Small Normal Mode Helical Antenna Research Articles

Normal-Mode Helical Antennas With Near-Field Coupling: Improving Design Flexibility [Antenna Applications Corner

The near-field coupling between closely placed metallic structures has been utilized to cope with the dilemma of a normal-mode helical antenna (NMHA) design. For small NMHAs, the impedance-matching difficulty is mitigated by designing appropriated coupling feeds, either electrically or magnetically. For long NMHAs, the radiation pattern distortion is rectified by dividing the complete helix into properly designed small sections. With mutual section coupling, the currents on all of the sections can be nearly in-phase, dictating an omnidirectional radiation pattern. The design principle has been described as two-element models and a corresponding equivalent circuit analysis. The simulated and measured results show that, compared to conventional NMHAs, it is possible to design an NMHA of simultaneous good matching and radiation properties but with a much more flexible helix length and size.

Investigating Equations Used to Design a Very Small Normal-Mode Helical Antenna in Free Space

A normal-mode helical antenna (NMHA) has been applied in some small devices such as tire pressure monitoring systems (TPMS) and radio frequency identification (RFID) tags. Previously, electrical characteristics of NMHA were obtained through electromagnetic simulations. In practical design of NMHA, equational expressions for the main electrical characteristics are more convenient. Electrical performances of NMHA can be expressed by a combination of a short dipole and small loops. Applicability of equations for a short dipole and a small loop to very small normal-mode helical antennas such as antennas around 1/100 wavelengths was not clear. In this paper, accuracies of equations for input resistances, antenna efficiency, and axial ratios are verified by comparisons with electromagnetic simulation results by FEKO software at 402 MHz. In addition, the structure of the antenna equal to 0.021 λ is fabricated, and measurements are performed to confirm the design accuracy.

Effects of Dielectric Materials on Impedance Characteristics of Embedded Small Normal Mode Helical Antenna

The normal mode helical antenna (NMHA) is widely selected on RFID and wireless communication applications and often placed in a dielectric material for protection and miniaurization design. In this paper, the effects of dielectric materials with finite dimension on resonant frequency and impedance characteristics of embedded small NMHA were investigated. Three parameters related to the dielectric material were considered. Significant results were obtained by varying each one of these electrical dielectric parameters. It provides an effective guidance for designers to tune the antenna to desirable complex impedance when surroundded dielectric materials are employed.

Deterministic Equation for Self-Resonant Structures of Very Small Normal-Mode Helical Antennas

For the easy design of very small normal-mode helical antennas (NMHAs), an equation that helps determine the self-resonant structures of these antennas is developed. For this purpose, the expression for the capacitance of an NMHA is established. The accuracy of this design equation is confirmed by comparing the results obtained using the equation with the simulation results.

Consideration of the local SAR and radiation characteristics of a helical antenna using a cylindroid whole body phantom at 150 MHz

AbstractA cylindroid whole body phantom is proposed to evaluate the local specific absorption rate (SAR) and radiation characteristics when a small normal‐mode helical antenna (NHA) of a 150‐MHz wireless terminal is in the vicinity of the abdomen of a human body. For their investigations, the distance between the NHA and phantom was set to the value in actual usage conditions. The thermography method is employed to evaluate the local SAR. It was confirmed that the local SAR of a wireless terminal with a transmission output of 5 W did not exceed 5 W/kg even in impedance‐matched conditions. In addition, to check the interrelationship between the local SAR and radiation characteristics in actual usage conditions, an analysis that considered impedance mismatch loss was employed. The results show that the rise of the local SAR decreased by deterioration of the radiated power due to mismatch loss when accompanying a phantom approach. Although the local SAR for a mismatch condition did not exceed 1.2 W/kg regardless of the distance, the maximum gain deteriorated at least 10 dB compared with the matching condition. The authors show that by setting the target value of the local SAR to at most 2 W/kg and controlling the matching condition or available power of the power supply, the gain can be improved 5 dB or more. © 2003 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 87(1): 48–60, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecja.10084