Mid Sweden University

Is it possible to produce a planar dipole-like antenna with a reduced conductive area without any loss in either robustness or performance? The objective is to reduce the amount of expensive conductor to be used when applying a meshing technique to the printing of antennas. In this context, robustness means that the characteristics are maintained when the antenna is damaged, for example if it is scratched. This is particularly important for radio frequency identification tags in logistic systems. A general antenna robustness evaluation methodology, based on numerical simulations of a large number of randomly damaged antennas, is used for the antenna comparisons. The antenna performance degradation, based on the return loss (S11) at 868MHz, is monitored for some basic planar antennas. Finally, we show that it is possible to produce robust low-cost antennas using wire replacements for the solid planar antennas and thus, provided that the robustness requirement is moderate, replace the solid antenna with a thin conductor analogue.

Antennas in RFID tags have often been designed in a single layer with copper as conductor and plastic foils as substrate. There is currently a large interest in roll to roll production of RFID tags and silver based inks have been developed for use in printed RFID antennas. Silver ink based single layer antennas works well and is providing 70% to 80% of the reading range compared to copper solutions. However, more advanced antennas are needed to provide less sensitivity to the environment of RFID tags .i.e. need for placing tags on metal or near water. In this work we present a study of multilayered antennas so called patch antennas, for 2.45 GHz RFID tags. The advantage of the patch antenna is that it can be applied to any kind of material, reflecting or lossy material, and still provide good antenna function. However, the patch antenna efficiency is strongly dependent on the material used. For low cost RFID tags in logistics there is a need to manufacture the antenna as a part of the packaging process. In the current work we have investigated the possibility to manufacture printed patch antennas of common packaging materials.

We address the question of robustness of damaged microstrip antennas, the damage being either penetrating, caused by fragment impact, or floating, caused by manufacturing imperfections. A simple analytic expression is derived to facilitate the prediction of robustness. To verify this expression a Monte Carlo method, based on a general 3D electromagnetic solver, is used to evaluate the robustness of the antennas. The simulations are verified by measurements and supplementary simulations in an alternative electromagnetic solver, using the finite difference time domain method (FDTD). The agreement between the simulated results and the analytic expression is found to be good in a qualitative comparison.

In a previous paper, (Olsson, Sidén, and Koptioug, "Robustness comparison of bow-tie and dipole antennas," IEE Proceedings Microwaves, Antennas and Propagation, vol. 151, no 6, Dec. 2004, pp. 525-529), it was shown that the dipóle antenna is more robust than the bow-tie antenna, with same radiator aperture area, to numbers of small randomly positioned holes penetrating the structures. In present paper it is shown that for a small number of holes the bow-tie antenna is more robust due to lower hit probability in high current density areas. The results show that the bow-tie feed is more robust for a small amount of damage, whereas the strip dipole feed (of width w and thickness t, with w ≫ t) is more robust in general terms, and particularly for larger number of damages.

In this work we present a simple printed moisture sensor fabricated using electronic inks on a multilayer paper structure. The sensor is based on a Carbon-Zinc type energy cell and provides power to a readout electronic circuit when activated by moisture. The sensors are based on a number of our filed patents according to which the sensor is used for both event detection and as a power source for the processing electronics. Typical applications are moisture and leakage detection in buildings, water pipe lines, smart packages and health care systems such as smart incontinence sensors. As the detector is triggered, it powers up an electronic circuit (polymer based or silicon based) that starts communication with the alarm server. In the simplest systems a sound or a light alarm is started to alert the user. In this work we present a characterization of some critical parameters of the sensor such as power driving capability, linearity, internal memory effects and saturation. In addition, we examine a specific application, when sensor is used as defrosting alarm for surveillance of frozen articles during transport.

It becomes more and more common to print tag antennas using electrically conductive ink for mass-produced Radio Frequency IDentification (RFID) tags. Electrical properties of the ink are mostly determined by conductive (e.g. silver) particles mixed into the ink solution. Since silver is relatively expensive it is desirable to minimize the amount of ink used per antenna. This paper illustrates how the printed conductor area of the antenna can be reduced by applying a grid pattern to an existing antenna geometry and to what extent the gridding can be performed without significantly degrading of the antenna electrical properties. Two common antenna structures are used as an example. It is also shown that by slightly modifying the original antenna geometry it is possible to even further reduce the amount of used ink.

The paper describes the application of a computer intensive methodology to evaluate antenna susceptibility to random physical damage for a dipole and a bow-tie antenna. For each randomly generated damage pattern antenna parameters are simulated and a statistical comparison is performed. Computer intensive statistical methods like the Bootstrap, used here for postprocessing, make fewer initial assumptions than classical statistical methods when modelling a problem, and are robust against small deflections from these assumptions. The bias-corrected and accelerated (BCa) Bootstrap method is used for confidence interval estimates. The dipole antenna is found to be more robust to numbers of small (similar to 100th of a wavelength) circular holes penetrating the structure than the bow-tie antenna.