TEM Horn Research Articles

Ultra‐wideband TEM horn design for ground penetrating impulse radar systems

AbstractThe partial dielectric‐loaded TEM horn (PDTEM) structure, which maintains ultra‐wideband antenna characteristics over a bandwidth ratio greater than 20:1 in order to improve the impulse‐radiation characteristics for ground penetrating radar systems is described in this paper. It is shown that high‐gain and low‐input‐reflection levels over the operational band are attainable for TEM horns with proper dielectric, absorber, and resistive loadings. © 2004 Wiley Periodicals, Inc. Microwave Opt Technol Lett 41: 333–336, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20133

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.

Analysis of conical quasi-TEM horn with a hard corrugated section

There is an interest in using electromagnetic bandgap (EBG) surfaces to realize quasi-TEM waveguides and horns. TEM horns will have a uniform aperture distribution and a corresponding high aperture efficiency and low cross polarization. In the present paper a conical quasi-TEM horn with a hard-walled section excited in the TE/sub 11/ mode from a regular waveguide is analyzed. The hard wall is formed by longitudinal dielectric-filled corrugations. The analysis is based on a stepwise cylindrical approximation of the conical surfaces with subsequent application of the mode-matching method in combination with the method of factorization. The modal field expansion of the longitudinally corrugated cylindrical sections is found by using the asymptotic boundary condition of the corrugated surface when the corrugation period approaches zero. The indicated approach refines the data for the limiting aperture efficiency corresponding to the open-ended waveguide, and reveals strong influence of the hard section length as well as the interaction between the aperture and throat on the horn performance. It is shown that in the preferable case when the corrugations occupy the whole inner horn surface, it is possible to provide the crosspolar level below -30 dB and aperture efficiency from 85% to 95% in some frequency band up to 20% depending on the horn parameters.

TEM horn optimized for transient radiation

AbstractTheory and measurements are presented to show that careful shaping of antenna metalization can be used to design TEM horn antennas for ultra‐wideband applications involving high‐fidelity impulsive electromagnetic fields. The effect on the radiated pulse, spectral bandwidth, and transfer function is discussed. A numerical time‐domain formulation is described to compute the impulsive radiated far‐field waveform, and some experimental results are included to indicate the usefulness of the technique. The ringing action of the antenna is recognized as the crucial parameter constraining the usefulness of the design. © 2001 John Wiley & Sons, Inc. Microwave Opt Technol Lett 30: 161–164, 2001.

Time-domain simulation and characterisation of TEM horns using a normalised impulse response

A common way of describing antennas in the time domain is by means of their impulse response. When the time-domain antenna equations are expressed in terms of the normalised impulse response (normalised IR), they become very simple to use, because all frequency-dependent antenna characteristics are included in the normalised IR. This paper describes a method for measuring the normalised IR experimentally, using a vector network analyser. The normalised IRs of different air and dielectric-filled TEM horn antennas are compared and discussed. The normalised IR is found to be a powerful tool for simulating antenna behaviour directly in the time domain. Thanks to the introduction of the virtual source, (i.e. an apparent point in the antenna from which the radiated field degrades by a factor 1/r), the time-domain antenna equations can also be used near the TEM horns, although still in the far field of the antenna. Some examples of time-domain simulations and system modelling using the normalised IR are presented. In each example, the simulations are compared with measured data.

Iterative Step-Like Reconstruction of Stratified Dielectric Media from Multifrequency Reflected-Field Data

We describe in this paper a frequency domain microwave method to reconstruct the permittivity profile of layered dielectric half-space. The inversion algorithm is based on the iterative Newton–Kantorovitch procedure applied to the Riccati nonlinear differential equation. To obtain a correction term to the initial profile, the integral equation relating to a small change of the profile function and corresponding change of the reflection coefficient is solved twice, with different systems of expanding functions for the unknown profile. The first set of functions is utilized to determine positions of the reflecting interfaces between layers. The second set represented by the Heaviside step functions provides the profile correction term completing the iteration. The proposed approach is valid for the essentially band-limited input data, that is, when both low- and high-frequency information is not available. This alleviates practical implementation especially from the viewpoint of antenna design, dielectric rods fed by TEM horns are used as transmitting/receiving antennas. Some numerical and experimental examples are presented for the input data in the frequency range of 1–4GHz, which demonstrate validity and robustness of the method for the complicated highly contrasted dielectric profiles.

