Electromagnetic Horn Research Articles

Classical axis-displaced dual-reflector antennas for omnidirectional coverage

The aim of this work is to discuss the synthesis and performance of classical dual-reflector antennas suited for an omnidirectional coverage. The reflector arrangements are axially symmetric with surfaces of revolution generated by axis-displaced conic sections, established from geometrical-optics (GO) standpoints to achieve omnidirectional radiation characteristics. Closed-form equations are derived for the design of all possible reflector configurations. The vector GO aperture field is also obtained, yielding an approximate analysis by the aperture method. Some pertinent geometrical characteristics and efficiency curves are then presented and discussed for several antenna configurations fed by transverse electromagnetic coaxial horns (for vertical polarization). A practical antenna design is conducted and analyzed by the method-of-moments technique, demonstrating the accuracy of the efficiency analysis yield by the aperture method for moderately large antenna apertures.

A compact former of high-power bipolar subnanosecond pulses

The system comprises either a nanosecond solid-state operating switch (SOS)-generator (see, e.g., Rukin, 1995) or a RADAN pulsed power source (see, e.g., Mesyats et al., 1993) that charges resonantly a short pulse-forming line (PFL) through a decoupling inductor, an active converter, a matched load, and several built-in voltage and current probes. The active converter comprises an additional 1-ns PFL charged through a peaking spark gap (SG) and a secondary decoupling inductor by the first PFL, and two SGs (chopping and peaking gap). The peaking SG and active converter are set in one body and operate in N/sub 2/ at pressure up to 6 MPa. Driven by RADAN, the converter generates bipolar subnanosecond pulses having peak-to-peak amplitude up to 270 kV across 46.6-/spl Omega/ load. The pulsewidth of each half wave of bipolar pulses on full-width at half-maximum is 280 ps, with the risetime of the first half wave of about 160 ps. The SGs' gaps can be regulated without system depressurizing (a technique adopted from Shpak et al., 1996). Circuit analysis accounting for the distributed character of the components and numerous parasitic parameters is presented. Mapping of main SG parameters (gap distance and pressure) was performed. Waveforms probed at different locations of the pulser systems, from the SOS-generator and from the RADAN to the load, are shown. The bipolar subnanosecond pulses had very stable rise, while the fall jitter was around 50 ps. The experimental results are in fair agreement with the simulation. Pulsers were tested with a transverse electromagnetic horn antenna. An effective potential of 910 kV was obtained.

Frequency Dependence of the Soil Electromagnetic Properties Derived from Ground-Penetrating Radar Signal Inversion

The accuracy at which the subsurface electromagnetic properties can be identified from full wave inversion of ground penetrating radar (GPR) signals relies on the appropriateness of the model describing their frequency dependence. In this paper, we focus on the characterization of the frequency dependence of the dielectric permittivity and electric conductivity of a sandy soil subject to different water contents from inversion of GPR measurements. Based on previous studies of Lambot et al. the methodology relies on an ultrawide band (UWB) stepped-frequency continuous-wave (SFCW) radar combined with an off-ground monostatic transverse electromagnetic (TEM) horn antenna. Forward modeling of the radar signal is based on linear system transfer functions for describing the antenna, and on the exact solution of Maxwellrsquos equations for wave propagation in a horizontally multilayered medium representing the subsurface. Model inversion, formulated by the classical least-squares problem, is carried out iteratively using advanced global optimization techniques. The frequency dependence of the electromagnetic properties of the sandy soil is characterized by performing inversions of the radar signal in different and subsequent limited frequency bands, in which the electromagnetic parameters are assumed to be constant. We observed that over the entire frequency band considered in this study (1–3 GHz), the dielectric permittivity of the sand remains constant with frequency, whatever the water content is. In contrast, the electric conductivity increases significantly from 1GHz to 3 GHz, and this effect increases with water content. The frequency dependence of the electric conductivity may be adequately described using a simple linear relationship. This approach is advantageous since it limits the number of parameters to be optimized in the inverse modeling procedure.

