Antenna design

Figure 1 represents the multiband antenna which is designed and fabricated on Rogers RTDuroid5880 substrate with permittivity ε _r = 2.2, tanδ = 0.0009. Figure 1(a) shows a tilted view of the proposed multiband antenna with a compact dimension of W mm × L mm × h mm. The radiating patch is fed by microstrip which is connected to 50 Ω SMA connector on one face of the substrate and the other side is printed with the ground plane. Figure 1(b) shows the radiating patch which is connected to four-quarter wavelength stubs intended to resonate at 1.9, 2.4, 3.5, and 5.5 GHz. The radiating patch is also etched by an inverted C-shaped slot which is responsible for a resonant frequency of 7.5 GHz intended for Downlink satellite system. The ground plane consists of two rectangular stubs and chamfered ground edges for better impedance matching of the final version of the multiband antenna. All the parameters are optimized by HFSS simulator and are given below in Table 1.

Fig. 1. (a) Tilted view of the proposed circularly polarized multiband antenna. (b) Front view.

Table 1. Optimized parameters of the proposed antenna

Figure 2 shows the design steps of the proposed antenna working in a single band. Figure 2(a) represents the radiating antenna with a single ellipse of radius R = 7.00 mm and an eccentricity ratio of 0.67. VSWR shows Antenna 1 resonating at 8.25 GHz with a bandwidth covering 7.17–9.59 GHz represented in Fig. 2(c). In order to improve the resonating frequency, an ellipse of radius R with an eccentricity ratio of 0.67 is embedded on antenna 1 as shown in Fig. 2(b) with a new resonating frequency centered at 4.11 GHz and covering a bandwidth of 3.16–5.25 GHz.

Fig. 2. (a) Step 1. (b) Step 2. (c) VSWR.

Figure 3 shows the antenna configuration which is developed to obtain five bands of operation and also illustrates the respective VSWR. Antenna 1 depicts a basic monopole antenna resonating at 4.10 GHz with a return loss of −25.96 and a covering bandwidth of 3.16–5.25 GHz. To obtain the first band of operation working in WWAN as shown in Fig. 3(b), a quarter wavelength stub is attached which resonates at 1.91 GHz covering a bandwidth of 1.73–2.02 GHz with a corresponding return loss of −18.29. The stub length is calculated by L _WWAN = W ₁ + L ₁ + W ₂. Furthermore, to obtain a dual band of operation, Antenna 2 is modified to Antenna 3 represented by Fig. 3(c) which is obtained by adding a quarter wavelength corresponding to the application for Bluetooth which is calculated by L _Bluetooth = W ₃ + L ₂. Antenna 4, shown in Fig. 3(d), corresponds to tri band of operation which includes WWAN, Bluetooth, and WiMAX band and a corresponding quarter wavelength stub is calculated by L _WiMAX = W ₄ + L ₃. Figure 3(e) represents the conversion of Antenna 4 to Antenna 5 with an additional band working in WLAN which is obtained by embedding the quarter wavelength stub given by L _WLAN = W ₅ + L ₄.

Fig. 3. (a) Antenna 1. (b) Antenna 2. (c) Antenna 3. (d) Antenna 4. (e) Antenna 5. (f) Antenna 6. (g) Antenna 7. (h) VSWR.

A final version of the proposed antenna is shown in Fig. 3(f). Downlink satellite system band is obtained by inserting a C-shaped slot on Antenna 6 which resonates at 7.4 GHz with a return loss of −13.57 and a corresponding slot length is calculated by L _{Downlink Satellite} = W ₆ + 2L ₅. Antenna 7 shown in Fig. 3(g) is the improved version of Antenna 6 which is obtained by chamfering the edges of the ground plane and inserting two stubs of dimensions W ₇ × L ₆ mm² and W ₈ × L ₇ mm². Design steps with bandwidth coverage and respective return loss are tabulated in Table 2 which is given below.

