Small volume, finite conductivity and high frequencies are major imperatives in the design of communications infrastructure. The radiation efficiency ηr impacts on the optimal gain, quality factor, and bandwidth. The current efficiency limit applies to structures confined to a radian sphere ka (where k is the wave number, a is the radius). Here, we present new fundamental limits to ηr for arbitrary antenna shapes based on k 2 S where S is the conductor surface area. For a dipole with an electrical length of 10 −5 our result is two orders of magnitude closer to the analytical solution when compared with previous bounds on the efficiency. The improved bound on ηr is more accurate, more general, and easier to calculate than other limits. The efficiency of an antenna cannot be larger than the case where the surface of the antenna is peeled off and assembled into a planar sheet with area S, and a uniform current is excited along the surface of this sheet.

Low cost methods of antenna production primarily aim to reduce the cost of metalization. This might lead to a reduction in conductivity. A systematic study on the impact of conductivity is presented. The efficiency, gain and bandwidth of cylindrical wire meander line, dipole, and Yagi-Uda antennas were compared for materials with conductivities in the range 10 3 to 10 9 S/m. In this range, the absorption efficiency of both the dipole and meander line changed little, however the conductivity significantly impacts on radiation efficiency and the absorption cross section of the antennas. The extinction cross section of the dipole and meander line antennas (antennas that Thevenin equivalent circuit is applicable) also vary with radiation efficiency. From the point of radiation efficiency, the dipole antenna performance is most robust under decreasing conductivity. Antennas studied in this study were fabricated with brass and graphite. Radiation efficiency of the antennas were measured by improved Wheeler cap (IWC) method. Measurement results showed a reasonable agreement with simulations. We also measured the extinction cross section of the six fabricated prototypes.

The fundamental limit for small antennas provides a guide to the effectiveness of designs. Gustafsson et al, Yaghjian et al, and Mohammadpour-Aghdam et al independently deduced a variation of the Chu-Harrington limit for planar antennas in different forms. Using a multi-parameter optimisation technique based on the ant colony algorithm, planar, meander dipole antenna designs were selected on the basis of lowest resonant frequency and maximum radiation efficiency. The optimal antenna designs across the spectrum from 570 to 1750 MHz occupying an area of 56mm×25mm were compared with these limits calculated using the polarizability tensor. The results were compared with Sievenpiper's comparison of published planar antenna properties. The optimised antennas have greater than 90% polarizability compared to the containing conductive box in the range 0.3 < ka < 1.1, so verifying the optimisation algorithm. The generalized absorption efficiency of the small meander line antennas is less than 50%, and results are the same for both PEC and copper designs.

Induced modes due to discontinuities inside the waveguide are dependent on the shape and material properties of the discontinuity. Reflection and transmission coefficients provide useful information about material properties of discontinuities inside the waveguide. A novel non-resonant procedure to measure the complex conductivity of narrow strips is proposed in this paper. The sample is placed inside a rectangular waveguide which is excited by its fundamental mode. Reflection and transmission coefficients are calculated by the assistance of the Green’s functions and enforcing the boundary conditions. We show that resistivity only impacts one of the terms in the reflection coefficient. The competency of the method is demonstrated with a comparison of theoretic results and full wave modelling of method of moments and finite element methods.

Carbon nanotube (CN) antennas have applications in the THz electromagnetic spectrum. Nanotubes have a highly dispersive and frequency dependent conductivity model. In this article, we compare the poles and zeros in the input impedance of CN antennas at different lengths. We used model-based parameter estimation to approximate the input impedance of the antenna with a rational function in the complex frequency domain. Despite dispersive conductivity of CN, the imaginary part of the poles and zeros are respectively the integer multiples and odd multiples of the imaginary part of the first pole and zero. However, the real part of poles is almost constant, while the pattern was not observed for the real part of zeros. We also show that CN dipoles operating between 43 and 53 GHz are well matched if the source impedance is much higher than 50 ohms, and even higher than 12.9 kX. The fundamental resonances (f 0 ) of CN dipoles plotted versus their inverse-half-length (1/L) are linearly related, but the intercept of the fitted straight line is non-zero unlike that for perfect electric conductor (PEC) dipoles. This leads to non-linear variation in wavelength scaling of CN dipoles. The resonant CN antennas are relatively much shorter than PEC dipoles. K E Y W O R D S carbon nanotube, model-based parameter estimation, wavelength scaling Int J RF Microw Comput Aided Eng. 2017;27:e21103.

IT&MMWAVES Company MalaysiaAbstract-In this paper new array element for use in dual band dual polarized antenna will be introduced that has high efficiency (more than 95%) and has high isolation in both 900 MHz and 1800 MHz bands. Also radiation pattern of this antenna is according to IEC recommendation.

Fundamental antenna limits of the gain-bandwidth product are derived from polarizability calculations. This electrostatic technique has significant value in many antenna evaluations. Polarizability is not available in closed form for most antenna shapes and no commercial electromagnetic packages have this facility. Numerical computation of the polarizability for arbitrary conducting bodies was undertaken using an unstructured triangular mesh over the surface of 2D and 3D objects. Numerical results compare favourably with analytical solutions and can be implemented efficiently for large structures of arbitrary shape.1 Standard tessellation language Stl is the industrial standard for handling triangulated meshes

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