Hakki Nazli

An Improved Design of Planar Elliptical Dipole Antenna for UWB Applications

In this letter, an enhanced planar elliptical dipole antenna design for ultrawideband (UWB) communication and impulse radar systems is presented. The printed-circuit-elliptical (PCE) antenna has been investigated to be an effective radiator for UWB applications. To enhance gain and return loss bandwidth of the antenna, elliptical slots are used on the dipole arms. The gain performance of the antenna has been increased by means of elliptical slots in the frequency range from 2.7 to 11 GHz. The standing wave ratio is less than 2 (SWR < 2) along 94.4% of operation bandwidth from 1.1 to 11 GHz. The radiation pattern in E- and H-plane for certain frequencies, the return loss, and the gain performance are presented with the experimental and simulation results. Moreover, the time domain analysis of the antenna is presented. The antenna introduces low-level ringing and pulse distortion. Consequently, the antenna is very useful for impulse and UWB communication systems.

Detection of Ammonia in Liquids Using Millimeter Wave Spectroscopy

Detection of ammonia plays a vital role for counter-bioterrorism applications. Using millimeter wave absorption measurements, ammonia dissolved in water solution is analyzed and compared to water-only solution. The inversion of ammonia molecule results in split rotational spectral lines and transitions of these lines can be detected. Two-port measurements were carried out with vector network analyzer and measurements revealed that ammonia presence can be identified, especially between 30–35 GHz.

Experimental investigation of different soil types for buried object imaging using impulse GPR

Performance of electromagnetic sensors which are used to detect buried objects, is varied according to properties of soil. Most common sensor pair is EMI (Electromagnetic Induction) and GPR (Ground Penetrating Radar) for buried object detection systems. Besides the surface roughness and overlying vegetation, electrical conductivity (σ, (S/m)), electric permittivity (ε, (F/m)), and magnetic susceptibility affects the detection performance of sensors. The electromagnetic properties of soil cause wave attenuation and change reflection coefficient of electromagnetic wave reflected from soil surface. For this reason, different soil types create various effects on sensor data, therefore performance of GPR and EMI sensors may decrease. Moreover, burial depth estimation can be performed if the soil properties are known exactly. This paper contains the effects of different soil types on impulse ground penetrating radar data, experimentally.

Millimeter-wave detection of landmines

Millimeter wave absorption relative to background soil can be used for detection landmines with little or no metal content. At these frequencies, soil and landmine absorb electromagnetic energy differently. Stepped frequency measurements from 20 GHz to 60 GHz were used to detect buried surrogate landmines in the soil. The targets were 3 cm and 5 cm beneath the soil surface and coherent transmission and reflection was used in the experimental setup. The measurement set-up was mounted on a handheld portable device, and this device was on a rail for accurate displacement such that the rail could move freely along the scan axis. Measurements were performed with network analyzer and scattering data in frequency domain were recorded for processing, namely for inverse Fourier Transform and background subtraction. Background subtraction was performed through a numerical filter to achieve higher contrast ratio. Although the numerical filter used was a simple routine with minimal computational burden, a specific detection method was applied to the background subtracted GPR data, which was based on correlation summation of consecutive A-scan signals in a predefined window length.

Effects of different soil types on strip-map GPR SAR images

In this study, we present generation of Strip-map Synthetic Aperture Radar (SAR) images using impulse GPR system, and investigate effects of different soil types on SAR images. The SAR images of buried objects have been interpreted via 2D inverse Fourier transformation. GPR buried target data have been collected from three soil pools having different dielectric constants and B-scan images have been reconstructed from the received data using mean A-scan signal subtraction method. In order to reconstruct SAR images, the time domain data collected from multiple observation points have been transformed to 2D spectral domain. Non-uniform data have been interpolated over spatial Cartesian grid by using uniform interval. Thus, the SAR images have been reconstructed via 2D inverse FFT of interpolated data on ky-kz plane. When examined mathematical background of SAR algorithm, the values of different dielectric constants change the wave number of k. This can lead to deterioration of the SAR imagery. In this study, we investigate the Effect of the dielectric constant of different soils has been examined on SAR images. Finally, resolution difference between background removed B-Scan data and SAR images is considered.

