Ultra-miniature UHF antenna for nomadic device

Abstract

IntroductionMore and more wireless applications, dedicated to nomadic and mobile devices, operating in the UHF frequency band, are expected in the coming years. They include digital terrestrial TV broadcast (DVB-T/H, MediaFlo) or new communication applications using the UHF spectrum resulting from the digital dividend and based on technologies such as WiMAX of future generation of cellular networks. [1,2]. However, considering the antenna recommendations from the EICTA/MBRAI [3], designing a compact antenna for small devices is a real challenge. Several technologies are already used to address such a problematic: 3D metallic shapes [4], use of substrate material with high dielectric constant, use of broadband or tunable matching network and tunable components [5]. In this paper, a new design based on the twisted slot antenna [6,7] is proposed covering the full UHF band . Realized on a 130*230mm 2 PCB board, the design combines artifacts such as folding the metallic shape using the third dimension and using a tunable component for frequency tunability in order to cover the full UHF band. The achieved ultra-miniature antenna size is lower than λ 0 /51* λ 0 /118* λ 0 /138 at the lowest frequency for a simulated peak antenna gain higher than EICTA/MBRAI recommendations. Principle of the Twisted Slot AntennaThe radiating slot antennas are now used in various applications [6,7]. One major advantage is their easy and low cost etching on a PCB in the same manufacturing flow than the RF part by removing a part of the ground plane. For portable/nomadic devices operating in the UHF band, a straight resonating slot (size ~λ g /2) is too long to be integrated on a PCB with a small size factor and including other electrical functions. As sketched in Figure 1a the meandering of the slot leads to the reduction of the antenna efficiency due to E field cancellation. In [6], it has been proposed to compact the slot antenna by meandering it while keeping good radiation efficiency. Indeed the radiated E field is associated to current orientations, reversing the current direction leads to reversing the radiated E field. A sort of bridge is added in the half guided wavelength slot (called the short circuit slot (SCS)) to reverse the electric fields of the slot, as shown in Figures 1b and 1c. A cut slot (called open circuit slot (OCS) is necessary in the ground plane on both sides of the SCS in order to not short-circuit the reversed E fields. Twisted Slot Antenna in UHF bandStarting from the U shaped twisted slot design shown in Figure 2, a more compact structure is proposed. Placed in a corner of the PCB, the SCS is folded along the length of the OCS. In addition, only one part of the OCS is extended (the other length of the OCS is very small). The U shaped slot is designed in the UHF band (with HFSS) on a 130*230mm 2 FR4 substrate ( Figure 2) with thickness of 1.4mm. The real and imaginary 978-1-4244-4968-2/10/$25.00 ©2010 IEEE parts of its impedance show two resonances (Figure 3-a /green curve). The best radiation efficiency is obta...