Order Waveguide Modes Research Articles

Analysis of longitudinal slots in rectangular waveguides

An accurate computation of the admittance characteristics of longitudinal shunt slots in a rectangular waveguide is presented. A moment method with entire basis functions is used for solving the electric field in the slot. The waveguide wall thickness is accounted for by introducing higher order waveguide modes in the short waveguide connecting the inner and outer regions. Particular attention is given to the resonant length of the slot, which is a critical parameter in the design of waveguide slot array antennas. Computed resonant lengths compare very well with measured results.

Tunable RF-excited CO<inf>2</inf>waveguide laser with variable guide width

The resonator mode frequencies and the interline and intraline tuning behavior of CO 2 waveguide lasers should depend strongly on precise guide dimension. We describe a compact grating-tuned RF-excited device with a rectangular Al/Al 2 O 3 waveguide whose width can be varied during operation. It yields ≃ 50 mW with a continuous tuning range of ≃ 500 MHz on stronger lines; more interestingly, its line selectivity does seem to depend on guide width, as we predict. In particular, the losses of unwanted lines (and thus the tuning range within a desired line) are very sensitive to waveguide width, because the resonator mode associated with an unwanted line may have significant higher order waveguide mode content. This suggests that variable-width waveguides offer one solution to the problem of line-hopping, without prejudice to the power or transverse mode quality obtainable on the desired line.

K-Band Tracking System for Domestic Satellite Communication System

System design and operational results of a tracking system for a K-band Earth station antenna with a diameter of 11.5 m are discussed. Monopulse tracking technique using higher order waveguide modes is utilized. An investigation was conducted to clarify the rainfall effect and the effect of oscillator phase noise on the tracking receiver. An autotrack system is to be manufactured on the basis of these studies. Experiments were performed using the medium capacity communications satellite. No cross coupling was seen in the pull-in patterns. Sufficiently good results were obtained in tracking accuracy: less than 0.005° (peak value) under normal conditions, and less than 0.01° under an average wind velocity of more than 20 m/s.

Parallel end-butt coupling for optical integrated circuits

The method of parallel end-butt coupling has been used to couple GaAs laser diodes to Ta (2)O(5) thin film waveguides. Theoretical caluclations predict that a coupling efficiency into the lowest order waveguide mode of 90% is achievable if the thicknesses of the waveguide (t(g)) and the laser light emitting layer (t(L)) are equal. Experimentally, efficiencies as high as 45.1% have been measured for laser and waveguide combinations with t(g)/t(L) = 0.34. The tolerance to misalignment of the laser and waveguide has been theoretically and experimentally evaluated.

Finite-aperture waveguide-laser resonators

A general theory of finite-aperture waveguide-laser resonators is developed which represents the external reflectors by matrices which couple linearly polarized waveguide modes having the same azimuthal symmetry. The theory allows the determination of resonator efficiency, resonator frequencies, and laser near- and far-field patterns. Computations of the coupling loss for the fundamental waveguide mode as a function of mirror curvature, separation, and aperture are in good agreement with recent infinite-aperture calculations in the limit of apertures and indicate three low-loss configurations: 1) radius of curvature mirrors close to the guide; 2) large radius of curvature mirrors centered at the guide entrance: and 3) generally smaller curvature mirrors separated by half their curvature from the guide entrance. The importance of higher order waveguide modes in determining laser output power and far-field patterns is demonstrated experimentally and compared to theoretical predictions. Design guidelines for the construction of high-efficiency CO 2 , CO, and He-Ne waveguide-laser resonators are summarized in tabular form.

Coupling losses in hollow waveguide laser resonators

Hollow waveguide gas lasers of the type described by Smith have some inherent loss in coupling radiation from the guide into free space and back into the guide. This paper calculates that loss for the EH 11 lowest order waveguide mode as a function of mirror position and mirror radius. It is shown that some mirror positions and radii are optimum, in that they provide low coupling loss and are relatively insensitive to mirror position.

