Miniaturised integrated antenna set for RFID/UWB applications K. Mohammadpour-Aghdam, S. Radiom, R. Faraji-Dana, G.A.E. Vandenbosch and G.G.E. Gielen Described is a high-efﬁciency miniaturised antenna set including one scavenging antenna at 5.8 GHz and one UWB antenna at 5.5 GHz with at least 500 MHz bandwidth. This antenna set is integrated with a same wireless-powered RFID chip in 5.8 GHz. The proposed anten- nas are derived from a new planar antenna structure which can be designed towards any arbitrary input impedance within a given area constraint. Measurement results concerning read range and realised gain for the proposed antenna are presented and compared with simu- lation results, showing good agreement. Introduction: The authors presented an autonomous wireless-powered active RFID chip in [1]. The power of the active RFID tag is provided by the electromagnetic energy of the incoming RF wave at 5.8 GHz. The chip uses UWB impulse radiation for the transmission to achieve a high data rate along with a low transmit power consumption. This chip is a far-ﬁeld wireless-powered RFID tag with monolithically integ- rated on-chip antennas in standard CMOS technology. Measurements show an operating distance of 7 cm with these on-chip antennas [1]. Although an operating distance of 7 cm is state-of-the-art for an RFID tag with on-chip antennas, there are many applications that require a cov- erage distance in the order of one metre. The designed chip therefore has the capability of wire-bond connection with an external off-chip antenna to increase the coverage distance. In this Letter, an off-chip antenna set with integration capability with the designed chip is presented to increase the read range of the chip up to 42 cm. extension parts extension parts feeding loop feeding loop 1.5 1.3 RFID scav. antenna chip chip 6 1.7 1.1 UWB tx antenna 1.7 4 (mm) Fig. 1 Scavenging (at 5.8 GHz) and UWB (at 5.5 GHz) antenna details Strip width and parallel strip separation, 90 mm; loop length 6.2 mm for UWB and 3.5 mm for scavenging Design and simulation: The most important performance characteristic for an RFID tag is the read range [2] – the maximum distance at which the RFID reader can power up the tag and detect the signal from the tag. Because the reader sensitivity is typically high compared with the tag’s sensitivity which is around 211 dBm in this chip, the read range is set by the tag response threshold [1]. Considering the requirement for con- jugate matching and low-cost production, the impedance behaviour of the chip determines the overall structure of the antenna [3]. A new antenna structure optimisable towards an arbitrary input impedance and with high miniaturisation capability is used in this Letter to design two different antennas. This general structure covers most of the reported RFID antennas [2] and has three main parts: the small feeding loop and the extension part(s) forming a modiﬁed dipole. These latter two parts are combined together by a direct connection, while, in other cases, they could be connected by mutual coupling. This structure permits a nearly independent tuning of the resistance and reactance of the antenna by modifying the tag’s geometrical para- meters [3]. Fig. 1 shows the antennas for the scavenging circuit at 5.8 GHz and for the UWB transmitter around 5.5 GHz emanate from the structure proposed in [3] with a very small occupied area of 4 × 6 mm 2 for both antennas, including the RFID chip. This antenna set is shown with separation of the antenna feeding loop in dark grey and the extension parts that mainly work as radiators. The extension parts in the scavenging antenna have some folds to form a multi-turn meander line as discussed in [3], while in the UWB antenna they are like a fat dipole. This is a consequence of the frequency and impedance requirements for the UWB and scavenging antennas. The UWB antenna has to provide a frequency response with a linear phase response in the transfer function of the UWB link [4]. Fig. 2a shows the simulation results with Zeland IE3D [5] for the input impedance and the realised gain of the proposed scavenging antenna. As shown, the antenna shows an impedance of Z ANT ¼ 1 + j82V and a return loss better than 13 dB at the centre frequency, relative to the chip impedance of Z c ¼ 0.65 2 j82V. It has an impedance band- width of BW imp ¼ 38 MHz ¼ 0.66%, and a realised gain of 25.4 dBi at the centre frequency with BW 23dB,Gain ¼ 128 MHz ¼ 2.2%. In addition, for the UWB antenna the simulation results for the transfer function S 21 are presented in Fig. 2b. The realised gain for this antenna in the desired bandwidth is better than 211 dBi. frequency, GHz 5.0 frequency, GHz 4.5 –100 –10 –5 0 impedance, W 0 100 200 300 400 real imag. # X Y 1 5.8 82.3141 1 5.8 1.06118 5.0 5.5 6.0 frequency, GHz 5.5 6.0 6.5 7.0 5.5 6.0 –25 –20 –15 dB or dBi –10 –5 0 –95 –90 –85 –80 dB phase, deg –75 –70 –65 –60 –55 5.1 5.2 5.3 5.4 5.5 5.6 5.7 45 cm dB[s(2,1)] dB[s(1,1)] a b phase[s(2,1)] 5.8 5.9 6.0 –180 –135 –90 –45 x z y 45 0 90 135 180 CMG CMF Fig. 2 Simulation results for proposed antennas a Scavenging antenna Left: real and imaginary parts of input impedance against frequency (insert is reﬂection coefﬁcient against frequency); right: CMG and CMF against frequency b UWB antenna transfer function setup and simulation results for amplitude and phase against frequency Constructions and measurements: The proposed antenna set has been fabricated on a Rogers RT/Duroid 5880 substrate with a relative permit- tivity of 2.2 and a thickness of 0.78 mm. Fig. 3a shows the constructed antenna with a mounted RFID chip. To measure the read range of the scavenging antenna, a measurement setup as shown in Fig. 3b was installed in an anechoic chamber. A horn antenna with a signal generator and a power ampliﬁer mimic the tag reader by radiating the required RF energy around +36 dBm to the tag. The RFID chip with the scavenging antenna is placed on a wooden holder in the far-ﬁeld region of the horn antenna. The DC voltage at the output of the rectiﬁer circuit, which was wire-bonded to the terminals of the UWB antenna, was monitored with a digital volt meter. During the measurement the radiated power and the physical distance between the RFID and the transmit (Tx) antennas are changed. The read range is also sensitive to the tag orientation and to the material the tag is placed upon. Therefore, the polarisations of the tag antenna and the Tx antenna were matched during the measure- ments. Fig. 3c shows the measurement results of the output DC voltage against the transmit power for the scavenging antenna. For this graph, the tag was ﬁxed at 50 cm away from the Tx horn antenna and the Tx output EIRP was increased from 25 dBm to 40 dBm at 5.8 GHz while the DC voltage on the output of the scavenging circuit was recorded. As shown, for the power-on edge point of the chip, that is around 1.8 V, the required Tx power is +37.8 dBm. Therefore, the exact read range for a power of +36 dBm can be calculated to be 42 cm. This read range is equivalent to the realised gain of 26.8 dBi. ELECTRONICS LETTERS 20th January 2011 Vol. 47 No. 2