II. CPW-FED BROADBAND ANTENNA ON FR4

Figure 1(a) shows the conventional CPW-fed circular patch antenna without DGS and Fig. 1(b) depicts the geometry of the proposed CPW-fed circular patch antenna with DGS. These patch antennas have equal cross-sectional areas. The width of the central strip of CPW antennas is of 3 mm, and the gap between the central strip and ground planes is 0.5 mm. There are two symmetric ground planes with dimensions of 7 mm × 8 mm lying on both side of the central strip. Figure 1(b) shows antenna with DGS of two circular slots etched of radius, “r ₁ = 2.5 mm” each, in order to enhance the gain of the antenna. Since the antenna is made to design for higher frequencies for wireless and X-band applications, the size of the antenna is kept compact enough to fit the portable devices. The antenna is also fabricated on low cost “FR4” (flame retardant woven glass reinforced epoxy resin) substrate, which makes it cost-effective for various users.

Fig. 1. The geometric design of antennas (a) conventional CPW-fed circular patch antenna without DGS (b) proposed CPW-fed circular patch antenna with DGS.

The optimized design parameters obtained for the proposed patch antenna are L = 18 mm, W = 20 mm, g = 0.5 mm, W _f = 3 mm, W _g = 8 mm, L _g = 7 mm, L _f = 7.23 mm, r _c = 5 mm, r ₁ = r ₂ = 2.5 mm, and r = 2 mm. Dielectric substrate (FR4 epoxy) has relative dielectric constant ε_r of 4.4, loss tangent of 0.02, and thickness “h” of 1.6 mm.

These antennas have a single-layer metallic structure on one side of the substrate (top) whereas the other side (bottom) is without any metallization. The circular disc is fed via CPW feed, which in turn is connected through a standard 50-Ω Sub Miniature Version A Connectors (SMA) connector. Figure 2 shows the fabricated patch antennas.

Fig. 2. The fabricated prototypes of antennas (a) conventional CPW fed circular patch antenna without DGS (b) proposed CPW-fed circular patch antenna with DGS.

III. DESIGN OF CIRCULAR MONOPOLE ANTENNA

The lower band edge frequency of a conventional circular printed monopole antenna with radius “r” can be evaluated from following equation [Reference Ray16]:

(1)$$f_{\rm l}=\displaystyle{{72} \over {\left({2r+{r \over 4}+p} \right)y}}{\rm GHz}.$$

Here, “y” is a factor accounting the role of dielectric material in reducing the lower band edge frequency by increasing the effective dimensions of the antenna and “p” is the gap between the circle and the ground plane, which is p = 0.23 mm in our case and y = 1.15 (for FR4). The lower band edge frequency with our design for r _c = 5 mm comes out 5.45 GHz, which is near by the simulated and measured values.

The characteristic impedance of CPW line with finite-width ground plane on the FR4 dielectric substrate of height “h _d” (see Fig. 3) is given by Simons [Reference Simons17]:

(2)$$Z_o=\displaystyle{{30\pi } \over {\sqrt {\varepsilon _{re} } }}\; \, \displaystyle{{K\lpar k\rpar } \over {K\lpar k^{\prime}\rpar }}\Omega \;.$$

Here, “K” is the complete elliptical integrals of first kind. The arguments k and k′ are dependent on the geometry of the CPW line

(3)$$k=\displaystyle{c \over b}\sqrt {\displaystyle{{b^2 - a^2 } \over {c^2 - a^2 }}} \, {\rm and}\, k^{\prime}=\sqrt {1 - k^2 }=\displaystyle{a \over b}\sqrt {\displaystyle{{c^2 - b^2 } \over {c^2 - a^2 }}}$$

and the effective dielectric constant (ε _re) of the CPW line is given as

(4)$$\varepsilon _{re}=1+\displaystyle{1 \over 2}\lpar \varepsilon _r - 1\rpar \; \displaystyle{{K\lpar k\rpar } \over {K\lpar k^{\prime}\rpar }}\displaystyle{{K\lpar k^{\prime}_1 \rpar } \over {K\lpar k_1 \rpar }}.$$

Fig. 3. The CPW with finite dielectric height (h _d) and finite ground planes.

