Ioan Nicolaescu

Improvement of Stepped-Frequency Continuous Wave Ground-Penetrating Radar Cross-Range Resolution

This paper presents some experimental results obtained with newly developed stepped-frequency continuous wave (SFCW) ground-penetrating radar in a frequency range from 400 to 4845 MHz. This paper describes the procedures to remove the delays within the transmitter, the receiver, and the antenna system and analyzes the experimental results obtained after processing the measured data. The measured system footprint and the experimental results show that using an ultrawideband (UWB) Archimedean spiral antenna decreases the coupling signal but does not provide the needed cross-range resolution. Based on the measured dimensions of the footprint, a synthetic aperture procedure is used to improve the radar cross-range resolution from around 60 cm to about 6 cm. This paper shows the ability of the radar to detect dielectric objects and to exhibit their shape as it works with circularly polarized waves. What makes this paper different from prior work, which presents some experiments using a vector network analyzer set up to work as an SFCW radar, is that the data were acquired with a real system working with circular polarization and using two UWB Archimedean spirals. The calibration procedures as well as the synthetic aperture procedure were developed and validated based on measured data with the SFCW radar.

Superior Modes in High Permittivity Cylindrical Dielectric Resonator Antenna Excited by a Central Rectangular Slot

High permittivity materials allow good miniaturization of microwave components, but the dielectric resonator antenna using these materials has the drawback of a reduced bandwidth. To overcome this disadvantage we wanted to obtain a relatively wideband high permittivity DRA using the superior modes. Using HFSS simulations, we identified some of the superior modes that appear in cylindrical DRAs situated on a conductor ground plane and excited by a symmetric rectangular slot made in the ground plane. We identified seven superior modes and, for all of them, the measured and theoretically calculated resonant frequencies did not differ by more than 6%. Knowing the possible excited modes in the antenna and the relations for their resonant frequencies, one can predict the operating frequencies of a cylindrical DRA excited by a central rectangular slot.

System description of a stepped frequency CW radar for humanitarian demining

A multiple frequency stepped frequency continuous wave radar for detecting flush buried and surface laid antipersonnel landmines was designed and built. The system covers a frequency band from 400 MHz up to 4845 MHz. The range resolution is 3.4 cm and the maximum unambiguous range is 4.3 m. The antenna configuration is a bistatic one with two Archimedean spirals with opposite sense of rotation. The instrumented dynamic range is 40-50 dB per frequency channel. After synthesis of the range profile the dynamic range is significantly higher. The system parameters are aimed at imaging algorithms exploiting the phase evolution over the antenna footprint in particular. One example of the results achieved with and without synthetic aperture radar processing is presented.

Archimedean Spiral Antenna Calibration Procedures to Increase the Downrange Resolution of a SFCW Radar

This paper deals with the calibration procedures of an Archimedean spiral antenna used for a stepped frequency continuous wave radar (SFCW), which works from 400MHz to 4845MHz. Two procedures are investigated, one based on an error-term flow graph for the frequency signal and the second based on a reference metallic plate located at a certain distance from the ground in order to identify the phase dispersion given by the antenna. In the second case, the received signal is passed in time domain by applying an ifft, the multiple reflections are removed and the phase variation due to the time propagation is subtracted. After phase correction, the time domain response as well as the side lobes level is decreased. The antenna system made up of two Archimedean spirals is employed by SFCW radar that operates with a frequency step of 35MHz.

Subsurface imaging using measured near-field antenna footprints

Images of the subsurface are made for the detection of land-mines using a bistatic steppedfrequency continuous-wave spiral-antenna system. While the system moves along the surface, the emitted electromagnetic wavefields are scattered by objects in the subsurface and cause changes in the voltages measured at the receiver. These changes are formulated as a convolution of a sensitivity function and a complex contrast function. Within the Born approximation, this sensitivity function is equal to the inner product of the wavefield emitted by the transmitter and the field from the receiver operating in transmitting mode. For true amplitude imaging purposes, knowledge of the wavefields in the subsurface is needed. Since it is difficult to obtain a model which describes the radiation characteristics accurately, we measure the footprints of the antennae at one level in the near-field region and propagate the emitted wavefields using Huygens’ principle. We use both synthetic and experimental data to localize objects in a homogeneous space. First, we apply time-domain synthetic-aperture-radar (SAR) imaging in its most basic appearance. Next, we apply a single-step inversion algorithm to the data, where we use the measured radiation characteristics of the antenna system. This results in an increase in resolution.We refer to this method as ‘minimized back-propagation’.

Imaging of stepped frequency continuous wave GPR data using the Yule-Walker parametric method

Ground penetrating radar for humanitarian demining has a large bandwidth. It should discriminate small objects against a complex background. The stepped frequency continuous wave radar of IRCTR has a synthesized bandwidth of 4.5 GHz. Classical methods for generating an image of the subsurface include synthetic aperture processing for cross-range resolution and inverse Fourier transform to generate the range profile. This paper presents a novel scheme, based on Yule-Walker auto regression. This method is widely known as an estimator of the power spectral density based on time domain signals. But here it is interpreted as an estimator of the range profile based on frequency domain signals. The result is a significantly improved detectability and range-resolution if the signal is not too weak. The paper presents some results on the effect of the order of the Yule Walker model and a comparison with the MUSIC method

BNT advanced ceramics for slot coupled dielectric resonator antennas

Barium neodymium titanate (BNT) material was prepared and characterized. The very high dielectric constant, high Qxf product and controllable temperature coefficient recommend the BNT material for microwave applications. Slot coupled antennas with BNT resonators are investigated. The resonator shape effect on the antenna bandwidth is discussed.

Null steering arrays

Located at the interface between space propagation and electronic devices, the antenna is a very important element of any communications and radar system. Especially in radar systems there are many situations when one has to cancel some unwanted signals. These signals may be caused by accidental interference or by jamming. This papers deals with the possibilities of cancelling unwanted signals by steering nulls of the pattern to the jamming bearing.

Procedures to improve the performances of a Sfcw radar used for landmine detection

In the last decades a lot of research has been done to develop different types of ground penetrating radars to be used for antipersonnel landmine detection, archaeological investigations, search for underground pipes etc. They have to provide a resolution comparable with the dimension of the smallest landmine and the needed penetration for each application. To comply with high resolution requirement a ultra wide band system is required and it can be manufactured to work either in time domain or in frequency domain. In this paper two procedures to improve downrange and cross range resolution of stepped frequency continuous wave (SFCW) radar are described. The two procedures are applied to experimental data collected with SFCW radar and the improvements in the radar performances are described in the paper.

Fractal Antennas for Wireless Communications

Design, simulation fabrication and measurement of fractal antennas for wireless communications are presented in this paper Two types of geometries, i.e. Sierpinski gasket and Koch curve, were investigated. The proposed antennas are compact, can operate for multiple frequency bands and exhibit small mutual coupling.