An ultrawide-bandwidth tapered resistive TEM horn antenna

A novel stable beamwidth, ultrawide-bandwidth low-scattering antenna is presented. This antenna is a modified version of the conducting slotline bowtie hybrid (SBH) antenna with resistive sheets (Rcards) introduced into the guiding structure design. Since the resistive sheets can attenuate creeping wave fields, much smaller rolled edges are needed in the Rcard SBH antenna. Moreover, due to the Rcard's partially transparent nature with respect to electromagnetic fields, the radar cross section (RCS) associated with this new antenna is smaller than the original conducting rolled edge one. By employing an optimization process based on the concept of the genetic algorithm, one can easily design the Rcard SBH antenna to obtain a very stable antenna beamwidth and phase center in both the E- and H-planes across the entire operating frequency bandwidth. Because of its constant beamwidth, low RCS, and broad-band features, the Rcard SBH antenna is an ideal antenna for many applications such as a compact range feed. Calculated and measured results are presented to demonstrate the performance of this new antenna.

A New Type of Ultra-Wide Band Conical Antenna Ridged with a Novel Finline Radiator

A new type of ultra-wide band(UWB) conical antennas ridged by a novel taper finline radiator(TFR) used as radiation and receiving of the short pulse are developed. In which, a new UWB feed structure of a coplanar waveguide to finline and a TFR ridge are used initially. The experiment results show the characteristics of high fidelity, high efficiency, and high gain compared with some common broad-band antennas such as the biconical antennas, V-conical antennas, logarithm periodic antennas, and the other TEM horns.

The multiple sweep method of moments (MSMM) design of wide-band antennas

This paper presents the use of the multiple sweep method of moments (MSMM) in the design of electrically large wide-band antennas. The MSMM is an O(N/sup 2/) recursive method for solving the large matrix equations which arise in the method of moments (MM) analysis of electrically large bodies. Although the MSMM is a frequency-domain solution, it has a time-domain interpretation. The main point of this paper is to show that this time-domain interpretation can be used to identify the location and magnitude of points of current reflections, thus making the MSMM a useful tool for the analysis and design of wide-band antennas. The method is applied to the three-dimensional (3-D) problems of a long dipole antenna with a reactive load in the center of each arm and a TEM horn antenna with a bend.

Design and evaluation of a coaxial transmission line fixture to characterize portland cement concrete

Electrical characterization of portland cement concrete (PCC) is necessary for reliable operation of many electromagnetic nondestructive evaluation (NDE) techniques. Examples of such techniques include ground penetration radar (GPR). For that purpose, PCC specimens were cast, and their complex permittivity and magnetic permeability were measured over a wideband of frequencies (1 MHz to 10 GHz) using both time domain and frequency domain measurement techniques. As part of an ongoing research project, three set-ups to measure the dielectric properties of PCC were designed. The measurements are carried out over a frequency range of 0.1 MHz to 10 GHz. The three different set-ups are parallel plate capacitor (0.1 MHz–40 MHz), coaxial transmission line (0.1 GHz–1 GHz), and TEM horn antenna (0.5 GHz– 10 GHz). This paper discusses the development of the coaxial transmission line fixture. It was designed to allow the evaluation of large PCC specimens and obtain an average effect of PCC components. A time domain reflectometry (TDR) setup was used in the design of the fixture and in testing its performance. A calibration scheme was developed and used to calibrate the fixture. Using a two-port S-parameter model, the complex permittivity and magnetic permeability of the PCC specimens were evaluated by frequency domain measurement. Time domain measurement techniques were based on one-port S-parameter model. Evaluation of the set-up using especially molded PCC specimens showed that curing status can be detected at a lower frequency using the imaginary component of the relative permittivity.