Table 2. Antenna configuration with respect to bandwidth and return loss (Antenna 2 to Antenna 7)

Optimization of key parameters, surface current density

Distribution and axial ratio

Optimization of key parameters leading to change in performance characteristics affecting various parameters such as matching of input impedance is carried out by EM simulator (Full Wave Ansyss HFSS simulator). Shifting of resonating bands either on lower or higher frequency side is due to a variation of the physical length of the stubs and slots which in turn also affects the bandwidth. The said analysis is discussed below:

Figure 4 represents the optimization of key parameters for the proposed five-band antenna and analysis of axial ratio (AR). Figure 4(a) depicts the optimization of W ₂ for WWAN wireless application. Variation of W ₂ from 5.50 to 6.50 mm suggests shifting of resonance frequency from 2.15 GHz (return loss = −57.26) to 1.44 GHz (return loss = −32.57). However, for W ₂ = 6.00 mm, required bandwidth (1.67–2.01 GHz) is obtained with a maximum return loss of −48.36 at a resonance frequency of 1.90 GHz. Optimization for Bluetooth application band is shown in Fig. 4(b) which confirms that a variation of L ₂ from 15.5 to 16.5 mm leads to shifting of resonance frequency from 2.85 GHz (return loss = −38.02) to 2.40 GHz (return loss = −32.11). By fixing L ₂ = 16.00 mm, required bandwidth for Bluetooth (2.39–2.67 GHz) application is obtained with resonance frequency centered at 2.51 GHz (return loss = −36.98). Figure 4(c) shows the variation of L ₃ from 9.50 to 10.50 mm with resonance frequency also varying from 3.75 GHz (return loss = −32.15) to 3.28 GHz (return loss = −17.82), respectively. The optimized value for L ₃ corresponds to 10.00 mm with bandwidth (3.26–3.75 GHz) centered at 3.51 GHz including return loss (−23.03). Figure 4(d) represents the optimization for WLAN which corresponds to the stub length L ₄. Variation of L4 from 3.50 to 4.50 mm leads to shifting of resonance frequency from 6.10 GHz (return loss = −55.11) to 5.28 GHz (return loss = −26.03) and the optimized value for L ₄ = 4.00 mm achieves WLAN band (4.95–5.86 GHz) with resonance frequency centered at 5.54 GHz (return loss = −39.59). Figure 4(e) shows the optimization graph for Downlink Satellite system. Variation of L ₅ from 2.50 to 3.50 mm achieves shifting of resonant frequency from 6.95 to 7.80 GHz. For L ₅ = 3.00 mm, a necessary bandwidth (6.97–7.67 GHz) is obtained with a return loss equal to −25.98 corresponding to 7.35 GHz resonance frequency.

Fig. 4. Optimization of parameter (a) W₂ (WWAN), (b) L₂ (Bluetooth), (c) L₃ (WiMAX), (d) L₄ (WLAN), (e) L₅ (Downlink Satellite), (f) axial ratio.

A patch antenna can produce a circular polarization so long as two perpendicular modes are excited simultaneously with the same amplitudes and 90°' phase difference. In order to show the functionality of an antenna in circular polarization, the parameter of 3 dB AR is used. The AR of the proposed antenna is shown in Fig. 4(f). The 3 dB bandwidth of the proposed antenna is in 1.93 GHz positioned between 1.70 and 1.97 GHz. Linear polarized antenna working in different bands which can radiate equally around it in the horizontal plane and also enables a large proportion of their power radiated near the surface of the earth. This type of polarization finds its applications with the use of automobiles. Whereas, a handheld transmitter which finds movement around a room/warehouse where antenna often oriented in different degrees off axis, it becomes necessary for the fixed antenna to radiate hemispherical-shaped pattern and hence circular polarization plays a vital role with advantages such as combating the effect of multipath interference or fading of received signals. The use of CP antenna is advantageous as it can launch and receive CP electromagnetic waves and is relatively less sensitive to their respective orientations. The CP monopole antenna can be widely used in many aspects such as radar, navigation, electronic countermeasure system, and medical application. WWAN bands is intended to cover applications such as Digital Cellular System (1.71–1.88 GHz), Personal Communication System (1.85–1.99 GHz), and Universal Mobile Telecommunication system2100 (1920–2170 MHz) which needs to receive signals in all the directions and hence the antenna needs to be CP. The proposed antenna is CP for above said applications as AR is near to 1 dB. For remaining applications, including Bluetooth, WiMAX, WLAN, and X-Band Downlink system, the proposed antenna fulfills the requirement of linear polarization.

