When trying to satisfy the requirements in Table I, a possible choice consists in the combination of two antenna
modules used to separately match specifications in the two regions of interest, namely the radiative and reactive nearfield
regions (Fig. 1). The first module is a TWA (namely, a transmission line designed to increase the field on the
antenna surface with respect to a conventional low losses microstrip or CPW transmission line). A spiral or a
meandered structure is preferred to uniformly distribute the EM energy among all the field components [14]-[15]. The
TWA module has to be located in the middle of the reader antenna surface, which represents a premium location since it
is the region where a tagged item is more likely to be located. The region surrounding the TWA antenna can be used toplace one or two resonating antennas that are supposed to cover the radiative near-field region with a circularly
polarized field. Since most of the antenna surface has to be devoted to the TWA antenna, resonating antenna module
has to be realized with miniaturized antennas. It is worth noting that the miniaturization helps to meet the requirement
on low antenna gain. As an example, two possible configurations are sketched in Fig. 1. ANT1 is a TWA whose end is
connected to a resonating antenna that could be a loop-like antenna (Fig. 1a) or a broadside array made of two
miniaturized CP patches (Fig. 1b). Differently by TWAs closed on a matched load [14]-[18], in the proposed
configuration the power at the end of the lossy transmission line is used to radiate a field contribution in the radiative
near-field region. Moreover, the series-feeding helps to get a very low reflection coefficient at the antenna input, so
allowing a simpler design able to match requirements on the allowable antenna size. Depending on the particular
application, it is possible to optimize the radiating elements typology as well as their position on the antenna surface.
The degrees of freedom available in the proposed layout allow the antenna designer to easier meet most of the
requirements for desktop reader antennas; also, the proposed modular antenna can be easily scaled to fit with reader
cases of disparate size. The orientation and layouts of the two antenna modules have to be chosen by considering that
the mutual coupling between the radiators sharing the antenna surface is not negligible, as the radiating elements will be
close to each other and operate at the same frequency band. Moreover, miniaturized antennas usually are narrow band
antennas, and this will make difficult to design reader antenna covering both ETSI and FCC
(a)
When trying to satisfy the requirements in Table I, a possible choice consists in the combination of two antennamodules used to separately match specifications in the two regions of interest, namely the radiative and reactive nearfieldregions (Fig. 1). The first module is a TWA (namely, a transmission line designed to increase the field on theantenna surface with respect to a conventional low losses microstrip or CPW transmission line). A spiral or ameandered structure is preferred to uniformly distribute the EM energy among all the field components [14]-[15]. TheTWA module has to be located in the middle of the reader antenna surface, which represents a premium location since itis the region where a tagged item is more likely to be located. The region surrounding the TWA antenna can be used toplace one or two resonating antennas that are supposed to cover the radiative near-field region with a circularlypolarized field. Since most of the antenna surface has to be devoted to the TWA antenna, resonating antenna modulehas to be realized with miniaturized antennas. It is worth noting that the miniaturization helps to meet the requirementon low antenna gain. As an example, two possible configurations are sketched in Fig. 1. ANT1 is a TWA whose end isconnected to a resonating antenna that could be a loop-like antenna (Fig. 1a) or a broadside array made of twominiaturized CP patches (Fig. 1b). Differently by TWAs closed on a matched load [14]-[18], in the proposedconfiguration the power at the end of the lossy transmission line is used to radiate a field contribution in the radiativenear-field region. Moreover, the series-feeding helps to get a very low reflection coefficient at the antenna input, soallowing a simpler design able to match requirements on the allowable antenna size. Depending on the particularapplication, it is possible to optimize the radiating elements typology as well as their position on the antenna surface.The degrees of freedom available in the proposed layout allow the antenna designer to easier meet most of therequirements for desktop reader antennas; also, the proposed modular antenna can be easily scaled to fit with readercases of disparate size. The orientation and layouts of the two antenna modules have to be chosen by considering thatthe mutual coupling between the radiators sharing the antenna surface is not negligible, as the radiating elements will beclose to each other and operate at the same frequency band. Moreover, miniaturized antennas usually are narrow bandantennas, and this will make difficult to design reader antenna covering both ETSI and FCC(a)
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