Here,

(5)$$k_1=\displaystyle{{{\rm sinh}\left({\displaystyle{{\pi c} \over {2h_d }}} \right)} \over {{\rm sinh}\left({\displaystyle{{\pi b} \over {2h_d }}} \right)}}\; \sqrt {\displaystyle{{{\rm sinh}^2 \left({\displaystyle{{\pi b} \over {2h_d }}} \right)- {\rm sinh}^2 \left({\displaystyle{{\pi a} \over {2h_d }}} \right)} \over {\sinh ^2 \left({\displaystyle{{\pi c} \over {2h_d }}} \right)- {\rm sinh}^2 \left({\displaystyle{{\pi a} \over {2h_d }}} \right)}}}\comma$$

(6)$$\eqalign{k^{\prime}_1 &=\sqrt {1 - k^{\prime}_1 \; } \cr & =\displaystyle{{{\rm sinh}\left({\displaystyle{{\pi a} \over {2h_d }}} \right)} \over {{\rm sinh}\left({\displaystyle{{\pi b} \over {2h_d }}} \right)}}\; \sqrt {\displaystyle{{{\rm sinh}^2 \left({\displaystyle{{\pi c} \over {2h_d }}} \right)- {\rm sinh}^2 \left({\displaystyle{{\pi b} \over {2h_d }}} \right)} \over {\sinh ^2 \left({\displaystyle{{\pi c} \over {2h_d }}} \right)- {\rm sinh}^2 \left({\displaystyle{{\pi a} \over {2h_d }}} \right)}}}.}$$

These equations are used to ascertain the physical parameters of the proposed CPW designs and designed antennas are simulated using HFSS EM simulation software and results were verified in CST MWS. The CPW central metal strip width, length, and the gap (g) of the distance between the strip and the coplanar ground plane are fixed at 3, 7.23, and 0.5 mm, respectively, in order to achieve 50-Ω CPW feed line. The designed antenna with DGS has the circular patch of radius r _c = 5 mm, which has a circular slot of radius 2 mm in the lower side of circular patch to enhance impedance bandwidth.

IV. RESULTS AND DISCUSSIONS

The broadband CPW-fed antennas with and without DGS are designed and simulated on HFSS and CST MWS. The fabricated antennas are characterized with Rohde and Schwarz vector network analyzer ZVA40 for reflection coefficient versus frequency, which has been compared with the simulated results in Fig. 4.

Fig. 4. Simulated and measured reflection coefficients of proposed antennas.

The simulated results of reflection coefficient versus frequency are in good agreement. The conventional CPW-fed antenna has huge impedance bandwidth but the gain is not significant for higher frequencies above 13 GHz. In the proposed antenna, we have introduced DGS in symmetrical ground planes. This proposed antenna as compared to conventional antenna achieves a significant impedance bandwidth of 100% ranging from 5 to 15 GHz with respect to the central frequency at 10 GHz. In literatures, we do not find improvement in radiation pattern characteristics in 12–15 GHz band for a compact design, as reported in this paper, as most of the works are limited to Ultra Wide Band (UWB) regions. The compact size and achieved broadband from 5 to 15 GHz make these antennas a good candidate for upper UWB applications and futuristic UWB as well.

