共查询到20条相似文献,搜索用时 15 毫秒
1.
Expressions for directivity as a function of spacing, scan angle, and current distribution are presented for uniformly spaced linear arrays subject to the assumption that the element radiation intensity function of each radiator can be expressed in either of the following forms:(1 - sin^{2} theta cos^{2} Phi)^{P} orsin^{2P} theta sin^{2Q} Phi whereP andQ are zero or positive integers. By proper choice ofP andQ , these expressions can be used to approximate the element radiation intensity function associated with a linear array of isotropic radiators, parallel short dipoles, collinear short dipoles, and planar equiangular spiral antennas. Analytical results are given for a number of cases, and graphical results are presented for directivity in the direction of scan angle for a linear array of radiators with a uniform current distribution and an element radiation intensity given by the expressionsin^{2} theta sin^{2} Phi in the regiony geq 0 and zero in the regiony < 0 . 相似文献
2.
Rahmat-Samii Y. Parhami P. Mittra R. 《Antennas and Propagation, IEEE Transactions on》1978,26(6):789-796
The Fresnel reflection coefficient technique is employed to establish anE -field integral equation for the antenna current. A resistive loading of the formLambda(x) = Lambda_{0}/(1 - |x|/L) is used to load the antenna. An optimization technique is discussed for determining the value of critical loadingLambda_{0}^{c} , which enforces a traveling wave current on the antenna. Results are given for the critical loading parameters, antenna currents, input impedances and radiation patterns versus different antenna dimensions and ground permittivities and conductivities. Some representative time-domain results for such loaded antennas are also included. 相似文献
3.
Null synthesis by phase control for antenna arrays 总被引:5,自引:0,他引:5
The author presents a method for forming nulls in prescribed directions in the radiation pattern P0 of an antenna array by phase-only control. The method modifies the excitation phases of P0 , allowing the construction of a sequence of patterns {Pn } having increasingly deeper nulls. A solution is provided by terminating the sequence at a suitable step 相似文献
4.
《Antennas and Propagation, IEEE Transactions on》2008,56(9):2920-2928
5.
《IEEE transactions on circuits and systems. I, Regular papers》2009,56(5):911-919
6.
Sarkar T. Nebat J. Weiner D. Jain V. 《Antennas and Propagation, IEEE Transactions on》1980,28(6):928-933
A noniterative method for approximating signals by a linear combination of exponentials is presented. Although the technique results in a suboptimal approximation, the continuous dependence of the suboptimal exponentssim{s}_{i} on the integral square errorepsilon is such that lim(epsilon = 0) sim{s}_{i} rightarrow {s}_{i} , the best least squares exponents. The method is also useful for system identification, where the system is modeled by a black box and one has access only to the input and output terminals. A technique is demonstrated for finding the multiple poles of a system along with the residues at the poles when the system output to a known input is given. Advantages of the method are natural insensitivity to noise in the data and a capability for approximately determining signal order. Representative computations are made of the poles from the transient response of a conducting pipe tested at the ATHAMAS-I EMP simulator. 相似文献
7.
Dipole antenna in space-time periodic media 总被引:2,自引:0,他引:2
The problem of dipole radiation in sinusoidally spacetime periodic media is studied and solved. The space-time periodicity can be considered as due to a strong pump wave and is expressed as a traveling-wave type change in the dielectric constant or the plasma density (i.e.,epsilon(z,t) = epsilon_{0}epsilon_{r}[1 + epsilon_{1} cos (Kz - Omega t)] andN(z,t) = N_{0}[1 + N_{1} cos (Kz - Omega t)]) . The solution also covers the limit case of a sinusoidally stratified medium (Omega = 0 ). The solution is formulated in a matrix form, such that the basic results and diagrams apply, with minor changes, to the different cases studied: electric and magnetic dipole in a dielectric, plasma, and uniaxial plasma. The wave-vector diagram is used extensively in studying and presenting the different properties of the solution: caustics, effect of the disturbance (pump wave) motion, harmonics, radiation outside the allowed cone in a uniaxial plasma. Many dipole radiation patterns are given and their features explained physically. Finally, our solution and results are extended to the generally space-time periodic media whereepsilon(z,t) andN(z,t) behave aseta(z,t) = eta_{0}[1 + eta_{1}f(Kz - Omega t)] wheref(xi) is any periodic function. 相似文献
8.
