EMI considerations in selecting heat-sink-thermal-gasket materials

Specific design criteria are proposed to mitigate radiated emissions from a resonant enclosure excited by a heat sink acting as a microstrip patch antenna source. In this particular application, the EMI mechanism is assumed to be due to coupling from the dominant TM₀₁₀ ^x mode to one or more resonant modes associated with the enclosure dimensions. The enclosure is then presumed to radiate, at the enclosure resonance frequencies, through one or more apertures, slots, or seams. The EMI-reduction strategy consists of shifting the resonant frequency of the dominant-patch antenna mode by dielectrically loading the patch antenna with thermal-gasket material having a specified electric permittivity. Specific formulas and graphs are presented showing how to select the electric permittivity of the thermal-gasket material in order to obtain a given frequency shift. A comparison of experimental measurements with the predictions of the design criteria indicates that frequency shifts of up to approximately three times the bandwidth of the patch resonance can be predicted with reasonable accuracy. In at least two different commercial products that we are aware of, changing the electrically insulating heat sink gasket materials has solved specific radiated EMI problems     

Measurement of electromagnetic fields near a monopole antennaexcited by a pulse

To observe transient radiation fields, a technique for reconstructing electromagnetic (EM) waveforms using the complex antenna factor (CAF) have been developed. However, the CAF is originally defined assuming plane wave incidence, while the waveforms are measured in a vicinity of the radiating source. In order to examine the reconstruction technique using the CAF in the near-field measurements, the EM fields radiated from a monopole antenna excited by pulsed input voltage was reconstructed and compared with calculated results. For the geometry of the experiment, the waveforms of the reconstructed and calculated EM fields have shown good agreement. Therefore, the reconstruction technique using the CAF can be used for similar dimensions or at a greater distance when sufficient sensitivity is provided     

Optimal Radiated Waveforms From an Arbitrary UWB Antenna

Solutions are presented for the optimal electric field waveforms radiated by an arbitrary ultrawideband (UWB) antenna. Optimization criteria include maximization of the electric field amplitude at a particular time and location, or maximization of energy density over a specified time interval at a particular location. Assuming bandpass signals, constraints are placed on the total radiated energy, the Q of the antenna, and the size of the antenna. The solution is developed using a spherical mode expansion of the fields radiated by an arbitrary antenna enclosed by a spherical mathematical surface, and optimized using variational methods. A closed-form result is obtained for the case of amplitude maximization, while an integral equation must be solved numerically for the case of energy maximization in a time interval. An interesting result from these solutions is that the shapes of the optimal radiated field waveforms are largely independent of the size of the antenna. The solutions also indicate that the antenna characteristics that provide optimum field amplitude or energy in the transient case are identical to those associated with maximum gain in the CW case.     

Simultaneous Electromagnetic Measurement Using a Parallel Modulated Probe Array

A simultaneous measurement method using a parallel modulated probe array is proposed to measure the electromagnetic field radiated by antennas at different locations simultaneously. Each modulated probe is excited by a local signal with different frequency, so that the received intermediate frequency (IF) signal contains different frequency components which indicate the relative magnitude of the radio-frequency signal at positions of the probes, and all the IF components are measured simultaneously by a broadband microwave receiver. The effect of the local signal on the received IF signal and interaction effect between the probe array and the measured antenna are investigated. The accuracy of the method is shown by measuring the antenna radiation pattern. A 3D antenna radiation measurement system based on the present method is also developed. The performance of the system is demonstrated by measuring the radiation efficiency of antennas.     

Transient response of the duoconical monopole

Large area radiators such as the duoconical monopole are less frequency sensitive than thin antennas. The dispersive nature of antennas is important especially when they are used to transmit video pulses. This paper presents the numerical results obtained for the duoconical monopole under pulsed excitation. Results include the current distribution on the monopole at different instants of time, the radiated waveforms in different directions, and the waveforms of transmitted and reflected signals on the feeding coaxial line. Numerical results were obtained by applying the inverse Fourier transform to the steady-state time harmonic current distributions and radiation patterns. The above results are then used to find a simple incident voltage waveform that will radiate a "pulse" of "good" shape. The radiated field in different directions are then presented for different parameters of the incident double exponential voltage waveform.     

Short cylindrical antennas with enhanced radiation or high directivity

The feasibility of enhancing the radiated power or improving the directivity of a short cylindrical antenna by double impedance loading is investigated. An approximate solution for the current on a doubly loaded short antenna is developed, and typical current dlstributions, impedances, and radiation patterns of antennas appropriately loaded to implement enhanced radiation or high directivity are presented. Significant improvements in radiated power or directivity can be achieved with optimum impedance loadings. Theoretical predictions are verified by the results of an experimental study.     