We find a slight variation between the simulated and experimentally measured results at higher frequencies. This can be due to the dielectric losses introduced at high frequencies or uncertainty of the purchased dielectric constant of the substrate. In order to achieve a good impedance matching we need to optimize the gap between the radiating circular patch and ground plane [Reference Ray16]. Hence, we start analysis with varying length of the ground planes “L _g” keeping all the parameters same. Figure 5 shows the effect of ground plane length on the proposed antenna with DGS. As per the simulated results most desired wideband is obtained at L _g = 7 mm. For lower values lesser than 7 mm the antenna is not able to radiate properly for frequencies below 5 GHz and gives poor impedance matching. For value of L _g >7 mm, i.e. 7.2 mm as shown in Figure 5, the antenna has no significant value of S ₁₁. The parametric analysis is performed on the radii of the circular slots r ₁ (see Fig. 6), which acts as DGS and change the current distribution in the ground plane of the patch antenna. This change in surface current distribution increases the gain of antenna at higher frequencies. From Fig. 7, it is found that optimum radius of the circular DGS is 2.5 mm, which gives broad impedance bandwidth as compared to 2 mm, whereas on increasing the value by 0.5 mm makes antenna non-resonating for 9.8–10.2 GHz band.

Fig. 5. Effect of variation of length of ground plane.

Fig. 6. Effect of the variation of radius of the circular slot “r ₁” on ground plane (radius of DGS).

Fig. 7. Simulated result of the proposed antenna with various radii of main circular patch “r _c” and “r”.

Figure 7 shows the results of parametric analysis performance on the radii “r” and “r _c” to improve reflection coefficient versus frequency characteristics around 10 GHz. It is observed that when the radius of the circular slot “r” is 1 mm the impedance matching is poor and improves when it is increased to 2 mm. We have also found that the value of “r _c” below 5 mm results in decrease in impedance bandwidth, while increasing its value for “r _c” to 5.2 mm, results in poor impedance matching. Hence, we have opted the values of “r _c” = 5 mm and “r” = 2 mm.

We have also analyzed the current distribution in the antenna at various frequencies, as shown in Fig. 8. The current value is high along central metallic strip of the CPW feed and lower edge of the main circular patch “r _c” of the proposed antennas. In the ground planes, the current value is more on the upper edge near the circular patch “r _c” (along the x-direction). This explains that the impedance matching of the antenna is also dependent on gap between CPW central feed strip and ground plane as well as gap between the upper edge of ground plane and patch. These gaps are very important parameter for proper signal coupling from feed line to patch. This explains the importance of optimized dimensions of the ground plane [Reference Ray16].

Fig. 8. Simulated current distributions of the proposed antennas at various frequencies: (a) antenna without DGS, (b) antenna with DGS.

Figure 9 shows the radiation patterns of the proposed antennas (CPW-fed antenna with and without DGS) at selective frequencies of 5, 8, 10.5, and 15 GHz, respectively.

Fig. 9. Stimulated radiation patterns of the proposed antennas (x–z plane, omnidirectional-H plane, and bidirectional-E plane) (a) without DGS (b) with DGS at four different frequencies.

From the radiation patterns shown in Figure 9, we see that gain and radiation characteristics of the proposed antenna with DGS have improved considerably as compared to conventional antenna (without DGS) at 15 GHz. The gain has increased to 7 dB and the H-plane radiation pattern has become nearly omnidirectional. At higher frequency, the ripple occurs in radiation pattern due to edge reflections.

It is noted that the radiation patterns of the antenna with DGS in the H-plane (x–z plane) are nearly omnidirectional in entire band of 5–15 GHz and radiation patterns in the E-plane (y–z plane) resembles figure of eight and are bidirectional. Table 1 shows the comparison between peak gains of the conventional CPW-fed patch antenna and antenna with DGS at higher frequencies from 12 to 15 GHz. A significant improvement in gain is noticed with antenna having DGS. This is due to reduction of surface waves at these higher frequencies. Figure 10 shows the peak gain of the proposed antennas for the frequency band 5–15 GHz. Maximum gain of the antenna with DGS is 3.88 dB at 7 GHz. It can be seen that antenna with DGS has stable radiation patterns (see Figure 9) and achieves improved gain at higher frequencies (see Figure 10). The gain of the antenna is significantly high for practical applications.

Fig. 10. Peak gain of the proposed antennas.

Table 1. The peak gains of the proposed antennas at higher frequencies.