Mode Conversion in Tapered Waveguides At and Near Cutoff

The coupling coefficient between the TE/sub 11/ mode and the TM/sub 11/ mode in tapered circular waveguides is derived, and at cutoff frequency it tends to approach an infinity of the order of 0/sup -1/4/. It is surprising to discover that the corresponding coupling coefficient between the TE/sub 10/ mode and the TM/sub 12/ mode in tapered rectangular waveguides approaches instead a zero of the order of 0/sup 1/4/ at cutoff frequency. Accordingly, for the modes concerned, the choice of using circular or square waveguides as tapers for transition at and near cutoff frequency is significant in reducing mode conversion level. At and near cutoff frequency a "synthesized" square taper is better in that it is shorter than a "synthesized" circular taper for the same mode conversion levels. On the other hand, for frequencies far away from cutoff the choice is insignificant. Design procedures for "synthesized" waveguide tapers at and near cutoff are presented, and the results of measurements are in agreement with the theoretical calculations.     

An Analysis of a Hybrid-Mode in a Twisted Rectangular Waveguide

Analytic expressions of electromagnetic fields for the dominant hybrid-mode in a twisted rectangular waveguide are obtained. The fields exactly satisfy the boundary conditions on the guide walls in a helicoidal shape. By expanding these expressions for the fields in terms of the eigenfunctions of a straight waveguide, the hybrid-mode is found to be composed of a fundamental TE/sub 10/ -mode component, accompanied with TE/sub 01/, TM/sub 21/,TE/sub 21/, and TE/sub 03/ modes, as successive higher order components. The result of the modal power calculation reveals that there exists a frequency at which the transmitting power carried in the cross-polarized TE/sub 01/-mode component just vanishes. As a limiting case of the twisted waveguide, fields in a twisted strip line are discussed also, and the existence of a propeller-like equiphase surface is shown.     

On the Mode and Square Correspondence Between Circular Multimode Tapered Waveguides

In an axially straight multimode circular waveguide taper excited with a pure TE/sup /spl circle// /sub 11/ dominant mode, the first and only converted mode at and near cutoff is the TM/sup/spl circle// /sub 11/ mode. It is shown that in an axially straight multimode square waveguide taper excited with a pure TE/sup/spl square// /sub 10/ dominant mode, the TM/sup/spl square// /sub 12/ mode corresponding to the TM/sup/spl circle// /sub 11/ mode in circular case is not the only first converted mode at and near cutoff. The overall behavior or coupling mechanism of waveguides is similar whether the waveguide is rectangular, square, circular, or elliptical: i.e., the overall coupling coefficient at cutoff of a converted mode or modes approaches an ininfinity of the order 0/sup -1/4/.     

Mode Coupling And Power Transfer In A Coaxial Sector Waveguide With A Sector Angle Taper

We report a theoretical study of mode coupling and power transfer in a coaxial sector taper. The power transferred from the desired TE /sub 01/ mode into other propagating modes is calculated as a function of taper length and operating frequency. Power transfer via mode coupling involves at least three other modes: TE/sub 21/, TE/sub22/, and TM/sub21/. Power transfer as a function of final sector angle is also shown. At sector angles greater than 180° the taper is highly over-moded. This type of waveguide taper is utilized to feed a wide-band input coupler for gyrotron traveling wave amplifiers.     

Resonant Cavity Type Mode Transducer

This paper describes the operating principle and the properties of a resonant cavity type mode transducer which was newly devised. The theoretical equations necessary for designing the mode transducer from a TE/sub 10/ mode of a rectangular waveguide to an arbitrary mode of a circular waveguide have been derived, and a design method using the coupling parameters is discussed. The experiments were made for the rectangular TE/sub 10/-circular TE/sub 01/, mode transformation in the 50 Gc band. Showing an example (N=1), the transfer loss, input SWR and mode purity were 1.34 dB, 1.13, and 95 percent (power contents), respectively, at the resonant frequency of 50 Gc/s. The 3 dB bandwidth of the transfer loss was 83 Mc/s at the constant cavity length, but it can be made much larger if the cavity length is adjusted according to the frequency change. This mode transducer is unique in that various modes can be excited purely in the circular guide by merely varying the cavity length.     

