This paper describes an application of physical optics and the method of equivalent currents to the calculation of radar cross section (RCS) of a helicopter rotor. The problem is treated using a quasi-stationary approach. The calculation can be parameterized as a function of the locations of the radar transmitter and receiver in relation to the rotor center. Therefore, this offers the possibility of monostatic and bistatic simulations in the far field and near field. Blade geometry is taken into account using a triangular meshing generated by the I-DEAS meshing software. Digital applications are presented and the effects on the RCS spectrum of incidence, frequency, blade number, and the near field are analyzed.

Calculation and analysis of electromagnetic scattering by helicopter rotating blades

A simple and effective approach for the evaluation of the chamber quality factor is presented. It is based on the averaged periodogram and permits a complete spectral characterization of the efficiency of reverberating chambers. A meaningful set of experiments is performed and discussed.

A spectral approach for the determination of the reverberating chamber quality factor

This paper investigates rotor modulation on the gain and input impedance of an antenna mounted on a helicopter's fuselage. The rotor blades, which are rotating about an axis with a given angular velocity, continuously modulate the signal transmitted by the antenna and adversely affect the established communication links. In this study, particular emphasis is placed on wire antennas operating within the HF and VHF bands. Specifically, the Doppler spectrum of the magnitude and phase of the radiated field from a vertical and a horizontal short dipole underneath the rotors is computed and analyzed. The variation of the corresponding antenna gain as a function of rotor angle and frequency is also investigated. The same type of analysis is then extended to a 14-ft towel-bar antenna mounted on the tail boom of a 10:1 scaled helicopter model. In addition to gain, the variation of input impedance versus angle and frequency is computed within the HF band. Finally, the bit error rate (BER) assuming a quadrature phase-shift keying (QPSK) modulation is evaluated assuming linearly and circularly polarized receiving antennas.

Helicopter rotor-blade modulation of antenna radiation characteristics

During parameter acquisition of rotating components, wireless data transmission is an efficient way. In this paper, electromagnetic (EM) wave propagation characteristics in rotating environments are preliminarily studied in order to provide a strong theoretical support for developing high-performance radio monitoring systems in rotating environments. The mathematical formulation of signal wave propagation between the transmitting antenna and the receiving antenna in time-dependent rotating motion is derived. The Doppler frequency shift and the EM wave polarization characteristics in rotating environments are presented and analyzed. The two-ray propagation model in rotating environments is proposed, and the validity of the model is verified by experimental results. Taking tire pressure monitoring system (TPMS) as a typical example, key parameters affecting wireless data transmission from a rotating transmitting antenna are investigated, and system performance enhancement strategies are proposed. Power transmission tests were conducted on a TPMS operating at different frequencies and different rotating radii relative to wheel rotation axis. The test results show that the path loss and the path loss fluctuation sharpness increase with the operating frequency and the rotating radius. Moreover, the path loss for most rotation angle values decreases drastically at lower frequencies and lower radii. Wireless data transmission tests were implemented for both the improved TPMS and the old TPMS. The test results show that the receiving probabilities of successful data frame transmission are improved greatly.

Study on Electromagnetic Wave Propagation Characteristics in Rotating Environments and Its Application in Tire Pressure Monitoring

The rotation of lift- and thrust-generating airfoils causes a periodic variation in aircraft communication signals. This effect is called rotor-blade modulation (RBM). The RBM on the radiation characteristics of helicopter-mounted antennas are predicted using the uniform theory of diffraction (UTD) and the finite-difference time-domain (FDTD) method and are compared with scale model measurements and full-scale in-flight measurements. In addition, the effects of electric field polarization on RBM are studied using the FDTD.

Rotor-blade modulation on antenna amplitude pattern and polarization: predictions and measurements

The electromagnetic scattering of rotating blades is investigated both theoretically and experimentally. We have developed an analytical method based on physical optics (PO) and the method of equivalent currents (MEC), in conjunction with the quasi-stationary method. We have predicted the bistatic field scattered by a multiple skew-plated rotating fan. Comparisons with a numerical calculation based on the method of moments and with measurements have given very satisfactory results. A comparison with previously published results has also been carried out. Physical interpretations are given both in time and frequency domains. Our analytical model correctly predicts the spreading and magnitude of the frequency response as a function of the scatterer's skew angles, its rotation frequency, and the directions of incidence and scattering.

I. Tardy

Papers

Computational and experimental analysis of the scattering by rotating fans