探地雷达阻性加载天线的应用研究

阻性加载技术可以有效拓展天线带宽、降低时域振铃.该文以双纽线构成平面印制天线的辐射臂, 分析阻性加载对其频域和时域特性的影响.通过数值仿真, 讨论了天线在两种阻性加载方式下阻抗带宽和时域波形的变化.设计了一款指数型微带转平行双线的巴伦, 并对巴伦和天线进行了加工及测试.测量结果表明, 天线时域波形拖尾振铃小于10%;在0.8~6 GHz频带内, 天线单元回波损耗小于-10 dB.将天线与雷达收发系统集成后, 进行地下目标探测实验.雷达探测结果成像清晰、浅层目标分辨率较高.该天线具有超宽带、低振铃、紧支撑等特点, 可满足便携式探地雷达的工程化应用需求.     

High repetition rate LC oscillator

LC oscillators have been built which can produce multikilowatt RF pulses in the megahertz frequency range with repetition rates of tens of kilohertz. The L and C for these oscillators can be determined from the frequency requirement and the high-Q requirements. The high repetition rates are achieved using a high-pressure spark-gap switch together with a dc to ac inverter power supply. Closely spaced antenna elements can be used to increase the number of cycles in the radiated waveforms (radiated Q).     

Analysis of injection-locked antenna array including mutual coupling effects

In this paper, the injection locking performance analysis of dipole antenna array with each element loaded with a two-terminal oscillator is presented. The analysis is based on the nonlinear model of oscillator and the linear model of antenna array considering mutual coupling effects. The locking range of injection signal and the array radiated power are obtained by solving an equivalent multiport network. In general, the solutions include stable and unstable solutions. The Routh-Hurwitz stability criterion is then applied to remove the unstable solutions. Numerical results show that the array performance such as frequency locking range and radiated power by taking into account the array mutual coupling effects is quite different from that of an isolated antenna element. In addition, the influence of antenna element spacing upon array locking parameters in this paper is found to be consistent with other existing theories.     

脉冲激励的长方形发射天线的分析与波形优化

在脉冲激励天线中，脉冲波形对于辐射功率有着重要的影响。对长方形行波天线的激励电流脉冲波形与其辐射功率的关系进行了分析。提出了激励电流的优化问题，使得天线在给定方向上的辐射能量最大化。并且给出了优化问题的解法和几种参数下的优化波形。通过具体的数值计算，与其他参考文献中使用的波形相比，优化波形在主辐射方向上可以辐射出更大的能量。     

Radiation Characteristics of Traveling-Wave Antennas Excited by Nonsinusoidal Currents

In this paper, the idea about traveling-wave antennas excited by nonsinusoidal currents for high-resolution exploration is advanced. First the analytic expressions of the radiation waveforms in the far zone produced by a single transient of current traveling along a straight wire are derived in time domain. Then the radiation characteristics of traveling-wave antennas excited by nonsinusoidal current, such as the mean-power pattern functions and the directivity, are given, and the space waveform control and radiation efficiency are discussed. Based on previous results, the radiation waveforms of dipole antennas excited by pulse current are obtained from time- domain analysis. In a practical example, comparing these waveforms with corresponding waveforms obtained from frequency-domain analysis and measured data in [4] and [13], good agreements are obtained in various radiation directions. Some errors in [14] are also pointed out and corrected.     

Analysis of Radiation from a Traveling-Wave Current and Comparison with the Standing-Wave Sinusoidal Current Filament

The traveling-wave current (TWC) has served as one kind of "canonical" current distribution for simulating the behavior of a wire antenna, as has the standing-wave, sinusoidal current filament (SCF). Besides yielding closed-form solutions for their far-field patterns and radiated powers, the sinusoidal current filament and traveling-wave current can approximate the current on a wire antenna that is appropriately excited. The traveling-wave current's power distributions, as obtained using the induced EMF method and the FARS (far-field analysis of radiation sources) method, together with similar results for the sinusoidal current filament, are discussed in this brief note     

Analysis of terahertz waveforms measured by photoconductive andelectrooptic sampling

Terahertz (THz) waveform measurements by photoconductive (PC) sampling and free-space electrooptic sampling (FS-EOS) are analyzed and quantitatively compared. Our data suggest that a short dipole antenna used in a PC receiver contributes a flat frequency response when used without a substrate lens and a jω response when used with a substrate lens, for the specific THz frequency range and optical system investigated in our experiments. These findings are explained using results from basic antenna theory. Experiments testing our theory for a variety of THz waveforms (obtained by using different THz emitters and simple as well as shaped optical excitation pulses) and for different carrier lifetimes are also presented. Finally, we demonstrate near-field effects in the PC sampling measurements of broad-band THz waveforms and explore the evolution of THz radiation from the near field into the far field     

Theory of a corner-driven loop antenna immersed in a warm plasma

A linearized hydrodynamic theory and potential function technique arc used to formulate the theory of a corner-driven loop antenna immersed in a warm plasma. The theory explains some of the experimental observations obtained from impedance measurement of a loop antenna on the Ariel 3 satellite. The far-zone fields and the normalized radiated power for different antenna sizes are calculated. The results suggest that much of the radiated energy is radiated in longitudinal plasma waves     

