Radiation and matching characteristics of a novel dual-band dielectric loaded coaxial horn

Copolar and crosspolar radiation as well as matching characteristics of a novel dual-band coaxial hybrid-mode feed horn with partial dielectric loading [1] for reflector antennas have been numerically and experimentally investigated. In comparison with dual-band corrugated horns this horn has higher crosspolar level, but provides larger band separation and rather good electrical performances over a wide frequency range (20% and more) within each operational frequency band. Furthermore, the main concept of the coaxial feed system can be easily extended for multi-band cost-effective applications in reflector antennas with polarization diversity.     

Eigenmodes of sectoral coaxial ridged waveguides

The results of numerical investigation of sectoral coaxial ridged waveguides eigenmodes of two configurations (with a ridge on inner or outer wall) for different cross-section dimensions are presented. In particular, dependences of cutoff wave numbers on geometrical dimensions ratios for first four modes are investigated, electric field components distributions for these modes have been obtained and the optimization of sectoral coaxial ridged waveguides has been carried out to provide maximal single-mode operation frequency band. Two optimal configurations of waveguides with single-mode operation bandwidth ratio 5.6:1 are obtained. It is shown that smaller cross-section dimensions at the fixed single-mode operation frequency band has the waveguide with the ridge at the inner round wall. The size of the gap between the ridge and the round wall of optimal waveguide is identical for both configurations and is determined by the required ratio of cutoff frequencies of two lower TE modes. Calculations are conducted utilizing the mathematical model obtained in [1] by the integral equation technique with the correct account of singular behavior of the field at the ridge.     

Radiation of a coaxial line into a half-space

We present a new approach to the problem of radiation by a coaxial line into a half-space. We obtain boundary-integral equations for the current densities over the walls of the coax and its opening to the half-space. Using the modes of the coax for basis and testing functions, we convert the integral equations to an infinite system of linear equations, the unknowns being the coefficients of the current density expansions. We demonstrate how, by solving a small subsystem, we can obtain the fields everywhere in space.     

Radiation properties of a pigtail-terminated coaxial transmissionline

The method of moments is used to study the shielding effectiveness of a pigtail-terminated shielded wire (coaxial line) above a perfectly conducting ground plane. Numerical computations are performed in the RF frequency range, i.e. 25-1000 MHz. Pigtail wires on either or both ends of the shield are considered. Results are compared to those from the ungrounded case (floating shield). Results show that the presence of pigtail wires as ground connections at RF frequencies is undesirable and may further enhance radiation. The impedance terminations have a small effect on the shielding effectiveness of the antenna structure     

Eigenmodes of coaxial quad-ridged waveguides. Theory

The electrodynamics eigenmodes boundary problems’ solutions for the eigenmodes of coaxial quad-ridged waveguides are presented. These solutions have been obtained by integral equations technique utilizing the proposed system of orthogonal basis functions, which take correctly into account singular behavior of the field at the ridges’ edges. The formulas obtained provide possibilities to calculate cutoff wave numbers and electric and magnetic fields distributions for TEM, TE and TM modes in the presence of the ridges either on the inner or on the outer conducting cylinder. Analysis of the dependence of numerical solutions convergence for cutoff wave numbers on the number of basis functions and partial modes has been carried out. It has been shown that for calculation of cutoff wave numbers with residual error less than 0.1% it is enough to utilize the 7 proposed orthogonal basis functions and the 21 partial modes.     

