses have low degrees of freedom for optimizing.To solve this drawback,single-sideband time-modulated array antenna with digital attenuators is proposed.By analyzing the formulas of the multi-step time-modulated pulse,the method to optimize the sideband level is given.On the basis,a single-sideband time-modulated array antenna suitable for narrowband signals is proposed by suppressing the undesired sidebands and obtaining more pure frequency spectrums.It employs4-step waveform of modulating pulse and its sideband level is-23.52 d B and allowable signal bandwidth without aliasing effects is sixteen times of modulating frequency.Then,the single-sideband time-modulated array antenna suitable for wideband signals is proposed by employing optimization algorithm to realize the optimal sideband level and suppress the aliasing effects.It employs 4-step and 8-step waveforms of modulating pulse and its sideband level is-28.76d B and-32.78 d B with lower signal distortion from aliasing effects in the situation of wide-bandwidth signals.These two designs are both verified by experiments and a comparison between them are also presented.3.Time-modulated amplitude-phase weighting array antenna:Single-sideband time-modulated array antenna are mainly phase-only weighting without amplitude weighting.The time-modulated amplitude-phase weighting array is proposed to suppress the sidelobe.The theory of the time-modulated amplitude-phase weighting array antenna is given.The effect of time amplitude weighting modulation to the sideband level and the method to improve it are also discussed.Based on the technology of high-efficiency single-sideband time-modulated time modulated array,an 8-element time-modulated amplitude-phase weighting array antenna is designed,whose sidelobe level is lower than-30 d B and sideband level is lower than-16 d B.4.Low-profile ultra-wideband log-periodic array antenna:Two kinds of log-periodic array antennas are proposed to realize low-profile and compact ultra wideband(UWB)antenna.Considering the elements in low-profile log-periodic array antenna with top-hat which limits the impedance matching within the ultra-wide bandwidth,a 2–18 GHz conformal low-profile log-periodic array is proposed which employs three types of element,including:monopoles of different heights for the high band,top-hat monopoles for the middle band,and folded top-hat monopoles for the low band.Its profile height is 0.053?_L at the lowest operating frequency.This array antenna is placed on a conducting cylindrical surface and all the top hats is conformal.A transition structure between the array antenna and cylindrical conductor is used to improve the performance.Meanwhile,a planar compact helical log-periodic array is proposed,whose elements employs printed circuit board(PCB)technology to reduce the overall size and low profile.It operates at 2-6 GHz and its size is only 58%of that of the conventional array.Both of these two designs are manufactured and measured in experiments.