II. STANDARD LPDA ANTENNA

In the proposed design, the operating frequency of 5.8 GHz is selected, which is defined as backscatter frequency in RFID systems. Therefore, this frequency can be used in readers and tags (e.g. semi-active or passive). The SHF band is desired, due to its highest data-transfer rate and its ability to be used for long range between the tag and the reader. Additionally, the antenna size is smaller in this band compared with other authorized frequency bands for RFID application [Reference Karmakar4, Reference Chen5].

Considering the existing trade-off between gain and antenna size along with the requirement of compactness in RFID systems, 7 dB directivity is chosen for this design. To achieve 7 dB directivity from a standard LPDA antenna, scaling factor (τ) and spacing factor (σ) were determined by Carrel's table [Reference Balanis21]. The obtained values are: τ = 0.8 and σ = 0.14, which give the apex angle of 19.56° by

(1)$$\alpha = \mathop {\tan ^ {-1}} \Big [\displaystyle{{1 - \tau} \over {4\sigma}} \Big] = 19.65^ \circ. $$

The lengths of the longest and shortest dipole elements in LPDA antennas are determined according to the following bandwidths. Desired bandwidth (B), active region bandwidth (Bar), and designed bandwidth (Bs), which are calculated by (2)–(4).

(2)$$B = \displaystyle{{f_{max}} \over {f_{min}}} = 1.52,$$

(3)$$B_{ar} = 1.1 + 7.7(1 - \tau )^2\displaystyle{{4\sigma} \over {1 - \tau}} = 1.97,$$

(4)$$B_S = B \times B_{ar} = 3,$$

where f _min and f _max in (2) are the lower and upper operating frequencies of the desired bandwidth, respectively. The number of LPDA antenna dipole elements (N) are then determined by

(5)$$N = 1 + \displaystyle{{{\rm log} \; (B_S)} \over {{\rm log} \; (1/\tau )}} = 6.$$

The length of first dipole element (L ₁) in the LPDA antenna is calculated by (6), and then other five series dipole lengths are determined by (7) and tabulated in Table 1.

(6)$$L_1 = \displaystyle{c \over {f_{min}}},$$

(7)$$L_{n + 1} = \tau \times L_n,$$

where c is the free-space velocity of light and (L _n) is the length of the nth element. Separation between every adjacent element (d _n) is yielded by

(8)$$d_n = 2 \times \sigma \times L_n.$$

Table 1.

Dimensions of the standard LPDA antenna for τ = 0.8, σ = 0.14.

All design parameters for the standard LPDA antenna operating at 5.8 GHz are given in Table 1. The classical LPDA antenna, shown in Fig. 1, is printed on a RO4003 slab with thickness of 1.6 mm. A 50 Ω SubMiniature version A (SMA) connector is used to feed the antenna. Simulation of the standard LPDA antenna was carried out in High Frequency Structure Simulator (HFSS) and predicted results are summarized in Table 2.

Fig. 1.

Geometry of the standard LPDA antenna.

Table 2.

Results of the standard LPDA antenna.

III. PROPOSED ANTENNA

In Section II, the initial LPDA antenna operating at 5.8 GHz for RFID application was designed. Despite the straightforward design methodology, the presented conventional LPDA antenna fails to perfectly satisfy all RFID requirements. The structure is relatively big and the F/B ratio is 17.1 dB, which can still be improved. The input impedance is also not perfectly matched (S ₁₁ = −15.5 dB). Therefore, to improve the antenna performance and make it quite practical for RFID uses, three techniques are proposed as follows:

1. (1) Miniaturization: introduction of the dynamic scaling factor (τ _n) and a constant separation distance for the purpose of miniaturization.

2. (2) Folding technique: introduction of a folding technique to increase the F/B ratio of an antenna.

3. (3) Coupling feed mechanism: introduction of a proximity coupling technique to optimize the input impedance matching.

A) Miniaturization

In order to miniaturize the conventional LPDA antenna designed in Section II, a constant separation between every two adjacent dipole elements is proposed. In other words, instead of using (8) to calculate the gap between dipoles, a constant distance of G = 1 mm is considered between dipole elements in the proposed LPDA antenna configuration. This re-arrangement reduces the boom length of the antenna by 51.5%, resulting in shifting operating frequency to a higher frequency. To re-tune the centre frequency, a dynamic scaling factor (τ _n), as opposed to constant scaling factor in the conventional LPDA antenna, is proposed. More precisely, variable scaling factors are used to achieve the required bandwidth and gain. The dipoles lengths are then updated by

(9)$$L_{n + 1} = \tau _n \times L_n.$$

The chosen scaling factors for dipoles are tabulated in Table 3 as following;

Table 3.

Scaling factors (τ _n) used between the adjacent elements.

