Unlike conventional 50 Ω-matched antennas, the most useful parameter for the definition of RFID tags’ bandwidth is the realized gain, rather than the return loss, as also in the design of ultra-wideband antennas. In fact, the tag bandwidth depends on the required minimum read range in the specific application and, once fixed the power constraints, it can be therefore related to the system parameter τGtag. The stability of the realized gain over the frequency gives indeed the rangewidth of the reader-tag system.

In general, it is well known that antenna miniaturization yields layouts with reduced bandwidth. Although RFID applications involve narrow frequency bands, bandwidth issues are nevertheless important since tag impedance may be easily detuned by the coupling with the objects to be tagged, as well as by the interaction with the surrounding environment, and the resulting reading range could be globally degraded.

At the purpose to introduce a general bandwidth definition, not dependent on the particular application, a perfectly matched isotropic antenna ([τGtag]0=1) is selected as reference. The tag band is then defined as the frequency range, [fmin, fmax], where the following condition holds:

  (1)

f_c being the middle-band frequency. Under his condition, the activation range reduces to not more than the 70% of the activation range of the reference antenna. This definition permits to compare the features of different tags whatever the transmitter power and the microchip sensitivity are. Whether the tag is designed to be attached over a high-loss target, as in the case of human body, a different reference antenna (lower) gain my be chosen.

To discuss the bandwidth capability of miniaturized tags, an MLA and an IFA antenna, having maximum size not exceeding 5 × 5cm (about λ/7 at 870 MHz), and matched to a high impedance phase angle microchip (Zchip=15-j450Ω), are now analyzed with respect to frequency changes. With reference to Figure 1 the IFA vertical conductors occupy most of the available space with the purpose to achieve the required input resistance. The MLA antenna has been automatically optimized as previously discussed. In particular, under the only constraint of best realized gain, the solution found by the genetic optimizer is slightly smaller than the imposed maximum size. No bandwidth constraint have been considered.

Figure 1: IFA and GA-optimized MLA tags (sizes in [mm]) matched at 870 MHz to the microchip impedance Zchip=15- j450Ω. The maximum external size are 5×5cm (≈0.14λ). Trace size: 1mm.

Figure 2 shows the tags’ features (maximum gain, power transmission factor and realized gain) with respect to the frequency variation. The MLA exhibits a gain larger than the IFA’s one. The gain is nevertheless only slightly frequency dependent and therefore the bandwidth performances are manly affected to the impedance matching. The MLA geometry permits a better realized gain (τGtag =1.46) at 870 MHz than the IFA design (τGtag =1.33) but the activation range improvement at that frequency if only of 5%.

According to the above definition, the band features of the considered examples are reported in Tabella 1, where Δf=fmax - fmin and B=Δf / fc. Although the two antennas show a nearly similar narrow band, as expected by the relevant size reduction, the IFA layout has a slightly broader relative bandwidth. It is however to consider that the MLA layout possesses a larger number of degrees of freedom, if compared to the standard IFA, and therefore additional constraints may be included in the MLA optimization procedure at the purpose of bandwidth enhancement.

Figure 2: Maximum gain and power transmission coefficients, vs. frequency, for the IFA and MLA tags having the same overall size (5×5cm). The horizontal thin line for τGtag =0.5 permits to appreciate the tag bandwidth within which the activation distance is not less than the 70% the activation distance of a reference perfectly matched isotropic antenna.

Tabella 1: Bandwidth features of the 870MHz MLA and IFA tags having the same maximum external size 5×5cm.