Scattering at Circular-to-Rectangular Waveguide Junctions

A formally exact solution is given for the problem of scattering at a circular-to-rectangular waveguide junction and at a thick diaphragm, with a centered circular aperture, in a rectangular waveguide. The method uses normal TE and TM mode expansions of the waveguide fields and traditional mode matching of the transverse electric and magnetic fields at the junction boundary. Exact closed-form expressions are obtained for the electric field mode-matching coefficients which couple the TE(TM) modes in the rectangular guide to the TE(TM) and TM(TE) modes in the circular guide. Numerical results are presented for the case of TE/sub 10/ mode propagation in the larger rectangular guide with all other modes cutoff. Convergent numerical results for the equivalent shunt susceptances of such junctions are obtained when about 12 modes (eight TE and four TM) are retained in the circular waveguide or in the circular aperture of the diaphragm. The results are graphically compared with formulas and curves due to the quasi-static theory of Bethe and the variational theory given in the Waveguide Handbook [2].     

Wide Band Metallic Waveguide With In-Line Dielectric Rods

Conventional rectangular metallic waveguides are seldom used at frequencies higher than twice the cutoff frequency because of higher mode propagation. Single-mode propagation is available for a metallic waveguide with arrayed dielectric rods at the center of the waveguide in the frequency under twice the cutoff frequency region using the TE₂₀ mode, and in the frequency over twice the cutoff frequency region using the TE2o mode. If the metallic waveguide and dielectric loss tangent are assumed to be WR-90 (f_c ap6.55 GHz) and 3times10^-4, respectively, then the attenuation constants are smaller than 0.25 dB/m in the frequency range from 7 to 10 GHz, 15 to 16.5 GHz, and from 17.2 to 21 GHz.     

Effects of Trains on Cutoff Frequency and Field in Rectangular Tunnel As Waveguide

Effects of trains in a rectangular tunnel on the cutoff frequency and field were determined at the range of VHF, UHF, and SHF bands by the finite-element method. According to this study, the tunnel is a transmission channel of high-pass type waveguide. The tunnel and the trains were assumed to be infinitely long and fully conductive. Generally speaking, the trains in the tunnel lowered TE/sub nm/ wave cutoff frequencies and raised TM/sub nm/ cutoff frequencies. Closer monitoring, however, has shown that the above results may be reversed, depending on the conditions. The field is represented by contour lines. Thus, its change is clearly shown by a change in the distribution of the lines caused by the train in the tunnel. Although the train changed field distribution for both TE/sub nm/ and TM/sub nm/ mode, greater changes were usually observed in higher order mode fields.     

Helix Leaky Waveguide

The propagation characteristics of leaky waves in a helix waveguide covered with a slitted cylinder are presented by a method of transverse network representation. The main interest is in helix waveguides with small pitch angles, characterized by a hybrid mode consisting of TE/sub 01/, and a small amount of TM/sub 01/ modes. The leaky wave discussed in this paper may then be regarded as a perturbation of the TM/sub 01/ wave by the slitted cylinder outside the helix. The radiation, metal, and dielectric losses are calculated numerically at a frequency of 50 GHz. The relation between the radiation loss and aperture angle of slit is very different from that of an ordinary leaky waveguide composed of a slitted cylinder without helix, especially when the distance between the helix and shield cylinder is about a quarter of the radial wavelength. The metal and dielectric losses are the same order as radiation loss, however the dielectric loss decreases as the power factor /spl epsiv/"/ /spl epsiv/' increases. The measured total attenuation constant averages about 5 dB/km, almost twice the theoretical value.     

The Channel Waveguide (Correspondence)

A waveguide propagating the TE/sub 10/ mode can carry more power than the normal rectangular waveguide if it has a symmetrically placed channel in the E-plane as shown in fig. 1. The greater height of the channel in the center of the waveguide will allow a higher voltage to be applied before dielectric breakdown occurs. The TE/sub 10/ cutoff wavelength /spl lambda//sub c/ was investigated using the methods of Iashkin and Cohn to find out if the cutoff wavelength of the channel waveguide was equivalent to that of the rectangular waveguide /spl lambda//sub cr/.     

