The Effect Of Plasma Sheath On Beam Pointing Property Of Phased Array Antenna

A space vehicle reentering the atmosphere at a hypersonic speed produces a layer of plasma sheath around the aircraft.The plasma sheath is a kind of lossy and dispersive medium,which is distributed in the near field of phased array antenna.It interacts with the electromagnetic wave radiated by the antenna,which affects the beam forming effect of phased array antenna,causing the beam-pointing deviation.As a result,the antenna beam of the aircraft can't be accurately aligned with the space-based TT & C satellite.There are two reasons: firstly,the amplitude attenuation and phase delay of electromagnetic caused by the plasma sheath have great differences in different directions,which changes the distribution of field strength,making the coherent superposition of radiation fields of different elements different from the free space.The array beam forming effect is changed.Secondly,the plasma sheath produces a strong reflection on electromagnetic wave,which increases the coupling between the array elements and lead to the deviation of the beampointing angle.In response to this issue,the work carried out in this paper mainly includes:Firstly,the propagation characteristics of electromagnetic waves in a inhomogeneous plasma sheath are studied.The amplitude attenuation and phase delay characteristics of electromagnetic waves radiated from different directions of the antenna through the plasma sheath were analyzed.According to electromagnetic wave superposition theorem,the formulas for phased array antenna array factor calculation under plasma sheath are deduced.Then,the influence of electron density and collision frequency of different plasma on the direction of factor beam is given.Secondly,based on the reflection characteristics of the plasma sheath on the electromagnetic wave,the impedance characteristics and mutual coupling characteristics of the phased array antenna covered by the plasma sheath were studied.The results show that under high electron density and low collision frequency,the plasma sheath can enhance the selfcoupling of the array elements and the mutual coupling of adjacent array elements.For oblique diagonal elements,the plasma sheath will reduce its mutual coupling.Then,the calculation method of the phased array antenna pattern covered by the plasma sheath was considered and the correctness was verified.Finally,according to the product theorem of pattern,taking into account the mutual coupling factors of the array elements,an electromagnetic analysis model of the phased array antenna covered by an inhomogeneous plasma sheath is established,and the radiation characteristics of different spatial directions of the phased array antenna covered by the plasma sheath are calculated.According to the calculation result of the pattern,the relationship between the parameters of the plasma sheath and the beam pointing deviation angle of the phased array antenna is obtained.The calculation results show that the gain of the main lobe of the phased array antenna pattern decreases as the plasma electron density increases.When the plasma collision frequency is close to the operating frequency of the phased array antenna,the deterioration of antenna pattern is the most serious caused by the plasma sheath.The greater the plasma electron density,the closer the collision frequency to the phased array antenna operating frequency,and the larger the beam pointing deviation of the phased array antenna caused by the plasma sheath.In this paper,the research results of this paper can provide reference for the design of phased array antenna for aircraft and provide compensation data for beam pointing error correction of phased array antenna covered by plasma sheath.