An octa-band planar monopole antenna for portable communication devices

Due to the rapid development of wireless communication systems, good numbers of services and devices use different frequency bands and protocols. To concurrently cover all these services, the antenna in communication devices should operate over multiple frequency bands. The use of wide and multi-band antennas not only reduces the number of antennas necessary to cover multiple frequency bands but also lessens the system complexity, size, and costs. To operate over eight frequency bands to cover sixteen well-established narrow service bands, a planar monopole antenna is proposed for portable communication devices. The proposed antenna is comprised of an inverted F-shaped monopole patch with a rotated L-shaped strip and an F-shaped ground strip with a rotated L-shaped branch. The studied antenna can excite at multiple resonant modes which helps it to achieve eight measured operating bands of 789–921 MHz, 1367–1651 MHz, 1995–2360 MHz, 2968–3374 MHz, 3546–3707, 4091–4405 MHz, 4519–5062 MHz and 5355–6000 MHz. The achieved measured operating bands can cover sixteen popular narrow service bands for 4G/3G/2G, MWT, WiFi, WiMAX, WLAN, and sub-6 GHz 5G wireless communication system. The studied antenna achieved good gain, efficiency and exhibits stable radiation characteristics. Moreover, the antenna does not use any lumped element and left ample space for other circuitries which makes it easier to use in portable devices such as tablets, laptops, etc. with low manufacturing cost.

www.nature.com/scientificreports/ in Ref. 9 a hepta-band antenna with a size of 70 × 150 × 6 mm 3 was reported. The designed antenna consists of a typical inverted-F antenna and an asymmetric T-shaped slot and can operate over 790-990 MHz and 1650-2920 MHz bands. In Ref. 16 , a 3D loop antenna was presented for multiband smartphone applications. It consists of a loop radiator and a large system ground plane and can operate over 0.8-1.1 GHz and 1.7-2.58 GHz bands. The antennas reported in Refs. 3,6,9,16 can cover seven service bands (the GSM850, GSM900, DCS, PCS, UMTS, LTE2300, LTE2500), but WMT, WiFi, WiMAX, WLAN, and 5G bands are not covered. An octa-band antenna for LTE mobile phones was presented in Ref. 4 . The studied antenna consists of two monopole branches, two ground branches, and a matching circuit. Having a size of 79 × 142 × 7.5 mm 3 , it achieved dual impedance bands of 665-965 MHz and 1610-2800 MHz. In Ref. 7 , an octa-band monopole antenna was presented for WWAN/LTE applications. The designed antenna is comprised of a folded metal plate with a lumped-element high-pass matching circuit and can achieve dual operating bands of 0.69-0.98 GHz and 1.63-2.74 GHz. For mobile phones in Ref. 8 , an octa-band antenna was studied. The presented design consists of a coupled line, a monopole branch, and a ground branch. Using the λ/4-, λ/2-, and 3λ/4-resonant modes, the reported antenna can achieve dual operating bands of 0.67-1.02 GHz and 1.65-2.92 GHz. For LTE/WWAN operation in Ref. 12 , a linear open slot antenna was reported. With an overall dimension of 75 130 × 6 mm 3 , the studied antenna achieved dual operating bands of 698-960 MHz and 1710-2690 MHz. In Ref. 13 , an octa-band antenna that operates over 0.675-1.05 GHz and 1.6-2.8 GHz bands was reported. It is comprised of two branches and a matching circuit and possesses an overall size of 70 × 127 × 6 mm 3 . A wideband monopole antenna for an octa-band mobile phone application was reported in Ref. 14 . The studied antenna consists of a T-shaped driven strip and a dual-branch parasitic ground strip and achieved dual impedance bands 690-970 MHz and 1680-2740 MHz. A compact multiband monopole antenna for WWAN/LTE mobile phones was proposed in Ref. 15  In this paper, an octa-band printed monopole antenna is presented for 2G, 3G, 4G, WiFi, WiMAX, WLAN, and 5G communication applications. The studied antenna comprises an inverted F-shaped monopole radiator and an F-shaped ground strip. To excite multiple resonant modes, two rotated L-shaped strips are respectively added to the radiator and ground strip and thus the antenna can achieve eight operating bands of 824-905 MHz, 1.436-1.584 GHz, 1.994-2.318 GHz, 2.998-3.324 GHz, 3.574-3.72 GHz, 4.123-4.363 GHz, 4.562-5.024 GHz and 5.404-6.00 GHz. The proposed antenna not only has a simple planar profile but also be able to cover sixteen narrow service bands. Besides, one prime advantage of the studied antenna is that it does not use any lumped element to match the antenna and kept ample of free spaces for other circuitries.

