소형 안테나 ( Small Antennas )

New meander line antennas with improved gain are proposed as low-profile self-resonant tags for application in passive radio frequency identification. Antenna shape and size is optimized by genetic algorithm, taking conductor losses into account. Examples are presented for application at 869 MHz with antennas of different materials and sizes.

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Gain-optimized self-resonant meander line antennas for RFID applications

The theoretical limit for small antenna performance that was derived decades ago by Wheeler and Chu governs design tradeoffs for size, bandwidth, and efficiency. Theoretical guidelines have also been derived for other details of small antenna design such as permittivity, aspect ratio, and even the nature of the internal structure of the antenna. In this paper, we extract and analyze experimental performance data from a large body of published designs to establish several facts that have not previously been demonstrated: (1) The theoretical performance limit for size, bandwidth, and efficiency are validated by all available experimental evidence. (2) Although derived for electrically small antennas, the same theoretical limit is also generally a good design rule for antennas that are not electrically small. (3) The theoretical predictions for the performance due to design factors such as permittivity, aspect ratio, and the internal structure of the antenna are also supported by the experimental evidence. The designs that have the highest performance are those that involve the lowest permittivity, have an aspect ratio close to unity, and for which the fields fill the minimum size enclosing sphere with the greatest uniformity. This work thus validates the established theoretical design guidelines.

Experimental Validation of Performance Limits and Design Guidelines for Small Antennas

The usefulness of fractal Hilbert curves in antenna geometry is explored here for the first time. Apart from being simple and self-similar, these curves have the additional property of approximately filling a plane. These properties are exploited in realizing a “small” resonant antenna. This approach has resulted in an antenna size smaller than λ/10 and still resonant, with performance comparable to a dipole whose resonant length is close to λ/2. Numerical predictions of the input impedance of the antenna have been compared with experiments. The effect of additional fractal iterations on the reduction of the resonant frequency has been studied. The radiation characteristics of the antenna at the resonant frequencies provided show that this is very similar to the dipole characteristics. © 2001 John Wiley & Sons, Inc. Microwave Opt Technol Lett 29: 215–219, 2001.

Hilbert curve fractal antenna: A small resonant antenna for VHF/UHF applications

The space-filling fractal geometry of the Hilbert curve is examined in terms of its effectiveness in lowering resonant frequency. It is demonstrated that the complex geometry associated with the space-filling Hilbert fractal curve is inherently ineffective in lowering resonant frequency compared to other less complex geometries of the same size and total wire length. The effectiveness of the geometry in lowering resonant frequency is a direct function of the current vector alignment established by the wire layout.

The effectiveness of space-filling fractal geometry in lowering resonant frequency

For antennas to radiate at maximum efficiency, their dimensions must be on the same order as the radiated wavelength At frequencies below 30 MHz, antennas with efficient radiation are often too large for mobile and portable applications Smaller antennas can be made to radiate efficiently by use of matching networks For installation convenience and ease of adjustment, these networks are usually placed between the transmitter and the antenna input; but it has been found that for best radiation efficiency, matching network elements should be placed at points on the antenna structure Unfortunately, such matching networks must be tuned for each transmitting frequency and, when mounted on the antenna, they cannot easily be tuned A meander element antenna was found to present some electrical and mechanical properties allowing convenient placement of tuning elements when configured as an electrically small transmitting antenna Some simplified design guidelines were derived from experimental data

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T.J. Warnagiris

Papers

Performance of a meandered line as an electrically small transmitting antenna