Comparative analysis of short and long GPR pulses for landmine detection

Ground Penetrating Radar (GPR) is one of the most popular subsurface sensing devices. It has a wide range of applications such as landmine detection, archeological investigations, road condition survey and so on. Hardware and software requirements of the GPR system are strongly dependent on type of applications. Principally, lower frequencies provide deeper penetration and low resolution, but higher frequencies are able to detect shallow objects with high resolution. As a fundamental design criterion, there is a trade-off between penetration depth and vertical resolution. In impulse radar, pulse duration (frequency related) is a key parameter because it affects the system detection performance. Specially, optimization of the pulse duration is a challenging problem for landmine detection because the GPR performance has been strongly affected from mine types, varying terrain and environmental conditions. In this work, two GPR systems with pulse durations of 650 ps and 870 ps are compared for evaluation of their detection performance. The pulses are tested with extensive data sets collected from different soil types by using surrogate mines and other objects. Receiver Operating Characteristic (ROC) curves of the system is also calculated. It seems that the 650 ps pulse duration gives better performance than the 870 ps pulse duration for the shallow landmine detection.

Multi-Channel Transmit/Receive Metal Detector Coil Design for Vehicular Applications

Metal detector coil design has been matured to great extent over the years. However, vehicle mounted or remotely operated metal detectors require different specifications and these specifications dictate multiple transmit and receive coils operating in various settings. Unlike handheld operation, detector is more susceptible to metallic body of the vehicle. Moreover, ground calibration is also different than handheld devices. Coil geometries and intercoupling between them play a significant role in system design and performance. In this study, we study different coil geometries for vehicular applications. Starting from well-known coil geometries, we placed coils on dry, wet and ferrous soil to understand the interaction mechanism. Simulation studies are performed in frequency domain but the results are all applicable to time domain pulse based detector systems.

Tapered slot antenna design for vehicular GPR applications

Vehicular applications of UWB GPR demand multiple GPR sensors operating in a harsh environment. One of the key elements of in the sensor is its UWB antenna which has minimal inter-element coupling, low group delay, high directivity and less prone to environmental conditions. Tapered slot antennas (TSAs) provide good impedance match, but they need to be modified for above specifications. Parasitic slot loaded TSA with balanced feed is proposed and a multi-channel antenna array structure is formed. Structural parameters are numerically analyzed and a prototype is built. Measurements show good performance for UWB GPR applications.

High Fidelity Gaussian Monocycle Generation Using Microstrip Lines

The impact of delay line on Gaussian monocycle pulse generation is studied analytically in terms of pulse fidelity and spectral energy density. We devise the shape of the short-circuited stubs for Gaussian monocycle pulse generation for lossy dispersive medium. Prototypes are built, and measured monocycle achieves 340 ps pulse width with -25 dB ringing level.

Effect of pulse fidelity on detection of landmines

Detection of landmines based on complex resonance frequencies has been studied in the past and no distinctive results have been reported. Especially for low metal content landmines buried at depths greater than 9 cm, resonant frequencies become fairly distributed in the background and no specific frequency of interest can be used. However, in a typical impulse radar, spectral energy density of the transmitted pulse can be very broad and its peak can be located anywhere. Usually, a compromise is made between penetration depth and feature resolution for spectral energy peak allocation. Pulse amplitude, duration, symmetry, its spectral energy distribution, ringing level all affect depth and resolution metrics in a complicated way. Considering receiver dynamic range, we study two distinct pulses having different spectral energy density peaks and their detection ability for landmines with little or no metallic content. We carry out experiments to show that pulse shape/fidelity is critical to obtain desired contrast in post-processing of data.