Antenna design and analysis
The layout of the proposed octa-band antenna is shown in Fig. 1. It is comprised of an inverted F-shaped monopole radiator and a ground strip. As shown in Fig. 1a an unequal arm inverted-F-shaped patch with a rotated L-shaped branch is printed on the front side of 1.6 mm-thick FR4 substrate material with dielectric constant 4.3 and is fed by a feed line of 50 Ω characteristics impedance. The F-shaped ground with a rotated L-shaped branch is printed on the other side of the substrate as displayed in Fig. 1b. The width and length of the feedline are respectively w f and l f . The monopole radiator with the rotated L-shaped strip is connected to the feed line through a microstrip line while the ground strip is terminated with another microstrip line. To achieve good impedance matching between the radiator and ground strip, the width and length of each branch of the patch and ground are optimized by CST simulation software. The optimized design parameters of the studied antenna are as follows: L 1 = 41.6 mm, L 2 = 17.38 mm, L 3 = 11.80 mm, L 4 = 41. 8 Fig. 2a shows, the lower resonance at 2.178 GHz moves down to 2.084 GHz with an improved S 11 value of − 28.2 dB. The shifting of this resonance helps to improve the bandwidth of the lower band. However, the operating bands are still unsatisfactory. When the horizontal strip line CD and EF are added to the vertical ground strip (Ant-3), the impedance matching is remarkably improved as shown in Fig. 3b. Figure 3b shows that the addition of branches CD and EF to the vertical ground strip of Ant-2 excites six additional resonances at around 874 MHz, 1.144 GH.140 GHz, 3.723 GHz, and 4.983 GHz that can help to cover the GSM850, UMTS, GSM900, WiFi 860 GHz, LTE850, LTE880, LTE3500, LTE3700, 3.5 GHz for 5G sub-6 GHz band, WiMAX 3.5 GHz band. When the rotated L-shaped branch GH is added to the Ant-3(the proposed antenna), the resonances at 1.144 GHz, 3.140 GHz, and 4.983 GHz shifted downward and upward resulting in enhancement of operating bandwidth that helps to cover the entire 4.8 GHz (4.6-5.0 GHz) band for 5G sub-6 GHz communication system.

Current distributions.
To realize the working principle of the studied antenna, the current distributions at resonant frequencies of 863 MHz, 1.51 GHz, 2.25 GHz, 3.11 GHz, 3.65 GHz, 4.25 GHz, and 5.59 GHz are shown in Fig. 4a-g. For clarity, the current distribution on the monopole radiator and ground strip is displayed side by side. At 863 MHz, the current is strong at branch PQ of the ground strip and section XZ of the inverted F-shaped radiator. As Fig. 4a shows, the current is strong at point P and it is weak at point Q. This means that the resonance at 863 MHz is associated with 0.25-λ mode of the ground branch PQ which can be tuned with the help of XZ which is supported by Fig. 3b. At 1.51 GHz as shown in Fig. 4b, the current is more intense at section RP of the ground strong at both points R and P. Consequently, resonance at 1.51 GHz is associated with 1.5 × 0. 25-λ mode of section RP which can be adjusted with the help of section XY of the vertical monopole. As in Fig. 4c, at the 3rd resonance frequency of 2.25 GHz, the current is highly concentrated at section RPQ of the ground strip and XZ section of the radiator. The current is strong at points R and P and it is weak at point Q. Three nulls also have been observed at section RPQ which indicated that this resonance is related to higher-order modes of section RPQ. At 3.11 GHz as displayed in Fig. 4d, the current is more intense at the sections RPS and PQ of the ground strip and XZ section of the monopole radiator. Thus, the resonance at 3.11 GHz is generated by sections RPS and PQ section of the ground strip with the help of section XZ and it is associated with the 1-λ mode of section RPS of length l 5 . As displayed in Fig. 4e, the current concentration at section XZ of the monopole strip and PQ of the ground strip is