Modes of Propagation in a Coaxial Waveguide with Lossless Reactive Guiding Surfaces

An analysis of the modes that can propagate in a coaxial waveguide with Iossless reactive guiding surfaces is presented. The cases where both the surfaces are either inductive or capacitive and the case where one of the surfaces is capacitive and the other is inductive are discussed. The results show that, in general, there are two surface waves and an infinite number of waveguide modes. Whereas all the waveguide modes show the cut off phenomenon, the surface waves may either propagate down to zero frequency or get transformed into the lowest order waveguide mode at certain critical frequencies determined by the structure parameters.

VLF propagation across discontinuous daytime-to nighttime transitions in anisotropic terrestrial waveguide

Discontinuity effects are analyzed after referring the ionospheric surface impedance of daytime and nighttime waveguide portions to the same common altitude h . The properties of the waveguide junction are determined by field matching, and the resulting linear equations are rearranged in a form that permits a sequential determination of transmission and reflection coefficients. Simple dominant mode approximations of the transmission and reflection coefficients are also developed. Field calculations of reciprocal geometries illustrate the consistency of the formulation and reflection coefficients are found to be of negligible magnitudes. The discontinuous surface impedance tends to excite higher order waveguide modes of larger amplitudes than a ground based source in homogeneous waveguide.

Hybrid-Mode Analysis of Microstrip by Finite-Element Methods

A numerical analysis is presented in terms of finite elements of hybrid-mode propagation in closed microstrip. Two modes with zero frequency cutoff are described; one is a quasi-TEM mode and the other a surface mode. Also investigated is a third mode which corresponds to the lowest order waveguide mode in the absence of the strip.

An Accurate Finite-Difference Method for Higher Order Waveguide Modes

The study of new waveguide shapes requires an accurate knowledge of their higher order modes for bandwidth consideration, waveguide discontinuity analysis, and multimode launching and propagation studies. The finite-difference method solved by successive overrelaxation is a very accurate and general technique for dominant mode solution. The method makes minimum demand on computer store, the number of storage locations required being equal to the order of the matrix defining the system of linear equations used. Convergence criteria require that this matrix be positive semidefinite. For modes higher than the dominant, the method fails to converge as this condition is violated. Solution is obtained by redefining the problem such that the matrix is positive semidefinite for all modes. An algorithm is described which produces one row at a time of the new matrix, as required for successive overrelaxation, and thus reserves almost all computer store for the eigenvector. The eigenvector elements give field potentials at discrete points in the guide cross section. Iteration for higher order modes is successfully applied with all the computational benefits realized by previous finite-difference schemes for the dominant mode. Examples studied are the rectangular, circular, asymmetric ridge, and lunar guides. Results for higher order cutoff wave numbers are usually accurate to within 0.1 percent.

Mutual Coupling in Infinite Planar Arrays of Rectangular Waveguide Horns

The radiation admittance and effective power transmission coefficient are derived by a field matching method for arbitrary scan of an infinite array of open rectangular waveguides. Higher order waveguide modes, as well as higher order modes in the free space cell above the array, are included in the field matching problem. The convergence of the solution was studied, and the number of external free space and internal waveguide modes necessary for an adequate answer are given. Numerical results are obtained and compared with previous theory and with experiment for rectangular grids. Comparison is also made with experiment on a triangular grid which exhibited an anomalous notch in the element pattern. The experimental results in both cases seem to substantiate the theory, at least to the extent that could be expected from the small test arrays that were used for the measurements. The comparison indicates that the method of analysis includes all of the necessary features required for the study of mutual coupling effects in infinite arrays of rectangular waveguides having thick walls.

A High Efficiency Dual Frequency Multimode Monopulse Antenna Feed System

Higher order waveguide mode radiation can be incorporated into antenna feed systems to provide significant improvements in antenna performance with full polarization diversity. Multimode techniques may provide full monopulse capability from a single aperture and may simultaneously be used to broaden the monopulse reference pattern. The inherent adaptability of the basic multimode feed to dual frequency operation permits the introduction of independent dominant mode excitation at a lower frequency band. Multimode beam broadening techniques are then used to equalize the high frequency patterns with those in the low frequency band to provide a high efficiency common aperture dual frequency feed.