Modal Inversion in Circular Waveguides--- Part I: Theory and Phenomenology

Part I of this work investigates the phenomenon of mode reordering in circular waveguides containing one or more dielectrics. Specifically, conditions are established, be they ranges of dimensions, dielectric constants, or frequency, under which the sequence, TM/sub 01/, TE/sub 01/, TE/sub 11/, and TE/sub 21/ is obtained instead of the conventional TE/sub 11/, TM/sub 01/, TE/sub 21/, and TE/sub 01/. The desired effect is the reversal of the order of propagation between the normally dominant TE/sub 11/ mode and the circularly symmetric low-loss TE/sub 01/. Bandwidth regimes are drawn and the credibility of the dielectric model is examined and found to be satisfactory for the application of the phenomenon to the design of digital ferrite phase shifters operating in the TE/sub 01/ mode.     

E- and H-Plane Bends for High-Power Oversized Rectangular Waveguide

A study is presented of theoretical and experimental results of E- and H-plane bends for high-power oversized rectangular waveguide having cross-section dimensions in the range between 1..5 and 2.5 free space wavelengths. It is expected that waveguides having these dimensions will be able to transmit 50 to 100 kW of average power at X-band without water cooling. The transmission of at least 5.0 MW of peak power at X-band without pressurization is also a design objective. Dimensions for bends having low-mode conversion loss were determined by numerical integration of the coupled transmission line equations. The dominant TE/sub 10//spl square/ mode and four spurious modes were considered in these calculations. The results obtained for both constant curvature and sinusoidally shaped E- and H-plane bends are presented. A compact H-plane constant curvature bend is described for which the ratio of centerline radius to waveguide width is equal to 1.48. The measured mode conversion loss to the TE/sub 20//spl square/, TE/sub 30//spl square/, and TE/sub 40//spl square/ modes for an experimental model having a width equal to 2.25 inches was less than -20 dB in the frequency range from 7.0 to 11.0 Gc/s.     

Precise Design of a Bandpass Filter Using High-Q Dielectric Ring Resonators

A precise design is presented for a bandpass filter constructed by placing TE/sub 01delta/ dielectric ring resonators coaxially in a TE/sub 01/ cutoff circular waveguide. On the basis of a rigorous analysis by the mode- matching technique, the interresonator coupling coefficients are determined accurately from the calculation of two resonant frequencies f/sub sh/ and f/sub op/ when the structurally symmetric plane is short- and open-circuited. For the TE/sub 01delta/ ring resonator,the resonant frequency f/sub 0/, the temperature coefficient tau/sub f/, the unloaded Q(Q/sub u/), and the other resonances are also calculated accurately in a similar way. From the calculations, the optimum dimensions are determined to obtain the maximum Q/sub u/, as F/sub r/ = f/sub r/ /f/sub 0/ is kept constant, where f/sub r/ is the next higher resonant frequency the ring resonator using low-loss ceramics (epsilon/sub r/ = 24.3, tan delta = 5 x 10/sup -5/) has Q/sub u/ = 16800 at 12 GHz and tau/sub f/ = 0.1+-0.5 ppm/° C, while the rod one has Q/sub u/ = 14700. A four-stage Chebyshev filter having ripple of 0.04 dB and equiripple bandwidth of 27.3 MHz at f/sub 0/ =11.958 GHz is fabricated using these resonator; the measured frequency responses agree well with theory. The insertion loss is 0.9 dB, which corresponds to Q/sub u/ = 9800.     

Attenuation characteristics in confocal annular elliptic waveguidesand resonators

The perturbation method is used to obtain the attenuation constant and Q-factor of several TEM, TE, and TM modes in confocal annular elliptic waveguides (CAE-Ws) and confocal annular elliptic resonators (CAE-Rs). Normalized attenuation and Q-factor charts are given for a variety of possible combinations of the focal distance and the eccentricities. Comparisons between the first higher mode in a CAE-W and a coaxial waveguide with the same cutoff frequency and cross-sectional propagating area reveals a lower attenuation in elliptic geometry. Consequently, the Q-factor in a CAE-R is 20%-40% greater than a Q factor for a coaxial resonator with the same volume and resonant frequency     

A High-Power S-Band Filter (Correspondence)

This note describes the measured performance of a high-power S-band "waffleiron" filter. Techniques for designing varying-impedance corrugated waveguide filters in rectangular waveguide have been available for some time [1 ]. If propagation is restricted to the dominant TE/sub 10/ mode, these filters have wide well-matched pass bands and wide high-attenuation stop bands. Unfortunately, the filters may have spurious transmissions in the stop band when power is incident upon them in other modes, such as TE/sub 20/ or TE/sub 30/, if the guide wavelength of these modes at the stop band frequencies is equal to the guide wavelength of the TE/sub 10/ mode at the pass-band frequencies of the filter.     