Antenna analysis can be further characterized by studying the surface current density distribution which is shown in Fig. 5. Multiband performance is defined in terms of current distributions of the proposed multiband antenna at the frequencies of 1.92, 2.41, 3.54, 5.55, and 7.50 GHz as shown in Figs 5(a) and 5(e), respectively. The current surfaces around the stubs at the frequencies of 1.92 and 2.41 GHz are more vivid. The current distributions of the proposed antenna at the frequencies of 3.54, 5.55, and 7.50 GHz are simulated and shown in Figs 5(c) and 5(e). As can be clearly seen, the antenna radiation is affected more at the abovementioned frequencies and the surface currents are more visible in the vicinity of both slots and stubs, and accordingly, the effects mentioned above show that the proposed antenna operates in multi bands.

Fig. 5. Surface current distribution at (a) 1.92 GHz, (b) 2.41 GHz, (c) 3.54 GHz, (d) 5.55 GHz, (e) 7.50 GHz.

Results and discussion

Figure 6(a) shows the comparison simulated result with measures and Fig. 6(b) shows the fabricated prototype. Impedance mismatch between the feedline and its relative impedance (SMA connector) leads to dissipation of power which is characterized in terms of return loss which is positive which means more and positive [Reference Trevor18] leads to better impedance matching. A prototype is developed and fabricated on Rogers RTDuroid5880 substrate. This leads to simulated and fabricated results and are compared as shown in Fig. 6(a).

Fig. 6. (a) Return loss comparison. (b) Fabricated prototype. (c) Peak gain (dBi).

Excited proposed antenna shows multi-resonance characteristics resonating at 1.93, 2.51, 3.47, 5.52, and 7.43 GHz resulting five bands of operation. Lower frequency band resonating at 1.93 GHz covers the bandwidth of 1.67–2.13 GHz which also exhibits circular polarization. Also antenna resonating at 2.51, 3.47, 5.52, and 7.43 GHz covers the measured bandwidth of 2.36–2.70, 3.20–3.90, 5.10–6.00, and 0.10–7.90 GHz. Dissimilarity between simulated and measured results may be an error in the fabrication and soldering processes.

Figure 6(c) shows the peak gain comparison of simulated and measured results which are obtained for resonating frequencies of the proposed antenna 1.88, 2.55, 3.49, 5.55, and 7.51 GHz which corresponds to the peak gain of 7.22, 5.11, 3.30, 5.02, and 6.02 dBi. Gain at 1.88 GHz is high compared to other resonating frequencies as the addition of two stubs in the ground plane improves impedance matching resulting in higher gain at lowest resonating band.

Radiation patterns are used to signify how field power of an antenna is radiated from a fixed direction. Radiation pattern in E- and H-planes with co- and cross-polarization is shown in Fig. 7 with an operating frequency of 1.92, 2.41, 3.54, 5.55, and 7.50 GHz. It can be observed that the radiation pattern of the proposed antenna has less cross-polarization interference with respect to resonating frequencies in the operating band. The proposed antenna offers less cross-polarization interference which is due to thickness (h = 0.787 mm) and dielectric constant (ε _r = 2.2) of microwave substrate. Also, the reduced size of the ground plane (W _g × L _g = 20 × 3 mm²) ensures the less cross-polarization interference.

Fig. 7. Measured radiation pattern of E- and H-planes in dB at (a) 1.92 GHz, (b) 2.41 GHz, (c) 3.54 GHz, (d) 5.55 GHz, (d) 7.50 GHz.

Table 3 shows the comparison of earlier published work with the proposed work. As compared with reference [Reference Pedram, Nourinia, Ghobadi and Karamirad12] which covers five bands of operation, the proposed antenna size is reduced by 12% and also the gain of the proposed antenna is 7.22 dBi. All the above discussed advantages of the proposed antenna make it more suitable for different wireless applications.

Table 3. Comparison of the proposed antenna parameters with several existing antennas

* P – Proposed Antenna.

Bold signifies Maximum Gain, Number of Separated Bands, Size, Bands of proposed antenna so as to distinguish from other published works. *Signifies Proposed work.