Electromagnetic Inversion of GPR Signals and Subsequent Hydrodynamic Inversion to Estimate Effective Vadose Zone Hydraulic Properties

We combine electromagnetic inversion of ground penetrating radar (GPR) signals with hydrodynamic inverse modeling to identify the effective soil hydraulic properties of a sand in laboratory conditions. Ground penetrating radar provides soil moisture time series that are subsequently used as input in the hydrodynamic inverse procedure. The technique relies on an ultrawide band (UWB) stepped frequency continuous wave (SFCW) radar combined with an off-ground monostatic transverse electromagnetic (TEM) horn antenna. Ground penetrating radar signal forward modeling is based on the exact solution of the three-dimensional Maxwell equations for describing free wave propagation and on linear systems in series and parallel for describing wave propagation in the antenna. Water flow in the sand is described by the one-dimensional Richards equation using the Mualem–van Genuchten parameterization. Both model inversions are formulated by the classical least-squares problem and are performed iteratively using advanced global optimization techniques. Compared with time domain reflectometry (TDR), results demonstrated the appropriateness of the GPR integrated approach to measure soil moisture remotely. In particular, the approach was found to be less sensitive to the inherent small-scale heterogeneities. Hydrodynamic inversion of soil moisture data led to hydraulic parameters agreeing reasonably well with direct measurements. The observed discrepancies were attributed to the different characterization scales and samples. The overall integrated approach offers great promise to map the effective hydraulic properties of the shallow subsurface at a high spatial resolution.

Modeling of ground-penetrating Radar for accurate characterization of subsurface electric properties

The possibility to estimate accurately the subsurface electric properties from ground-penetrating radar (GPR) signals using inverse modeling is obstructed by the appropriateness of the forward model describing the GPR subsurface system. In this paper, we improved the recently developed approach of Lambot et al. whose success relies on a stepped-frequency continuous-wave (SFCW) radar combined with an off-ground monostatic transverse electromagnetic horn antenna. This radar configuration enables realistic and efficient forward modeling. We included in the initial model: 1) the multiple reflections occurring between the antenna and the soil surface using a positive feedback loop in the antenna block diagram and 2) the frequency dependence of the electric properties using a local linear approximation of the Debye model. The model was validated in laboratory conditions on a tank filled with a two-layered sand subject to different water contents. Results showed remarkable agreement between the measured and modeled Green's functions. Model inversion for the dielectric permittivity further demonstrated the accuracy of the method. Inversion for the electric conductivity led to less satisfactory results. However, a sensitivity analysis demonstrated the good stability properties of the inverse solution and put forward the necessity to reduce the remaining clutter by a factor 10. This may partly be achieved through a better characterization of the antenna transfer functions and by performing measurements in an environment without close extraneous scatterers.

Design of an exponentially‐tapered TEM horn antenna for the wide broadband communication

AbstractA transverse electromagnetic (TEM) horn antenna is designed and realized using exponentially tapered conductor flares for wireless communications. This exponentially tapered structure of the flares decreases the impedance mismatching a lot and reduces the size and cost of the TEM horn. The VSWR of the antenna is less than 2 in the frequency band of 74.4 MHz to 1.19 GHz, and its gain is more than 8.3 dBi over 200 MHz. Those performances show that the designed TEM horn antenna has wide broadband characteristics. © 2004 Wiley Periodicals, Inc. Microwave Opt Technol Lett 40: 531–534, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20025

A design study for the basic TEM horn antenna

In this paper, a design study was performed for the basic TEM horn antenna. In the theoretical model, care was taken to avoid the problems associated with the use of gap sources when calculating the reflection coefficient and input impedance of the antenna. All numerical calculations were made with a program based on the method of moments, and the accuracy of the numerical calculations was established with measured results for selected antennas. The transverse electromagnetic (TEM) horn antenna is a popular broadband antenna. The basic antenna has a simple design consisting of two triangular plates and a feeding structure. The horn is described completely by only three variables: /spl alpha/, /spl beta/, and s. /spl alpha/ is the angular width of each plate, /spl beta/ is the angular separation between the two plates, and s is the length, measured from the drive point to the corner of the plate. The geometry of the basic TEM horn antenna does not require a particular choice of a feeding method.