Eigenmodes of coaxial quad-ridged waveguides. Numerical results

The results of numerical investigation of coaxial quad-ridged waveguides’ eigenmodes of two configurations (with ridges on inner or on outer perfectly conducting circular cylinder) for different cross-section dimensions are presented. In particular, dependences of cutoff wave numbers on geometrical dimensions ratios for the first three TE modes and for the first TM mode have been investigated and transversal electric field components distributions for these eigenmodes and for theTEM mode have been obtained. Besides, the optimization of coaxial quad-ridged waveguides has been carried out at antiphase excitation in order to provide maximal single-mode operation frequency band for the first TE mode. As a result two optimal configurations of coaxial quad-ridged waveguides with single-mode operation bandwidth ratios 4.6:1 have been designed. It has been defined that the waveguide with the ridges at the inner conducting circular cylinder has smaller cross-section dimensions at the fixed single-mode operation frequency band. Calculations are conducted utilizing the mathematical models obtained in [1] by the integral equations technique taking correctly into account of singular behavior of the field at ridges’ edges. Reliability of the results obtained is confirmed by the calculations based on finite difference time domain technique, which is implemented in the software package CST Microwave Studio at mesh size λ/100.     

Broadening of primary feed patterns by small E-plane slots

It is shown that the radiation patterns of a circular waveguide with a corrugated flange can be broadened by using small E-plane slots. These slots tend to improve the amplitude patterns of the feed, but introduce some phase variations in the axial radiation.     

Radiation characteristics of dielectric-coated coaxial waveguideperiodic slot with finite and zero thickness

The leaky wave radiated from the dielectric-coated coaxial waveguide periodic slot with finite and zero thickness is investigated theoretically for the infinite and finite periodic structures. For the infinite periodic structure, mode-matching technique and integral equation method are applied to the analysis of finite and zero thickness slot cases, respectively. The integral equations are derived for the finite periodic structure by use of the Fourier transform and mode expansion and simultaneous linear equations are obtained. The effects of the slot thickness, the finite slot number and the dielectric coating are analyzed. Results for finite periodic slots are compared with those of the infinite extent structure and good agreement is found     

Radiation properties of circumferential slots on a coaxial cable

Both the Galerkin's and mode-matching methods are used to analyze the reflection and radiation properties of a coaxial cable with a circumferential slot on the outer conductor. The effects of higher order modes are also included. The accumulated effect of multiple slots on the reflected signal is also studied by using a Monte Carlo simulation     

Mutual coupling in arrays of coaxial waveguides and horns

A solution is presented for the mutual coupling between the modes of different-sized circular coaxial aperture arrays, which are used as direct radiators and feed antennas. This solution is expressed in terms of integral transforms and the mutual admittances for the cross coupling between all three mode types, TE, TM, and TEM, are obtained. The accuracy of this new integral formulation is demonstrated by comparing computed results with measured data for a three-element test array. Excellent agreement is obtained between theory and experiment for both coupling coefficients and radiation patterns. Results are also presented for a coaxial feed array for the Lovell radio telescope.     

Propagation and radiation properties of corrugated cylindrical coaxial waveguides

A theoretical study is given for the behavior of cylindrical coaxial waveguides having reactive (corrugated) surfaces. After a general analysis assuming independent values for the surface reactances, the propagation and radiation properties for specific surfaces are considered. These include the case where 1) both surfaces are smooth-walled; 2) one surface is smooth-walled with the other corrugated; and 3) both surfaces are corrugated such that they have equal values for the surface reactance.     

Radiation from a flanged coaxial line into a dielectric slab

The radiation from a flanged coaxial line into a dielectric slab is investigated in this paper. The Hankel transform and mode-matching technique are used to obtain simultaneous equations for the modal coefficients. The residue calculus is utilized to represent the solution in rapidly convergent series. Numerical computations are performed to illustrate the behavior of reflection in terms of the slab permittivity, frequency, and coaxial-line geometry     

Radiation patterns of pyramidal dielectric waveguides

The radiation patterns of pyramidal dielectric waveguides excited by pyramidal metallic horns are predicted and compared with patterns measured at 9 GHz. Satisfactory agreement is obtained, especially in the region of the main beam. At wide angles, the pattern corresponds quite closely to that of the launcher alone.     

