This paper presents a microwave approach to investigate the wake velocity distribution of hypervelocity projectile. The ballistic range measurement is carried out to model the transport of hypervelocity object on the ground. An X-band horn antenna sends a continuous sinusoidal wave with f c = 8.7 GHz to the hypervelocity object and detects the reflected signal. The continuous wavelet transform is utilized to obtain the Doppler frequency shift, and the wake velocity is calculated based on the theory of the Doppler effect. The validity of this experiment is confirmed by comparing the measured projectile velocity with that achieved by a laser-based velocity meter. As a contactless method, this approach does not interfere with the wake flow field. Therefore, the velocity distribution achieved in this paper exhibits better accuracy compared with that obtained from other contact approaches. INDEX TERMS Contactless, hypervelocity projectile, wake, velocity.
To investigate the behavior of plasma generated in the shock tube, microwave reflectometry is proposed to extract the permittivity r of plasma. To remove the influence of parasitic reflections caused by the surroundings, a calibration process is introduced and the unknown calibration coefficients are determined by utilizing microwave interferometry as the reference technique. The shock tube is modeled as a threelayered medium to calculate the reflection coefficient. A time-dependent reconstruction algorithm is applied and theoretically validated to eliminate the multiple solutions in the inverse problem. By comparing the permittivities extracted with microwave reflectometry and interferometry, the effects of plasma diffusion are demonstrated with a modified analytical model in the beginning time region of experiments. In addition, the nonuniform flow in the generated plasma located near the end time region is also observed. The determination of the effective time region for electron density N e and collision frequency v e extraction is discussed as well. Finally, the differences between microwave reflectometry and interferometry in terms of averaged N e < 1×10 17 m −3 and averaged v e < 1.5×10 9 s −1 are investigated in the effective time region.
Switchable rasorber operating between rasorbing and absorbing states can effectively improve its stealth performance within the passband. However, the existing switchable rasorbers utilize single‐order frequency selective surfaces (FSSs), limiting their transmission bandwidth. This work proposes a switchable rasorber with expanded passband utilizing high‐order FSS. First, the switching principle of high‐order FSS is presented, which is then used to design the rasorber with an extended transmission band. The simulated results and experimental verification of the proposed switchable rasorber are also presented. Compared with the existing switchable rasorbers, the proposed device demonstrates the widest fractional transmission bandwidth of 17% and a minimum insertion loss of 0.54 dB in rasorbing state. In the absorbing state, the passband is eliminated and the device exhibits a single absorption band with fractional bandwidth of 89.3%.
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