The input admittance of a small thin-wire circular loop antenna, driven by a slice generator, immersed in a dissipative medium, is considered. It is found that the solution given by Storer for the loop antenna in a lossless medium can be carded over readily by replacingzeta_{0} byzeta , andk_{0} byk . The numerical values of the normalized input conductance and input susceptance of a small loop antenna, namelybeta b leq 0.3 ,Omega = 10 , are calculated. It is to be noted that the input susceptance is practically independent ofk while the input conductance changes as much as seventeen times in this special case. 相似文献
9.
TheB^{3} Sigma-_{u} rightarrow X^{3}Sigma-_{g} system of S2 has many of the characteristics required of a tunable laser: a broad fluorescence spectrum and a Franck-Condon shift between theB andX states. In this paper, theupsilon' = 4 level of theB^{3} Sigma-_{u} rightarrow X^{3}Sigma-_{g} system of S2 is pumped by the absorption of the 308 nm XeCl excimer radiation. Rare gas-S2 collisions redistribute the population among theupsilon' levels, and the subsequent emission contains intense bands throughout much of the neat UV and visible region of the spectrum. Broad-band gain is reported on two blue-green bands (2,17) and (3,18) populated by rare gas relaxation. Absorption is reported on the near UV bandsupsilon' = 0 rightarrow upsilon" rightarrow 6,7,8 where the lower levels are filled by the transient population cascading down theX state manifold. A fluorescence study is presented which examines the potential advantages and limitations of this system. 相似文献
10.
《IEEE transactions on information theory / Professional Technical Group on Information Theory》1973,19(6):769-772
In this, the first part of a two-part paper, we establish a theorem concerning the entropy of a certain sequence of binary random variables. In the sequel we will apply this result to the solution of three problems in multi-user communication, two of which have been open for some time. Specifically we show the following. LetX andY be binary randomn -vectors, which are the input and output, respectively, of a binary symmetric channel with "crossover" probabilityp_0 . LetH{X} andH{ Y} be the entropies ofX andY , respectively. Then begin{equation} begin{split} frac{1}{n} H{X} geq h(alpha_0), qquad 0 leq alpha_0 &leq 1, Rightarrow \ qquad qquad &qquad frac{1}{n}H{Y} geq h(alpha_0(1 - p_0) + (1 - alpha_0)p_0) end{split} end{equation} whereh(lambda) = -lambda log lambda - (1 - lambda) log(l - lambda), 0 leq lambda leq 1 . 相似文献
11.
An in-situ technique to estimate the following parameters of a phased-array antenna is described: 1) the relative array-element excitation voltages, 2) the array-element tuning phases, and 3) the RF phase shifts at the array elements. This technique has several significant features. First, it involves the use of two auxiliary antennas. One is a remote continuous wave (CW) source directed at the phased-array antenna. The other is a passive antenna mounted close to the phased-array antenna. Its output is used to produce a reference phase for phase measurements. Second, it contains a technique to reduce the errors in phase estimates. Third, it takes note that beam steering uses phase sums of the form (phi_{k} + beta_{kl} ), wherephi_{k} is the tuning phase for thek th array elemenet andbeta_{kl} , is an RF phase shift of the array element, and pays special attention to reduce the errors associated with the estimates of these sums. Fourth, it assumes the use of a reasonably stable and strong CW source of commerically available quality. No other assumptions are made. Experimental results obtained with a 295-elementS -band space-fed phased-array antenna are given. 相似文献
12.
Lin X. Q. Bao D. Ma H. F. Cui T. J. 《Antennas and Propagation, IEEE Transactions on》2010,58(2):375-380
13.