A mode transformer using fin-line array for spatial power-combiner applications

A new TE/sub 10/-to-TE/sub 20/-mode transformer has been developed on the basis of a fin-line array for waveguide spatial power-combiner applications. The fin-line array is designed such that the input TE/sub 10/ mode is split in space into multiple segments, then subject to different signal paths, which effectively converts the TE/sub 10/ mode into a TE/sub 20/ mode, while eliminating the TE/sub 10/ mode at the output. Our Ku-band (14-15 GHz) design indicates that the proposed mode converter makes it possible to obtain the TE/sub 20/ mode with greater than 22-dB suppression of the TE/sub 10/ mode over the band of interest. This converter was used in the design of a waveguide spatial power amplifier involving four monolithic microwave integrated circuit power chips and an output power of 31 dBm has been obtained with a combining efficiency of 80%. A measurement technique has been also developed to measure the electric-field profile inside the rectangular waveguide. A good agreement between measured and simulated results has been observed, showing an effective suppression of the TE/sub 10/ mode, as well as a good TE/sub 20/-mode formation judging from its amplitude and phase.     

Solution of the Finite Length Septum Problem with Application to Periodic Mode Suppressors

A square waveguide with periodic septums is considered with a view towards determining the parameters of a structure which suppresses three of four possible propagating modes. The analysis is presented in two parts, the first of which is concerned with the isolated septum. Using available techniques, semi-infinite scattering matrices are determined for the semi-infinite septum for arbitrary TE/sub N0/, TE/sub N1/, and TM/sub N1/ incident modes. These are used to derive Fredholm matrix equations which yield the field everywhere near the finite length septum. The leading terms of the inverted equations are the far-field transmission and reflection coefficients. The solutions are evaluated for several frequencies, and fifth degree polynomials are fitted for the computation of S/sub 11/ and S/sub 12/. The analysis for the TE/sub 10/ mode is applicable to an arbitrary height waveguide and may be used without modification for the "finite length" septum in conventional waveguide. The second part is concerned with the periodic waveguide and assumes that the septums are far apart. A contour chart is introduced to visualize the performance of the periodic structure. The chart is especially useful when more than one propagating mode is involved and simplifies the design problem so that the parameters of practical structures may be obtained with little effort.     

High-performance circular TE/sub 01/-mode converter

This study presents the design and cold testing of a Ka-band TE/sub 01/-mode converter. A wave is efficiently converted from the TE/sub 10/ rectangular waveguide mode into the TE/sub 01/ circular waveguide mode. This converter comprises a power-dividing section and a mode-converting section. The field pattern and the working principle of each section are analyzed and discussed. A prototype was built and tested. Back-to-back transmission measurements exhibit excellent agreement to the results of computer simulations. The measured optimum transmissions are 97% with a 1-dB bandwidth of 5.8 GHz centered at 34.0 GHz. The angle-independent transmissions manifest high mode purity and the field pattern is directly demonstrated on a temperature-sensitive liquid-crystal sheet. In addition to exhibiting a high conversion efficiency, high mode purity, and broad bandwidth, this converter is also easy to construct and is structurally simple.     

0.1 THz rectangular waveguide on GaAs semi-insulating substrate

The realisation of a millimetre-wave rectangular waveguide fabricated using traditional monolithic technology is presented. The rectangular waveguide has a cutoff frequency of 100 GHz and an operating frequency of 105 GHz. The measured performance clearly shows that the dominant TE₁₀ mode of propagation is supported     

对称单负材料平面波导的波导模

基于经典电磁理论建立了芯层为单负材料的对称三层平板波导的导波方程,采用图解法研究了该波导中的TE和TM波导模,分析了TE波导模和TM波导模会受波导参数μ1/μ2和R的影响,给出TE模和TM模解的横向场分布图.结果显示,在单负材料的对称三层平板波导中只存在慢波导模,TE慢波模只能在磁负材料中传播,TM慢波模只能在电负材料...