II. PROPOSED ANTENNA CONFIGURATION

Figure 1 shows the geometry and prototype of the asymmetric coplanar inverted L-fed proposed antenna having length Ls and width Lsw. The feed of proposed antenna is designed using standard design equations of ACS [Reference Simons16, Reference Garg, Bhartia and Bahl20]. The ground plane dimensions of the antenna are optimized by iteration for good impedance matching. The antenna is designed and printed on an FR4 epoxy substrate having dielectric constant 4.4 and thickness 0.8 mm. It has to be noted that the overall antenna dimensions (Table 1) in terms of area is greatly reduced in the case of the proposed antenna using the ACS feed since it uses only a single lateral ground plane.

Fig. 1. (a) Geometry of proposed antenna and its prototype (b) side view.

The percentage reduction of area is almost about 50% when compares with similar CPW-fed antennas because it is using single lateral ground plane comparing with twin lateral ground plane in the CPW. So ACS-fed antennas are the promising candidates for the future generation antennas.

Figure. 2 shows the design evolution of the proposed antenna. Initially started from a CPW-fed C-shaped strip-fed geometry and finally we arrived on the proposed ACS L-fed design with miniature size. Figure 2 shows the comparison of return loss characteristics of the proposed antenna with its possible variants. From Fig. 2 we can identify the changes and size reduction when the design development phase of the proposed antenna.

Fig. 2. (a) Design evolution of proposed antenna, (b) simulated return loss results.

The design equations for the perfect matching of impedance are given below:

(1)$$Z_o=\displaystyle{{60\pi } \over {\sqrt {\varepsilon _{eff} } }}\displaystyle{{K\lpar k\rpar } \over {K\lpar k^1 \rpar }}\comma \; $$

where from Fig. 1(b)

$$\eqalign{& k=\displaystyle{a \over b}\comma \; \cr & k^1=\sqrt {1 - k^2\comma \; } }$$

and K(k)/K(k ¹) is the elliptical integral of first kind which is given by

(2)$$\displaystyle{{K\lpar k\rpar } \over {K\lpar k^1 \rpar }}=\left\{{\matrix{ {\displaystyle{\pi \over {\ln \displaystyle{{2\left({1+\sqrt {k^1 } } \right)} \over {\left({1 - \sqrt {k^1 } } \right)}}}}} & {0 \leq k \leq \displaystyle{1 \over {\sqrt 2 }}\comma \; } \cr {\displaystyle{1 \over {\pi \ln \displaystyle{{2\left({1+\sqrt k } \right)} \over {\left({1 - \sqrt k } \right)}}}}} & {\displaystyle{1 \over {\sqrt 2 }} \leq k \leq 1\comma \; } \cr } } \right.$$

(3)$$\varepsilon _{eff}=\displaystyle{{\varepsilon _r+1} \over 2}.$$

III. RESULTS AND DISCUSSION

The simulated, parametric study and measured results of the proposed antenna have also been observed. The measured and simulated results of return loss characteristics for optimized set of antenna parameters are presented in Fig. 3. From the simulated results it is found that the proposed antenna yield dual-band resonance centered at 2.45 and 5.8 GHz which covers IEEE 802.11 WLAN bands 2.4 GHz (2.4–2.48 GHz) and 5.8 GHz (5.725–5.825 GHz) which is useful for RFID applications. The proposed antenna fabricated on an FR4 substrate by simple chemical etching and screen-printing process. The measurement is carried out using HP8510C vector network analyzer. The measured results show good agreement of dual-band operation of the proposed antenna which ranges 2.42–2.5 GHz in lower frequency band and 5.7–5.95 GHz in upper frequency band

Fig. 3. Comparison between simulated and measured return loss.

Table 1. Parameters of the proposed antenna.

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The simulated current distribution of the proposed antenna shown in Fig. 4, which shows the current perturbation of the proposed design is more in the ground plane for lower band of resonance and in upper band of resonance current through exciting strip is prompting. The parametric analysis of the proposed antenna by varying width of the ground plane Lw and leg length of exciting strip L1 are shown in Figs. 5 and 6, respectively. The ground width has great influence on the lower frequency band 2.4 GHz. Here it is found that when the width of ground plane increases the lower resonance tends to shift and goes on decreasing. The optimum results obtained on the Lw = 4 mm. The parametric study on changing the value of leg of exciting strip L2 also carried out. It is clear that L2 has an effect on upper resonance and optimum performance obtained when L2 = 6 mm. So the parameters can be used for tuning the proposed antenna in different nearby applications

Fig. 4. Current distribution of proposed antenna (a) 2.4 GHz and (b) 5.8 GHz.

Fig. 5. Effect of ground plane width of the proposed antenna.

Fig. 6. Effect of variation on leg length (L2) of inverted L exciting strip.

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The simple RLC series lumped equivalent model for the proposed antenna for the dual band of resonance shown in Fig. 7. The basic circuit parameters are obtained from designed equations and simulated by mentor Graphics IE3D modua. This analysis carried out by large number of iterations and data fitting, finally obtained an optimized circuit parameters shown in Table 2. For obtaining the equivalent circuit extracts R and L values from IE3D modua for each resonance and cascaded to form the complete equivalent circuit. The comparison between the simulated and lumped equivalent modal of the proposed antenna shown in Fig. 8, which shows good conformity between both results

(4)$$C=\displaystyle{{5f_c } \over {\pi \left({f_c^2 - f_p^2 } \right)}}{\rm pF\comma \; }$$

(5)$$L=\displaystyle{{250} \over {C\left({\pi f_p^2 } \right)}}nH\comma \; $$

(6)$$R=\displaystyle{{2Z_0 } \over {\sqrt {\displaystyle{1 \over {S_{11} \lpar f\rpar ^2 }} - \lpar 2Z_0 \lpar fC - \displaystyle{1 \over {wL}}\rpar \rpar ^2 - 1} }}\Omega s.$$

Fig. 7. Lumped RLC equivalent model of proposed antenna.

Fig. 8. Comparison between simulated and lumped equivalent model.

Table 2. Calculated values of equivalent circuit lumped elements.

The measured radiation patterns of the E- and H-planes for 2.4 and 5.8 GHz are given in Fig. 9. The results show good and omnidirectional radiation pattern at the H-plane and bidirectional patterns at the E-plane. The small asymmetry in the patterns is because of asymmetry in the proposed antenna feeding configuration. The polarization of the antenna is also experimentally determined and it is found that antenna is polarized along the X-axis for dual band of operation. The cross-polarization level of the proposed antenna is minimum comparing with co-polarization levels

Fig. 9. Measured radiation patterns of the proposed antenna (a) E-plane at 2.4 GHz, (b) E-plane at 5.8 GHz, (c) H-plane at 2.4 GHz, and (d) H-plane at 5.8 GHz.

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The comparison between area and gain of the proposed antenna with some of the published literatures is shown in Table 3. Here it is observed that the proposed structure has prevailing profile reduction comparing with the existing ACS-fed multiband WLAN antennas. The gain characteristics also shows good agreement with existing geometries. The proposed designs yield peak gain of 1.6 dBi at lower band and 3.4 dBi at upper band of resonance, which is shown in Fig. 10.

Fig. 10. Measured and simulated gain of the proposed antenna at lower frequency band and upper frequency band.

Table 3. Comparison between size and gain of proposed and existing literatures.