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This letter proposes a simple design approach to gain stable omnidirectional radiation pattern in printed ultrawideband (UWB) antennas. Monitoring of the current distributions in different frequencies shows that a combination of standing and traveling modes exists on the antenna. By determining the dominant mode at higher and lower frequencies, the corresponding optimum size of antenna that guarantees a stable omnidirectional radiation pattern and a good return loss is obtained. Using the proposed method, two compact antennas with circular ground and miniaturized fractal ground are designed. Simulation and measurement results show a good impedance matching and an excellent omnidirectional radiation pattern for proposed antennas.Index Terms-Monopole antenna, omnidirectional, pattern stability factor (PSF), radiation mechanism, ultrawideband (UWB).

accurate estimation of the unknown wall parameters is of great importance in through wall radars (TWR). Time delay only measurement is an efficient method to derive real part of dielectric constant (ε') and thickness (d) of wall. However the imaginary part of dielectric constant (ε'') in lossy media cannot be obtained by this method. To derive ε'' and also loss tangent, we upgrade this method using the amplitude of echoes, in addition to their time delays. Since the loss in the medium does not affect the time delays, ε' and d is achieved precisely and using them loss components can be computed. In this paper, permittivity, thickness and loss tangent of a plaster slab is measured using the proposed method.Index Terms-Ultra wide band (UWB), through wall radar, wall parameters estimation.

Time delay estimation is of great significance in multipath propagation to recover overlapped signals and identify the channel characteristics. However, achieving a high accuracy in this purpose may pose many problems in ultra-wideband (UWB) applications. In UWB systems, capturing a signal with high sampling rates cannot readily be done; hence classical methods for time delay estimation substantially lose their precision. To overcome this challenge, the authors incorporate a robust estimation approach and supplementary sampling process in a unified algorithm to retrieve time delays from signals with low sampling rates. Toward that pursuit, a model based least squares estimator is proposed as the main approach to calculate time delays and a modified method based on multiple signal classification (MUSIC) is also presented for comparison aim. Then, the authors have developed the algorithm by embedding two additional pre-processing steps of under sampling and interpolation to achieve a higher sampling rate and a better resolution. To show the high accuracy of work, root mean square error is computed in different values of time delay. Simulation and experiment results show the considerably higher precision of the proposed algorithm in comparison with presented MUSIC type method and also previously proposed methods in literature.

The frequencies lying between 300 Hz to 3 kHz have been designated as Ultra Low Frequency (ULF) with corresponding wavelengths from 1000 Km to 100 Km. Although ULF has very low bandwidth it is very reliable, penetrating and difficult to jam which makes it a great choice for communication in underwater and underground environments. Small and portable ULF antennas within a diameter of 1 meter would operate under an electrical length on the order of 10−4 to 10−6 wavelengths in free space, making them very inefficient because of fundamental limits on radiation from electrically small antennas. To overcome this problem, Mechanical Antennas or ‘Mechtennas’ for Ultra Low Frequency Communications have been proposed recently. For efficient generation of ULF radiation, we propose a portable electromechanical system called a Magnetic Pendulum Array (MPA). A proof of concept demonstration of the system at 1.03 kHz is presented. The theory and experimental results demonstrate that such a system can achieve a significantly higher quality factor than conventional coils and thus order of magnitude higher transmission efficiency. The concept can be easily scaled to the ULF range of frequencies.