JOLT: a highly directive, very intensive, impulse-like radiator

Ultrawideband (UWB) systems that radiate very high-level transient waveforms and exhibit operating bandwidths of over two decades are now in demand for a number of applications. Such systems are known to radiate impulse-like waveforms with rise times around 100 ps and peak electric field values of tens of kilovolts per meter. Such waveforms, if properly radiated, will exhibit an operating spectrum of over two decades, making them ideal for applications such as concealed object detection, countermine, transient radar, and communications. In this paper, we describe a large, high-voltage transient system built at the Air Force Research Laboratory, Kirtland AFB, NM, from 1997 to 1999. The pulsed power system centers around a very compact resonant transformer capable of generating over 1 MV at a pulse-repetition frequency of /spl sim/ 600 Hz. This is switched, via an integrated transfer capacitor and an oil peaking switch onto an 85-/spl Omega/ half-impulse radiating antenna. This unique system will deliver a far radiated field with a full-width at half-maximum on the order of 100 ps, and a field-range product (rE/sub far/) of /spl sim/ 5.3 MV, exceeding all previously reported results by a factor of several.     

Symmetric electromagnetic field of a circular loop antenna over the conductive cone of finite length

The diffraction properties of the field of circular loop antenna over the conic surface of finite length have been investigated. The relationships of the radiated power as a function of the antenna position on the symmetry axis of the cone were established. The directional patterns and field distribution in the near-field zone were obtained for different geometrical parameters of the diffraction system. The results obtained were compared with similar relationships for semi-infinite cones.     

Transient response of the infinite cylindrical antenna and scatterer

The transient response of an infinite cylindrical antenna and scatterer is obtained by time gating a time-dependent electric-field integral equation for thin wires of finite-length. The driving point current and broadside radiated field approximate the driving point voltage in the antenna case, while the current excited in the scattering case approximates the time integral of the incident field. This approximate integral relation is used to estimate currents excited on cylindrical structures without employing the complete integral equation solution procedure.     

Aperture-Sparsity Analysis of Ultrawideband Two-Dimensional Focused Array

We investigate the effects of aperture sparsity on the focusing performance and the angular-resolution capability of a two-dimensional focused array antenna excited by ultrawideband (UWB) impulse waveforms. The UWB-focusing array is characterized by a planar square aperture and a design parameter referred to as array spatial bandwidth. Spatial bandwidth is a function of the number of array elements, inter-element spacing, and frequency bandwidth. Performance analysis is carried out by generating computer plots of three-dimensional and two-dimensional antenna patterns for different values of array spatial bandwidth that hold for large aperture sparsity and large aperture density. The antenna patterns are peak-amplitude pattern, peak-power pattern, and energy pattern, whose narrow beamwidth and low sidelobe level are robust against aperture sparsity that may be caused by removed or failed elements. The half-power beamwidth (HPBW) of the antenna patterns, the focal distance, and the far-field distance of the UWB-focused array are expressed in terms of array spatial bandwidth. Computer simulation results show that UWB-focused-array beamforming based on impulse waveforms achieves efficient focusing of the radiation energy in the radiation-near-field region and beyond, and yields improvement in focusing performance and angular resolution for increased values of array spatial bandwidth. Such practical advantages are achieved without encountering grating lobes, large sidelobe level, or distortion of the radiation beam pattern that often limit the performance of the conventional narrowband phased array antennas.      

A new ultrawide-bandwidth dielectric-rod antenna for ground-penetrating radar applications

A new ultrawide-bandwidth dielectric-rod antenna is presented with its application in detecting shallow targets, such as antipersonnel (AP) mines. The lowest hybrid mode is launched and guided along a circular-dielectric waveguide. The end of the waveguide is tapered to a point where electromagnetic waves are radiated out with field behavior similar to that radiated from a Hertzian dipole in the forward direction. The low antenna clutter and weak antenna-ground interaction are two unique features. Its near-field radiation properties are investigated by directly probing the fields and by numerical simulation with a three-dimensional finite-difference time-domain technique. Both measurement and numerical simulation results are presented for the detection of buried AP mines using a prototype dielectric-rod antenna operated at a frequency range from 1 to 6 GHz.     

Quadratic antenna systems and noise excited antennas

A quadratic receiving antenna system is defined as a multiport antenna whose outputs are passed through square-law detectors, or, in pair, through correlators (product detectors). These detected signals are then linearly combined with different weights to give the output or response of the receiving system. A reciprocity relation is derived which relates the electromagnetic field radiated by a multiport transmitting antenna excited by quasi-monochromatic noise sources to the response of a quadratic receiving antenna system. The noise sources exciting the transmitting antenna may be independent or they may be correlated. In general, the radiated field is partially polarized. The weights used in combining the signals from the square-law detectors and the correlators in the associated receiving system are determined by the intensities of, and the correlations between, the noise sources exciting the transmitting antennas. Practical use has been made of the derived reciprocity relation. The radiation properties of noise-excited multiport antennas have been determined by measurements made on the associated quadratic receiving system.