higher. Thus, the resonance at 3.65 Hz is mainly generated by section XZ of the monopole radiator and tuned to lower frequency by section PQ which is demonstrated in Fig. 3. The current is strong at both points X and Z and weak at points P and Q and there are nulls in between them. Therefore, the antenna works at around 0.5-λ mode of section XZ and around 0.75-λ mode at the section of PQ. At 4.25 GHz as shown in Fig. 4f, the current at sections XZ of the monopole and RPS and PQ of the ground strip is stronger. The current at points X, Z, R, P, Q are stronger and weak at S, and numbers of nulls have been observed in section RPS. Ignoring the radiation from the microstrip line, it can be said that at 4.25 GHz the antenna excites at 0.5-λ mode of section XZ and higher modes of section RPS and PQ. As displayed in Fig. 4g the current concentration at 5.59 GHz is higher at sections XZ of the monopole and RPQ of the ground strip. Therefore, the resonance at this frequency is generated by section XZ and shifted to the lower band by section RPQ as shown in Fig. 3. The current is strong at points X, Z, R, P, and Q. A null has been observed in between points X and Z and numbers of nulls have been observed in between points R and Q, thus it can be commented that 0.5-λ mode of section XZ and higher-order modes of section RPQ are excited at 5.59 GHz. As conclusions, using an inverted F-shaped monopole radiator and the 0.25-λ, 0.5-λ, and 1-λ modes of vertical and lower arms of the F-shaped ground strip, the studied antenna achieved desired multiple operating bands which was reported in Refs. 8,[11][12][13] . Moreover, the equivalent circuit of the proposed antenna is designed based on transmission line theorem, where each metal part has the effect of inductance and gap between two conductor acts as the capacitor, presented in Fig. 5.
Parametric studies. To better comprehend the behavior of the anticipated antenna, the effects of l f , W 3 , and  Fig. 6. When the value of l f varies from 59.36 to 69.36 mm, at the 1st, 2nd, and 3rd bands, there is little change in the resonant frequencies and operating bandwidth. At the 4th band, the bandwidth is decreased, and resonance frequency moves towards the lower band with the worst matching. In the 5th band, the matching deteriorated and resonance frequency moves from 3.29 to 2.98 GHz with the decrement of bandwidth. In the 6th band, the bandwidth slightly decreases and resonance frequency moves downward with lower S 11 . At the7th band, bandwidth is increased and the resonance frequency moves from 4.33 to 4.15 GHz. At the 8th band, bandwidth is decreased and the resonance frequency down to 4.58 GHz from 4.67 GHz. The resonance frequency of the 9th band shifts towards the lower band and the band-      Figure 7 shows the simulated S-parameters for different values of W 3 , the length lower branch of the inverted F-shaped monopole radiator. It can be observed from the plot that W 3 has no significant effect on the 2nd, 3rd, and 9th operating bands. The resonance frequency of the 1st band moves towards the lower band as the value of W 3 increases and the matching becomes worse for values higher and lower than the optimized one. The bandwidth of the 4th band at 2.25 GHz decreases with increasing W 3 whereas the bandwidth of the 5th band at 3.11 GHz increases with increasing W 3 . For the values lower and higher than the optimized one, the matching at the 6th band at 3.65 GHz becomes worse. The bandwidth of the 7th band at 4.25 GHz increases with decreasing W 3 despite of poor S 11 value. When the value of W 3 varies from 57.35 to 77.35 mm, the resonance frequency of the 8th band is shifted from 4.44 to 4.56 GHz and the bandwidth is decreased. Great dependence of the 10th band at 5.59 GHz on W 3 has been observed in the plot.
Trading off the bandwidth and the impedance matching, a value of 67.35 mm is taken as the final value to fabricate the studied antenna.