On the Characteristic Impedance of the TEM Horn Antenna

Some time ago, Carrel presented an analytical formula, based on conformal mapping, for the characteristic impedance of the transverse electromagnetic horn antenna . Later, results from this formula were shown to be in error, and Lambert et al. offered a new analytical formula, also based on conformal mapping . This formula also contains an error that is easily corrected. Independently, Yang and Lee offered an analytical formula . In this paper, we compare these analytical formulas to a direct numerical solution. Finally, we compare the "microstrip approximation" for the characteristic impedance to the exact solution. Graphs are presented for the characteristic impedance versus the angles of the horn that should be useful for the purpose of design.

Four‐element TEM horn array for radiating ultra‐wideband electromagnetic pulses

AbstractIn this paper, ultra‐wideband arrays using interconnected transverse electromagnetic (TEM) horns are studied. The paper analyzes the radiation characteristics of four‐element arrays using the finite‐difference time‐domain (FDTD) method. The computed results for arrays are compared with those for the single TEM horn and two‐element array. Some properties of the four‐element array, such as the input impedance, voltage standing‐wave ratio, and energy pattern, are presented in this paper. © 2001 John Wiley & Sons, Inc. Microwave Opt Technol Lett 31: 190–194, 2001.

Scanning and impedance properties of TEM horn arrays for transient radiation

A general concept for ultrawide-band array design using interconnected transverse electromagnetic (TEM) horns is described. At high frequencies (wavelength small compared to unit cell dimensions), the mutual coupling between elements is small and, consequently, the input impedance depends only on the lattice dimensions and not on either scan angle or frequency. At low frequencies (wavelength large compared to unit cell dimensions), the mutual coupling is purposefully made large, by interconnecting the elements to maximize the low-frequency performance. This paper presents the results of analyses using a periodic hybrid finite-element approach to calculate input impedance and scanning performance of generic TEM horn arrays. The limiting case, the planar bicone, is shown to have the frequency-independent property of a self-complementary antenna, making it a useful case for establishing the effects of feed region geometry. Although it radiates bidirectionally, it has the interesting property that its broadside-scan frequency response in the array environment is absolutely flat up to the grating lobe onset limit. A TEM horn array is more unidirectional, but as a consequence suffers both oscillatory variations in the input impedance with frequency and increased limits on minimum achievable rise time.

An appreciation of J.C. Bose's pioneering work in millimeter waves

The pioneering work in the area of millimeter waves, performed by J. C. Bose, a physicist from Calcutta, India, during 1894-1900, is reviewed and appraised. Various measurement techniques and circuit components, developed by him a hundred years ago, are still being used. The development of the electromagnetic horn, the point-contact detector, and the galena (semiconductor) detector of electromagnetic waves are attributed to the original research of J.C. Bose.

Calculation of surface impedance effects on transient antenna radiation

A numerical time domain approach to radiation and scattering problems is introduced for surfaces of arbitrary conductivity. This approach is based upon a method of moments solution of the electric field integral equation. The problem is posed as a second‐kind integral equation in discrete time for those regions of the surface with finite conductivity; it is shown that this formulation possesses desirable stability properties. The numerical method introduced is used to determine the far field radiation of both directive and nondirective transient antennas excited by temporally compact, ultrawideband pulses when the antenna surfaces are subject to a Wu‐King surface impedance profile. Such loads are shown to suppress the “late time ringing” resulting from reflected surface currents without significantly affecting the initial radiated waveform. In the case of nondirective transient radiators, such as the bicone or bow tie antenna, the presence of a surface loading has no effect on the peak radiated field strength. In the case of a directive transient radiator, such as a transverse electromagnetic horn, the introduction of a surface impedance reduces the radiated field strength somewhat. The difference in the physical behavior of these two classes of antenna is accounted for.

Study on the phase center of electromagnetic horns

The characteristics of the phase centers of electromagnetic horns having nearly symmetric pattern are presented analytically. It is proved that the phase center on the ϕ=45° plane coincides with the average phase center, and if a single plane phase center is used to define the horn phase center approximately, the ϕ=45° plane phase center will be the best choice.

