Thorsten W. Hertel

On the dispersive properties of the conical spiral antenna and its use for pulsed radiation

Conical spiral antennas can have an input impedance and gain that are nearly frequency independent over a wide bandwidth. However, these antennas normally have dispersive properties that produce significant distortion when they are used to radiate a pulse. We examine this dispersion in detail and the possibility of compensating for the dispersion so that the antenna can be used for pulse radiation. First, a simple, qualitative model for this antenna is described. This model provides physical insight into the causes for the dispersion. Next, the antenna is examined using an accurate, full electromagnetic analysis done with the finite-difference time-domain method. Results from this analysis support the conclusions reached with the simple model and provide additional insight into the dispersion. Finally, an approach for compensating for the dispersion in the antenna is described, and the interesting features of the pulse radiated from this antenna, after compensation, are discussed.

Analysis and design of two-arm conical spiral antennas

The two-arm, conical spiral antenna is analyzed using the finite-difference time-domain method. The analysis is validated by comparison with measurements of the input impedance and the realized gain. A parametric study is performed with the analysis, and the results from the study are used to produce new design graphs for this antenna. These graphs supplement and extend the existing, mainly empirical, design base for this antenna. Two resistive terminations, intended to improve the low-frequency performance, are examined. One is a termination formed from two lumped resistors, and the other is a new termination formed from a thin disc of resistive material. These terminations are shown to improve the front-to-back ratio and axial ratio for the antenna.

On the convergence of common FDTD feed models for antennas

The finite-difference time-domain (FDTD) method is routinely used to calculate the input admittance/impedance of simple antennas. The value of the input admittance/impedance depends on the level of discretization used in the method, and should converge to a final value as the discretization becomes finer. In this paper, the level of discretization necessary for convergence is studied using two common feed models: the hard-source feed and the transmission-line feed. First, the simplest and most naive methods for introducing the voltage and the current in these models are considered, and the results for the admittance are shown not to converge. Next, improved methods for introducing the voltage and current in these models are constructed. The results for the admittance are then shown to converge, and guidelines are offered for the level of discretization needed for convergence. In addition, two general problems associated with the computation of the admittance are discussed: the agreement between admittances computed with different simple feed models, and the agreement between these admittances and measurements.

Cable-current effects of miniature UWB antennas

Since the FCC legalized the operation of ultra-wideband (UWB) enabled products in the US, many novel, very broadband antenna designs have emerged. The paper discusses the cable-current effects of a common, miniature printed monopole antenna with a shaped ground plane and of a miniature printed dipole antenna. A CPW-fed pseudo monopole antenna is used to demonstrate that cable currents affect the low frequency performance when the antenna is fed by a coaxial line. A UWB dipole antenna is presented with significantly reduced cable-current effects. It is fed with a CPW line which transitions rapidly into a CPS line. The surface area of the PCBs for the dipole and the pseudo monopole are about the same. It is shown that using a balun and avoiding a finite-sized ground plane reduce the effect of cable currents on the antenna performance.

The insulated linear antenna-revisited

In the past, the insulated linear antenna has been analyzed with an approximate transmission-line theory. The range of validity for this theory has not been established. In this paper, the finite-difference time-domain (FDTD) method is used to analyze the insulated monopole antenna. The validity of the FDTD analysis is established by comparison of results with accurate measurements for a variety of antennas. The FDTD analysis is then used to determine the accuracy of the approximate transmission-line theory. Graphs are provided to quantify the errors in the approximate theory as functions of the geometry and the electrical properties of the monopole antenna.

