Radiation Efficiency Of Small Antennas Research Articles

OPTIMIZATION OF THE WHEELER CAP TECHNIQUE FOR EFFICIENCY MEASUREMENT OF RFID ANTENNAS MATCHED TO COMPLEX LOADS

This paper proposes an improved mathematical formulation of Johnston’s approach tomeasure the radiation efficiency of an antenna, based on the Wheeler Cap (WC) technique. Theproposed modifications allow the measurement of the radiation efficiency of small antennas matched tocomplex loads implemented on Radio Frequency IDentification (RFID) tags. The studied structure is alow-cost, silver-printed, differentially-fed RFID dipole antenna. The antenna is printed on a flexible PET(polyethylene terephthalate) paper that is conformable on various objects. Link budget measurementsvalidate the accuracy of the formulation, which can be applied to any dipole antenna matched to anRFID chip with a complex input impedance.

Improved Wheeler Cap Method Based on an Equivalent High-Order Circuit Model

The conventional Wheeler cap method usually produces accurate radiation efficiency of small antennas when the antennas under test (AUTs) operate as a simple series or a parallel RLC resonance circuit. However, this method often gives unreliable radiation efficiency if the AUT has a complicated operating principle such as circular polarization (CP), multiple resonances, or broad-band properties. In this paper, we propose an improved Wheeler cap method based on the equivalent high-order circuit model including transformers to provide accurate radiation efficiency, although the AUT does not operate as a simple resonance circuit. For building equivalent high-order circuit models, a method for estimating the initial values of a genetic algorithm (GA) is also proposed, which effectively reduces the searching space and improves the convergence of the optimization. To verify the proposed method, we measure the radiation efficiency of a CP microstrip antenna, a UHF RFID tag antenna, and a triple-resonance microstrip antenna.

Calculated and Measured Absorption Cross Sections of Lossy Objects in Reverberation Chamber

Reverberation chambers can be used to measure radiation efficiency of small antennas when these are located close to lossy objects. The lossy objects represent a heavy loading of the chamber. This loading is characterized by the mean absorption cross section of the lossy objects. This paper describes how this mean absorption cross section can be calculated from the scattered far field of an object by using the forward scattering theorem, or from a more laborious near-field evaluation. Results for lossy spheres and cylinders are calculated by using three different codes, based on spherical mode expansion, finite difference time domain techniques, and moment methods, respectively. The results for the cylinder are compared with measured levels in a reverberation chamber.

Detection of a polarization imbalance in reverberation chambers and how to remove it by polarization stirring when measuring antenna efficiencies

AbstractIt has previously been shown that the radiation efficiency of small antennas can be measured with good accuracy in reverberation chambers. The results are obtained by averaging several measurements of the transmitted power between the antenna under test and a fixed antenna, both located inside the chamber. Further investigations have shown that the results depend strongly on the orientation of the antenna under test and thereby on its polarization. The present Letter explains why it is like this, and how this imbalance can be removed by polarization stirring, that is, by using three orthogonal fixed antennas instead of one. © 2002 Wiley Periodicals, Inc. Microwave Opt Technol Lett 34: 145–149, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10398

Surface-patch modelling of a Wheeler cap for radiation efficiency simulation of a small loop antenna with NEC2

The Wheeler cap method is an easy and accurate way to measure the radiation efficiency of small antennas, and has been analysed by other investigators using a wire-grid model. However, it is more reasonable to use a surface-patch model for the cap instead of the wire-grid model. This would avoid the inevitable leakage from the wire grid. The authors confirm the validity of the simulation using the surface patch for the Wheeler cap with NEC2 (numerical electromagnetics code 2 developed by Lawrence Livermore National Laboratory).