B) Folding technique

A novel approach, named folding technique, is proposed to improve F/B ratio, which is a very important index for directional antennas. Different passive and active methods have been proposed to improve this index [Reference Huang and Hsu22–Reference Cheng, Ding, Wang and Shao24]. The introduction of reflectors at lower frequency bands and directors at higher frequency bands was investigated in [Reference Huang and Hsu22] for the F/B ratio enhancement. The well-known split ring resonators and complementary split ring resonators were proposed to be employed as radiating elements to improve the F/B ratio [Reference Kim, Yoon, Yoon, Lee and So23]. In a different approach, application of active devices was also proposed for such a purpose [Reference Cheng, Ding, Wang and Shao24]. Here, we propose a passive simple technique which neither enlarges the antenna nor needs defecting the ground of the antenna. In our technique, the antenna is folded relative to the x-axis, as depicted in Fig. 2, improving the F/B ratio from 17.1 to 21.5 dB at the resonance frequency (5.8 GHz). This enhancement versus frequency is later shown in Section IV, where the results will be discussed.

Fig. 2.

Proposed antenna. (a) Layout of the antenna. (b) Photograph of the antenna.

C) Coupling feed mechanism

One of the main challenges with the conventional printed-LPDA antennas is to achieve a good impedance matching without sacrificing the compactness and the overall performance of the antenna. Baluns can be used for a better impedance matching at the price of larger size [Reference Zhang, Wu, Liu, Yu and Chen25–Reference Zhang and Zhihong27]. Coaxial cables were also mostly used in wideband applications to feed these types of antennas [Reference Chauloux, Colombel, Himdi, Lasserre and Pouliguen28, Reference Bishop, Miller, Zeppettella, Baron, Tuss and Ali29]. A coupling method using multi-substrates was also utilized in [Reference Amini and Oraizi30]. Here, we propose a simple feeding system which not only increases the size and complexity of the antenna, but also improves the impedance matching in terms of bandwidth and level. In our feeding system, a coupled feed with an open stub, as shown in Fig. 2(a), are used to provide more flexibility for a better impedance matching. The input reflection coefficient is dominantly controlled by the length of the coupling path (Lin). Smith chart in Fig. 3 shows the input impedance values of the RFID antenna for different lengths of Lin in a frequency range of 5.5–6.1 GHz.

Fig. 3.

Input impedance of the proposed antenna for frequency swept from 5.5 to 6.1 GHz versus coupling path length (Lin).

Based on Smith chart results, shown in Fig. 3, the best impedance matching close to 50 Ω at the operating frequency of the LPDA antenna (5.8 GHz) is achieved for Lin = 5.2 mm. The recorded impedance value from SMD port (Zant) is equal to 52−j1.4.

The geometrical parameters of the final LPDA antenna are given in Table 4. The layout and the fabricated prototype of the RFID antenna are shown in Figs 2(a) and 2(b), respectively. The antenna was fabricated on a grounded RO4003 substrate (εr = 3.38, δ = 0.002), with the dielectric and metal thicknesses of 1.6, 0.035 mm, respectively. The overall dimension of the substrate is 30 × 45 mm² (0.58λ0 × 0.87λ0).

Table 4.

Dimensions of the proposed antenna.

IV. DISCUSSION AND RESULTS

All simulations were carried out by Ansoft HFSS 14 software using Finite Element Method (FEM) analysis. Measured and simulated reflection coefficients are shown in Fig. 4. The measured S ₁₁ at the operating frequency of the RFID antenna is −28 dB with 3.4% 10-dB fractional bandwidth, showing an excellent input impedance matching. It can be seen from this figure that the return loss at 5.8 GHz has been increased by 12.5 dB due to the application of coupled feed. Similarly, The 10-dB return loss fractional bandwidth has also increased from 1 to 3.4%. Figure 5 compares F/B ratio of the designed RFID antennas for the standard and proposed cases. The folding technique has improved F/B ratio of the antenna from 17.1 to 21.5 dB, which is significantly higher compared with the other presented works [Reference Hsu and Huang8, Reference Syed and Neto31, Reference Estrada, Paez and Fajardo32].

Fig. 4.

The input reflection coefficient of the standard LPDA and the proposed antenna.

Fig. 5.

F/B ratio of the standard LPDA and the proposed antenna. The maximum value of 21.5 dB is obtained at the frequency of 5.8 GHz.

Measured radiation efficiency of the prototype at 5.8 GHz is 90%, while this is 85% for the standard case, as shown in Fig. 6. Far-field radiation patterns (gain) of the antenna in two planes at 5.8 GHz are shown in Fig. 7, exhibiting very good agreement between simulated and measured results in both planes. The measured gain at 5.8 GHz is 6.2 dBi. Also, the 3 dB beamwidth for the X–Z plane is 101° and for the Y–Z plane is 89°, showing a good directional characteristic. A performance comparison is made in Table 5 between the proposed antenna and some other works.

Fig. 6.

Antenna efficiency versus frequency of the standard LPDA and the proposed antenna.

Fig. 7.

Simulated and measured far-field radiation patterns of the standard LPDA and the proposed antenna at 5.8 GHz.

Table 5.

Performance comparison of the proposed RFID antenna and some of the latest previous works.