Transmission characteristics of a coaxial optical fiber line

A coaxial optical fiber line is proposed and studied as a long-distance transmission medium. The radial structure consists of a core surrounded by two clads. The inner clad has higher refractive index than the core and the outer clad. Guided modes are thus characterized by a concentrated field in the inner clad and evanescent fields in both the core and the outer clad. Using a rigorous modal analysis, characteristic curves for a monomode coaxial fiber line are presented. It is shown that the zero-dispersion wavelength can be placed at any desired value by proper choice of the refractive index contrast and the fiber dimensions. The proposed coaxial line compares well with the known W-profiled line with respect to ease of manufacture and dispersion characteristics     

Synthesis of offset parabola with open-ended waveguides feed

The synthesis of an offset parabola with array feeds consisting of open-ended waveguides located around the focal point of the reflector is investigated. The approach is based on simulated annealing which has been applied to the analysis to two-dimensional phased arrays. Two cases of aperture distributions, uniform and Taylor with 30-dB sidelobe level, are discussed. Both results seem to be satisfactory     

Linear analysis of cyclotron-cherenkov and cherenkov instabilities in dielectric-loaded coaxial waveguides

We present, based on the cold fluid theory, linear analysis of the Cherenkov and cyclotron-Cherenkov instabilities which are driven when a linear electron beam is injected into a dielectric-loaded waveguide immersed in an axial magnetic field. In the analysis we consider azimuthally symmetric TM_0n modes. We derive dispersion relations for three types of waveguide, and compare computationally obtained linear growth rates of both instabilities. For the type A, which consists of a metallic cylinder with dielectric liner on its inner surface, the growth rate of the Cherenkov instability is larger than that of the cyclotron-Cherenkov instability. For the type B, which consists of a dielectric core and an outer metallic cylinder, both growth rates are comparable. And for the type C, which consists of a metallic core with dielectric liner on its surface and an outer metallic cylinder, the growth rate of the latter instability is higher than that of the former instability. Finally, for the type C, obtained are dependences of the oscillation frequency and the growth rates of both instabilities on the following parameters: the beam energy, the beam current, the axial magnetic field, the dielectric constant, and the thickness of the dielectric.     

Higher order mode impedances and cut-off frequencies of overmoded coaxial waveguides

Using both theoretical and experimental methods the critical geometrical ratios for effective higher order mode superposition in coaxial waveguides are identified for SDMA beam forming antenna applications. Higher order mode impedances are also important in the design of such antennas and a closed form expression for the power-voltage definition of impedance is derived together with estimates for the voltage-current and power-current forms. An industry standard finite element solver is used to confirm the accuracy of these formulations for a 60?mm radius overmoded coaxial waveguide. In the second part of the paper, the mode chart is used to design a coaxially fed, overmoded air-spaced coaxial waveguide “T” junction supporting the TEM, TE ₁₁ and TE ₂₁ modes. A Green's function based model of the input impedance of this junction establishes that the power-voltage characteristic impedance occurs naturally in calculations requiring absolute impedance values. Finally, this input impedance model, which includes higher order and evanescent mode propagation, is demonstrated to agree well with experimental data for this junction.     

Radiation from dielectric-loaded arrays of parallel-plate waveguides

The problem of calculating the reflected and transmitted fields as a function of scan angle in an infinite phased array of parallel-plate waveguides is studied on an analytical basis. Two infinite sets of simultaneous linear equations are derived for the scattered fields internal and external to the waveguides. These sets of equations are attacked by a newly developed modified residue calculus method. Application of the results to the problem of designing rectangular waveguide arrays with improved impedance performance over a wide range ofH-plane scan angles is discussed. It is indicated that the present analysis may serve as a first step toward investigating the more general problem of phased arrays scanning at an arbitrary angle.     

Admittance of coaxial feed between two finite parallel conductors

It is known that, using a reduction process, the admittance contribution of the higher order modes of the current on a coaxial probe can be calculated in an analytical way. The authors prove that, once the lower order modes are determined, they can also be taken into account analytically. The final result is that no numerical effort at all is needed to determine the admittance of the coaxial feed in terms of the probe current modes.<>