Time-domain antenna characterizations

A set of time-domain characterizations that can efficiently describe wireband antennas is proposed. The experimentally measured responses of transverse electromagnetic horn antennas are used to evaluate the utility of these characterizations. Comparisons are made between the antennas' frequency-domain response and their time-domain characterizations. The comparisons show that the time-domain characterizations can provide significant insight into an antenna's behavior as well as providing a means to accurately compare two or more different antennas. >

Radiation patterns of few-moded horns and condensing lightpipes

We analyse condensing lightpipes of the form often used to couple an infrared detector to a relatively slow optical system. The response in a given direction is expressed in terms of the radiation patterns of the different waveguide modes propagating in the lightpipe and throat section. Such lightpipes are seen to be just a generalization of the electromagnetic horn antennas often used to illuminate radiotelescopes and other reflector antennas. The results of our analysis are compared with ray-tracing results for horns operating in the short wavelength geometrical optics limit: excellent agreement is obtained. This new analysis is equally valid in the long wavelength case pertaining for most FIR and submm detector systems, where only a few spatial modes propagate and the radiation pattern cannot be adequately determined by any other means.

ESR spectrometers using electromagnetic horns

A new ESR spectrometer using electromagnetic horns has been developed for practical applications to biological or material sciences. Based on theoretical considerations, the sensitivity, frequency dependence, and other characteristics of transmission-, reflection-, and induction-type spectrometers are examined by using standard samples. It is shown that (1) the ESR signals of the samples with a volume up to 25 ml can be measured independent of the sample shape, (2) the microwave frequency for the ESR observation can be freely selected in the range available, and (3) the sensitivity of the reflection-type spectrometer is 1.62 × 10 14 spins/gauss. As a special application of the spectrometers developed, ferromagnetic resonance of single-crystal magnetic bubble devices has been measured.

A Derivation for the Noise Temperature of a Horn-type Noise Standard

Noise sources consisting of an electromagnetic horn aimed at an absorbing material have been in use for many years. A satisfactory derivation of the noise temperature for such a configuration has been missing, however, preventing the use of this horn-type noise source as a primary reference standard. The derivation described in this paper models the various noise emitters within the source well enough to provide an accurate estimate of the noise temperature and a complete error analysis.

Correction to "UAT analysis of E-Plane near and far-field patterns of electromagnetic horn antennas"

Correction to "UAT analysis of E-Plane near and far-field patterns of electromagnetic horn antennas"

Transmission of electromagnetic waves in hollow tubes of metal

Electromagnetic energy may be transmitted through the inside of hollow tubes of metal, provided the frequency is greater than a certain critical value; this value is inversely proportional to the tube radius and to the dielectric coefficient for the tube interior. Calculations and measurements of the more important characteristics of this new kind of transmission system have been made, and the conditions for minimum attenuation obtained Terminal devices for connecting a hollow pipe system to a biconductor system and others, in the form of horns, for directly radiating radio waves, have been developed; these electromagnetic horns may also be fed with ordinary coaxial lines. Certain types of terminals act as sharply resonant hollow tube elements. Several independent communication channels may be established within a single pipe line by utilizing distinct types of waves for each channel in a unique kind of multiplex operation. A section of a hollow tube may be used as a high-pass filter. Although presupposing adequate technique for the generation and utilization of the shortest radio waves, this new system possesses several features, among which are a minimum dielectric loss, substantially perfect shielding, and a simplicity of structure.

UAT analysis of<tex>E</tex>-plane near and far-field patterns of electromagnetic horn antennas

The uniform asymptotic theory (UAT) of diffraction has been applied to arrive at expressions valid for both the near as well as far-field patterns of antenna with edges, namely an electromagnetic horn. It is found that the theoretically computed results agree very well with experiment. The agreement is found to be better than that reported by earlier authors in the far sidelobe and backlobe regions. The same theory is valid for near field as well with the pertinent values of radial distance where the uniform theory of diffraction (UTD) requires a slope diffraction correction.