فصلنامه رادار
سال دوم شماره 4 (پیاپی 6، زمستان 1393)

In this research, electromagnetic properties of graphene-epoxy nanocomposite up to wt% graphene, were investigated in X band. Results showed that these nanocomposites do not have significant permeability so that it can be considered equal to 1. However they exhibit high permittivity such that real part of permittivity of 3wt% graphene sample is 8.4 in GHz and 6.8 in 12GHz frequencies, which are respectively 2.9 and 2.3 times that of pure epoxy at the same frequencies. Also imaginary part of permittivity was obtained 5.8 in 8GHz and 4.4 in 12GHz that is respectively 144 and 68 times that of pure epoxy at the same frequency. Also, the average value of loss factor was measured 0.5 and 0.8 for 2 and 3wt% graphene sample respectively. An average effective electrical conductivity of 3S/m for 3wt% graphene sample was attained. Results showed that the epoxy-graphene nanocomposites are very convenient materials for electromagnetic absorption applications.

Nowadays، SAR imaging systems have become more interesting for scientists. high spatial resolution of recent SAR images has become a key factor for their accurate applications such as interferometry، radargrammetry and change detection. In these applications، SAR image matching procedure is a critical step for conjugate point extraction which is much more difficult than the optical images. The main reasons are both high radiometric speckle noise and high geometric distortions defining by layover، shadow، and foreshortening. Therefore، utilizing efficient methods to achieve a more favorable result in the SAR image matching، could help us to get better output in various applications. Among the SAR images، study about TerraSAR-X is so important، because TerraSAR-X sensor acquires high spatial resolution images. In this paper، we used various textural image features including contrast، correlation، dissimilarity، entropy، homogeneity، optimal gradient filter، mean، second moment، variance، high pass filter and edge images. The main aim of our research was to determine how much can these features improve the SAR image matching process? In our experiments، we used a small urban part of pair of TerraSAR-X single-look slant-range complex (SSC) images that were acquired over the city of Jam، southern Iran، in spotlight mode. The result shows that for some image areas with low texture، the conventional image matching algorithm (here Lucas-Kanade’s optical flow) is not able to detect corresponding points، while using the above textural features in image matching process causes to extract appropriate numbers of matched points.

Nowadays، in the modern wars، radars have faced with many threats. Therefore، low probability of interception (LPI) capability is considered as an important factor in military radars. On the other hand، research on the interception of their signals is started and so far several methods are presented for detection of these signals. By using of time-frequency techniques، we can estimate signal parameters such as carrier frequency، Modulation bandwidth، and its period، in addition of detection of LPI signal. In this paper، the proposed detector architecture uses time-frequency techniques including Wigner-Ville and Choi- Williams distributions for detection of some LPI radar waveforms with Phase Shift Keying (PSK) modulation such as Barker code، Frank code and P1-P4 codes to compare an interception receiver and matched-filter detector in LPI radar receiver with assumption of Additive White Gaussian Noise (AWGN). Then، Power Function graphs for proposed intercept receiver and LPI radar receiver are depicted and compared، which indicate that we obtain desirable results by using of this technique in the intercept receiver.

In recent years، the plasma antenna is a new technology for replacing the metal antennas that have been designed with better performance. In the steady state، the radiation pattern of a plasma antenna will be similar to the case of metallic antennas with the same form، size and performance at the same frequency and power. By sweeping the parameters in the plasma tube، we can change the operating frequency without changing in the antenna length. The most of designs have been investigated only on the basis of the practical experiment. In this paper، a step by step procedure is presented for the design and simulation of a plasma antenna andits material. Also، a new design for the coupler shows that in addition to transmit and receiving signal with the inner coupler، there is no electromagnetic interference (EMI) due to the outer shell. Based on the design procedure، the simulation and implementation of the antenna is performed and tested in VHF band. The measurement results are compatible with the simulation excellently.

Underwater target imaging using acoustic waves is possible by employing the method used in synthetic aperture radars. In recent years، synthetic aperture sonar has been the subject of several researches. Wherein imaging of underwater moving target is required، inverse synthetic aperture sonar (ISAS) idea can be used but specifications and limitations of underwater channel should be considered. In this paper، first we survey monostatic inverse synthetic aperture sonar and its difficulties for use; then we propose multistatic inverse synthetic aperture sonar (MISAS) with specific geometry and condition which can create better images of underwater moving targets than monostatic ISAS. Improvement in images quality is achieved by space diversity of multistatic ISAS which is the result of placement and multipath effect of only one transmitter in shallow waters.