Ultra-wideband source using gallium arsenide photoconductive semiconductor switches

An ultrawide-band (UWB) pulse generator based on high-gain (lock-on mode) gallium arsenide (GaAs) photoconductive semiconductor switches (PCSS's) is presented. Revised PCSS contact design shows improved performance in hold-off field, on-state switch potential, and switching jitter, while reducing the switch volume by 75% compared to previous designs. A compact laser diode module operates at 904 nm and triggers the PCSS at pulse repetition rates (PRR) of up to 2 kHz. The 625 W laser diode output power is found to be sufficient to produce switching jitter of 65 ps rms at a switched field of 80 kV/cm. The PCSS switching jitter is found to have a strong dependence upon the switched field when triggered with the laser diode module. The revised PCSS geometry is easily integrated into a compact, parallel-plate source used to drive a TEM horn impulse-radiating antenna. The radiated field has a rise time of 330 ps and an adjustable low-frequency spectrum.

FDTD simulations of TEM horns and the implications for staircased representations

Two-dimensional (2-D) TEM horns are modeled using the finite-difference time-domain (FDTD) method. The boundary walls are perfect electric conductors and one wall, which does not align with the Cartesian grid, is approximated using a staircased representation. By carefully comparing the FDTD results to those of the analytic solution, one can make conclusions about the coarseness with which a boundary can be represented. It is found that staircasing errors are small when the staircase diagonal (the hypotenuse of the right triangle created by the stairstep) is smaller than half a wavelength at the highest significant frequency in the excitation. This rule-of-thumb is put forward as a necessary condition for the discretization of general problems. Results are also provided for some simple FDTD schemes that are designed to reduce staircasing errors. By using large aspect-ratio cells, a grid can be constructed that satisfies the rule-of-thumb given above. While this approach eliminates general staircasing errors, some errors persist owing to the presence of step discontinuities immediately adjacent to the horn feed. These errors can be further reduced by using a cell-splitting approach. It is shown that the contour path FDTD technique can be used to eliminate nearly all staircasing errors, while some additional improvement is shown to be provided by using a stabilized contour path FDTD approach. Finally, a recently proposed conformal technique that permits simple implementation is shown to provide results comparable with those of the stabilized contour path approach.

Generation of high-power ultrawideband electromagnetic pulses in a system with a coaxial tem horn

A coaxial TEM horn was designed on the basis of results from nonstationary computer modeling using code KARAT. With its high dielectric strength, this antenna is capable of radiating high-power ultrawideband nanosecond pulses. The pulse source used was a compact generator built around a coaxial forming line with a built-in Tesla transformer, which shapes pulses up to 1 GW high at repetition frequencies up to 1 kHz. The amplitude of the pulses on a matched load was 20 kV at a duration of 4 nsec. Returns of ultrawideband signals from objects with simple geometric shapes were studied in laboratory experiments using this radiator.

TEM horn antenna for pulse radiation: An improved design

The finite-difference-time-domain (FDTD) method is used in an interactive mode to improve the performance of a TEM horn antenna for pulse radiation. The new design is shown to yield a small reflected voltage in the feeding transmission line, to radiate a far-zone field in the axial direction that is proportional to the derivative of the incident voltage, and to be fairly directive. Experimental results are in very good agreement with those predicted by the theory. © 1996 John Wiley & Sons, Inc.

TEM horn antenna for pulse radiation: An improved design

The finite-difference-time-domain (FDTD) method is used in an interactive mode to improve the performance of a TEM horn antenna for pulse radiation. The new design is shown to yield a small reflected voltage in the feeding transmission line, to radiate a far-zone field in the axial direction that is proportional to the derivative of the incident voltage, and to be fairly directive. Experimental results are in very good agreement with those predicted by the theory. © 1996 John Wiley & Sons, Inc.

