Most terahertz (THz) radar systems can only work in the near-field region, because the THz source power is limited and the size of the target scattered near field is up to tens of kilometers. Such conditions will result in the conventional radar range equation being unsuitable. Therefore, the near-field radar cross section (RCS) formula is given according to the numerical simulation on different targets. By modifying the parameters in the near field, including the gain of radar antennas and the RCS of targets, the generalized radar range equation is proposed. The THz radar working efficiency in the whole range and the simulation of the near-field RCS simulation model were employed to validate its effectiveness. Through comparison with the radar range equation, it can be concluded that the calculation results of the proposed equation are smaller in the near field, and the outcomes in the far field are identical. The proposed generalized radar range equation can be applied to the whole radiation area including the near field and the far field. Furthermore, more complicated real targets are calculated according to the generalized radar range equation and it can be extended from the submillimeter wave band to a much wider band range. Finally, the near-field radar theory is established, which shows its potential application to the radar cross section estimation in the extremely high frequency and fine design of THz radar systems.

Compact antenna test range can generate a region of uniform pseudo-plane wave called quiet zone, which can provide a test zone for obtaining far-field measurement data of high-performance antenna in the near field. The most important thing to be concerned in CATR is the evaluation of quiet zone. However, most of CATRs are mainly composed of reflectors, which are electric large size antennas, especially in millimeter and THz wave band. Thus, it’s time-consuming and inefficient to use full-wave simulation method like MOM or FDTD to analyze their performance. And high frequency solving techniques cannot calculate the exact radiated field in some cases. In this paper, an accurate analytical method to fast determine the quiet zone of CATR is proposed. The aperture electric field distribution on the surface of main reflector can be obtained by the modified near-field physical optics algorithm. According to the known aperture distribution, the near-field calculation formula is derived to calculate near fields of it. And the correctness of our proposed expression is verified via the comparison with the reflector antenna working in the millimeter waveband with full-wave simulated method. Our proposed expression can be applied to fast estimation of quiet zone. Different typical kinds of antennas applied in the compact range are employed for verification. And the deviations between quiet zone amplitudes calculated by proposed expression and amplitudes obtained from simulation or published paper are both less than 0.5dB. Our analytical method can ensure the calculation accuracy and improve the speed of quiet zone determination at the same time.

Near-field electromagnetic wave, which is radiated by tapered circular aperture antennas, has found more engineering applications in many diverse fields. Through the simulation of a reflector antenna whose efficiency is up to 51.4%, it can be found that the aperture distribution cannot be regarded as uniform distribution but as tapered distribution. Thus, expressions for near fields of uniform circular apertures cannot be applied to tapered circular aperture antennas in practical engineering. To alleviate such problem, near-field calculation formulas are proposed based on the Sommerfeld integrals with steepest descents technique, which ensures the accurate and efficient analysis of the near field of the tapered circular aperture antenna. And near-field characteristics of tapered circular aperture antennas are displayed. The maximum difference of normalized near-field amplitudes between tapered and uniform distribution is 0.35V/m. Three-dimensional near-field gain of circular aperture antennas is developed from the near-field radiation pattern. The proposed expression extends the range of calculation from the axial to more directions compared with the previous expression. Also, this expression is valid in both the near field and the far field. Using full-wave simulations, results obtained from the proposed expression are compared with the simulated results. And the deviation in near-field gain is less than 0.3dB, which indicates the convenience and effectiveness of our expression. The proposed near-field gain expression can be applicable to the near-field measurement and design of millimeter or terahertz radar systems that work in the near-field region.