On the transient radiation of energy from simple current distributions and linear antennas

Smith (1998) examined the radiation from two simple filamentary current distributions: traveling-wave and uniform. The radiated or far-zone electric field was computed for an excitation that was a Gaussian pulse in time. Two interpretations for the origin of the radiation were presented, based on the far-field results. The present article continues this investigation; however, the emphasis is on an examination of the near field and the related transport of energy away from the current filament. We examine traveling-wave and standing-wave current distributions, because these distributions are frequently used to model practical antennas. Exact analytical expressions are presented for the electric and magnetic fields of the assumed, filamentary current distributions when the excitation is a general function of time. For the filamentary distributions, the current and charge are confined to a line (a line source). There is no radius associated with the filament. The expressions for the fields apply in both the near and far zones, and are used to determine the Poynting vector. For an excitation that is a Gaussian pulse in time, exact analytical expressions are obtained for the energy leaving the filament per unit time per unit length, the total energy leaving the filament per unit length, and the total energy radiated. Graphical results based on these expressions are used to study the energy transport from the filamentary current distributions. The results for the standing-wave current distribution are compared with those from an accurate analysis of a pulse-excited, cylindrical monopole antenna, performed using the FDTD method.

The conical spiral antenna over the ground

The two-arm, conical spiral antenna (CSA) is a well-known antenna that has several desirable features when isolated in free space, such as uniform input impedance, gain, and circular polarization over a broad frequency range. This antenna is investigated when placed directly over the ground with its axis normal to the surface of the ground. In this configuration, the CSA may be useful for applications in which signals must be transmitted into the ground and/or received from within the ground, such as communication links to underground tunnels and ground-penetrating radars. First, the performance of the CSA in free space is reviewed, and then qualitative arguments based on the geometrical and electrical properties of the isolated antenna are used to predict the performance when this antenna is placed over the ground. Next, results from a complete analysis of the CSA over the ground, performed with the finite-difference time-domain method, are used to quantitatively verify these predictions. The paper ends with an illustrative example in which the CSA is used in a monostatic ground-penetrating radar.

Analysis and design of conical spiral antennas using the FDTD method

Conical spiral antennas were first developed by Dyson (1959, 1965) using empirical results. No analytical/numerical analysis has been performed to date for the full model with expanding arm width. The only numerical analysis has been by the method of moments (MoM) for the thin-wire conical spiral antenna which is not truly frequency independent. The objective of this paper is to analyze this antenna using the finite-difference time-domain (FDTD) method. A parametric study is used to improve upon the available design base. Various design parameters of the conical spiral antenna are provided as a function of the antenna geometry to supplement Dysons experimental results.

Pulse radiation from an insulated antenna: an analog of Cherenkov radiation from a moving charged particle

Cherenkov radiation arises when a charged particle moves with a constant velocity that is greater than the speed of light in the surrounding medium. This radiation has distinctive characteristics. Near the charge, the electric field is most intense along a conical surface with apex at the charge-the Mach cone. In the far field, the radiation occurs predominantly in one direction-at the Cherenkov angle. An insulated antenna consists of a metallic cylindrical conductor covered by a concentric sheath of dielectric. In use, this antenna is embedded in a medium whose permittivity is often much greater than the permittivity of the insulation. When the antenna is excited by a pulse of voltage, a pulse of charge appears to travel along its length. The apparent velocity of this charge is close to the speed of light in the insulation, which, because of the difference in the permittivities, is greater than the speed of light in the surrounding medium. Thus, the radiation from the pulse excited, insulated antenna should be analogous to Cherenkov radiation from the moving charged particle. In this paper, the pulse-excited, traveling-wave insulated linear antenna is accurately analyzed using the finite-difference time-domain (FDTD) method. Results are obtained for the charge on the conductor, the near field, and the far field. These results show the striking similarity of the radiation from this antenna to Cherenkov radiation from the moving charge.

Pulse radiation from an insulated linear antenna: an analogue of Cherenkov radiation from a moving charge

We analyze an insulated, traveling-wave linear antenna using the finite-difference time-domain (FDTD) method. The rotationally symmetric computational volume is truncated with the new perfectly matched layer (PML) for cylindrical coordinates (Maloney et al. 1997). The radiated field (far-field) is calculated from the field on a cylindrical surface surrounding the antenna using a near-field to far-field transformation. The FDTD analysis involves no approximations and directly produces results in the time domain.