Elkhatib T. A. Kachorovskii V. Y. Stillman W. J. Veksler D. B. Salama K. N. Zhang X.-C. Shur M. S. 《Microwave Theory and Techniques》2010,58(2):331-339
14.
TheN -level uniform quantizer on[-c,c] plus the assignment ofy_{0}deg = -(a _{s}+ c)/2 andy_{N+1}deg = (a_{s}+ c)/2 to signal values falling in the saturation regions[-a_{s},- c) and (c,a_{s}] , respectively, is shown to be the minimax(N + 2) -level quantizer with a nonsaturating input range[-c,c] . The performance criterion considered is the mean weighted quantization error and the input signals are only required to be amplitude bounded bypm a_{s} wherea_{s} > c > 0 . The worst case input signal marginal probability distributions are shown to be discrete. From the derivation of this result, the minimax error can be computed. An example is given which illustrates the performance of the minimax quantizer for several input ranges against different input signal probability distributions. 相似文献
15.
《Electron Device Letters, IEEE》1985,6(12):620-622
The absolute magnitude of the thermal drain current fluctuations and the associated effective thermal noise coefficient of 1-µm gate-length MESFET's have been measured under various bias conditions. At low drain-source voltages the magnitude of current fluctuations are in good agreement with the thermal noise theory which is based on the gradual channel approximation. However, under normal operating conditions (V_{ds} geq 1.5 V,V_{gs} approx 0 ), we find for the thermal drain noise currentimin{d}max{2} approx 1 - 2 times 10^{-22} A2/Hz with a noise coefficientP approx 0.1 in disagreement with the commonly used, theoretically predicted value P = 1.1. Our results are qualitatively consistent with a more comprehensive FET noise theory which properly takes into account high-field effects. 相似文献
16.
An analytical expression for the aperture field is obtained from an integral equation formulation and a subsequent use of a quasistatic approach in which the dominant part of the solution can be extracted from canonical problems. Unlike the conventional approach of replacing the aperture field by equivalent dipoles, our method retains the basic features of the propagating mode in the coaxial region, and the radiation characteristic of the coaxial cylinder. An equivalent network representation of a small aperture is then derived. The network equivalence of a symmetrical aperture with its axes oriented in the direction of the incident field is shown to consist of a lumped shunt capacitanceC_{b} and a series impedanceZ composed of an inductive elementL_{b} , a capacitive elementC_{T} and the input admittanceY_{binfty} of the external antenna, all in parallel. 相似文献
17.
DC and Transient Performance of 4H-SiC Double-Implant MOSFETs 总被引:1,自引:0,他引:1
《Electron Devices, IEEE Transactions on》2008,55(8):1824-1829
18.
The current distribution and input admittance of an infinitely long cylindrical antenna driven by a slice generator and immersed in an anisotropic plasma are investigated. The applied dc magnetic field is along the axis of the antenna. By superimposing the characteristic waves guided along the antenna, the current solution is obtained in the form of a one-dimensional integral, which is examined both analytically and numerically. WhenK_{perp} > 0 , the magnitude of the current decays slowly with the distance from the source, and its phase is nearly linear with a "propagation constant" equal tosqrt{K_{perp}}k_{0} for an antenna with very small radius. WhenK_{perp} < 0 , the magnitude of the current decreases rapidly away from the source, and its phase is no longer linear. The input admittance decreases with the radius of the antenna. 相似文献
19.
A new type of detector array is described. By means of a suitably designed metallic network, many detector elements (each individually small compared to wavelength) are assembled into an impedance-matched termination for radiation incident normally on the plane of the device. Residual reactance is tuned out by means of a movable backshort. An array of 400 bismuth-film microbolometers with a total area of 1 cm2has been tested at 215 GHz. A coupling efficiency of approximately 60 percent was observed. The detector has aD* of4 times 10^{8} cm . Hz1/2/W at room temperature with response time on the order of2 times 10^{-7} s. Similar arrays of Schottky and SIS diodes can probably be constructed. 相似文献
20.