Accurate analysis of TEM horn antennas for pulse radiation

In the past, various approximate theoretical models have been used to analyze TEM horn antennas. Because of the limitations of these approximate models, there has been, to date, only qualitative agreement of measurements for TEM horn antennas with the predictions of the theories. The finite-difference time-domain (FDTD) method is used to accurately analyze TEM horn antennas for pulse radiation. First, the metallic triangular-plate TEM horn antenna is considered. Computed results for the reflected voltage in the feeding transmission line and the time-varying radiated electric field are shown to be in very good agreement with new experimental measurements. Graphs of the electric field in the space surrounding the antenna (magnitude of field plotted on a color scale) are used to give a physical insight into the process of radiation. Next, the method is used to analyze two TEM horns that were previously designed for pulse radiation. The geometry and electrical properties of these antennas are more complicated than for the metallic, triangular-plate horn. One has shaped metallic plates with a resistive termination at the open end; the other has plates whose resistance varies continuously along their length. The computed results for these antennas are compared with previously made experimental measurements.

Comparison of two FDTD near-field to near-fieldtransformations applied to pulsed antenna problems

Recently, a time-domain, near-field to near-field transformation, based on the Stratton-Chu representation, was presented for use with the FDTD method. In the Letter, an alternative near-field to near-field transformation based on the Kirchhoff representation is described. Savings of both computation time and computer memory are achieved using the Kirchhoff formulation. Comparisons of the savings attained are illustrated through an example, the application of the transformation to a TEM horn antenna for pulse radiation.

GPR applications for bridge deck evaluations in Finland : Saarenketo, T; Soderquist, M K Non-destructive Testing in Civil Engineering, Liverpool (United Kingdom), 14–16 Apr 1993. Vol. 1, pp 211–226. Edited by J H Bungey. The British Institute of NDT (1993)

The usefulness of Ground Penetrating Radar (GPR) for examining areas of deterioration in bridge decks was tested in Finland, with the aim of assessing whether the method can be used to estimate pavement thickness, deterioration of the protective concrete and slab and damage to the water proofing. Some tests were also performed to locate cracks in the slab. The surveys were performed using ground coupled dipole bow-tie antennae with frequencies of 900 MHz, 1.0 GHz and 1.5 GHz, and 2.5 GHz TEM horn antennae were also tested. The measured GPR data were compared with information obtained from drill cores and damage mapping during repair of the bridges. The results show that GPR can be used to map areas of deterioration in bridge decks. The radar data image of the weathered areas varies depending on whether the damaged concrete is dry or saturated with water, but the level of deterioration is still difficult to determine from radar data alone in all bridges and some reference drill cores have to be taken. A pavement thickness of more than 40 mm can be determined with sufficient accuracy and failures between the concrete and asphalt pavement and in water proofing are also detectable. The problems with the method as used in Finland have concerned the manual interpretation of the GPR data, which is slow and time-consuming, but efforts are under way to develop software for automatic interpretation. (A) For the covering abstract see IRRD 861706.

Ground penetrating radar applications for bridge deck evaluations in Finland : Saarenketo, T.; Soderqvist, M.K. Insight, Vol. 36, No. 7, pp. 496–501 (Jul. 1994)

The usefulness of Ground Penetrating Radar (GPR) for examining areas of deterioration in bridge decks was tested in Finland, with the aim of assessing whether the method can be used to estimate pavement thickness, deterioration of the protective concrete and slab and damage to the water-proofing. Some tests were also performed to locate cracks in the slab. The surveys were performed using ground-coupled dipole «bow-tie» antennae with frequencies of 900 MHz, 1.0 GHz and 1.5 GHz, and 2.5 GHz TEM horn antennae were also tested. The measured GPR data were compared with information obtained from drill cores and damage mapping during repair of the bridges. The results show that GPR can be used to map areas of deterioration in bridge decks

Calculation of the characteristic impedance of TEM horn antennas using the conformal mapping approach

A new conformal mapping technique for determining the characteristic impedance of TEM horn antennas is presented. The original work by Carrel [1958], on noncoplanar fin antennas of this type, predicts incorrect impedance values which do not agree with experimental results. The present paper discusses fully the source of the error in Carrel and details the successive mappings involved in the new approach. Verification of the method is provided by experimental results and by comparison with an alternative numerical technique. >