The S-parameters with different values of W 6 , length of the lower branch of the F-shaped ground strip are shown in Fig. 8. The variation of W 6 has no significant effect on the 2nd, 3rd and 9th bands, while the other operating bands are affected by W 6 . The bandwidth of the 1st band at 863 MHz is increased with increasing W 6 and its resonant frequency moves towards the lower operating band. When the value of W 6 varies from 57.35 to 77.35 mm, the bandwidth of the 4th band at 2.25 GHz is decreased whereas the bandwidth of the 5th band at 3.11 GHz is increased due to improved matching. The matching at the 6th band becomes poorer as the value of W 6 is lower or higher than the optimized one. The matching at the 7th and 8th bands is improved with increasing W 6 along with shifting of resonance frequencies from the desired one. The bandwidth 10th band becomes smaller and the matching is poorer as the value of W 6 is lower or higher than the optimized value. To get the desired operating bands with improved matching the final value of W 6 is taken as 67.35 mm.

Experimental results and discussions
The studied antenna has been fabricated with optimized parameters and is shown in Fig. 9. The radiation characteristics of the studied multi-band antenna are measured in an anechoic chamber using near field antenna measurement system StarLab from MVG. The gain and efficiency of the proposed antenna are respectively shown in Figs. 11 and 12. It is observed from Fig. 10 that at the eight operating bands, the average measured gains are 2.04 dBi, 1.19 dBi, 2.57dBi, 3.48 dBi, 2.60 dBi, 3.05 dBi, 4.94 dBi, and 3.11 dBi, respectively. At the first, second, third, fourth, fifth, sixth, seventh, and eighth impedance bands, the maximum measured efficiency of the presented antenna are respectively 86.6%, 82.7%, 62.2%, 48.5%, 48.6%, 55.4%, 49.1%, and 44.8% and is acceptable for cellular communications [3][4][5]9,[19][20][21] . The efficiency of the proposed antenna in the 4th, 5th, 7th, and 8th bands is slightly less than 50% which is due to the excitation of surface waves. In the proposed antenna, the surface waves spread out around the excitation point and are reflected when they meet the ground plane and at the dielectric-to-air boundary. The fields remain trapped within the substrate and absorb part of the energy of the signal resulting in the decrement of efficiency 22 . Moreover, the low efficiency of the antenna is due to the www.nature.com/scientificreports/ higher loss of standard FR4 substrate used to fabricate the antenna. When a substrate with low dielectric permittivity is used, less energy is coupled into the surface waves, thus reduces the magnitude of the surface waves which in turn increases the efficiency. Moreover, the efficiency of the proposed antenna can also be increased using reflecting surfaces, active component, EBG structure, slot matching, and substrate with the low loss 23 . The measured normalized radiation patterns of the studied antenna at 863 MHz, 1.51 GHz, 2.25 GHz, 3.11 GHz, 4.25 GHz, and 5.59 GHz are depicted in Fig. 13. In the plot, the black solid lines represent the copolarized component (E φ ) while the red dashed lines represent the cross-polarized component (E θ ). At 863 MHz, 1.51 GHz, and 2.25 GHz shown in Fig. 13a-c respectively, the E-field pattern is almost omnidirectional and the H-field pattern is dipole-like with good omnidirectional radiation which indicates that at lower frequencies, the studied antenna exhibits stable radiation characteristics. This characteristic is similar to the observation in many wideband/multiband slot antennas reported in Refs. [24][25][26] . At higher frequencies of 3.11 GHz, 4.25 GHz, and 5.59 GHz, respectively displayed in Fig. 13d-f, comparatively large variations both in the E-field and H-field radiation patterns are observed. These rapid variations in the radiation characteristics are mainly due to the nulls of the surface current distribution on the ground strip. In both E-and H-plane, the cross-polarized level, and distortion are respectively higher at low and frequencies which may be due to the excitation of surface/ leaky waves. Surface waves propagate until they reach an edge of the substrate. When surface waves reach these boundaries, they are reflected and diffracted by the edges. The diffracted waves provide an additional contribution to radiation, degrading the radiation patterns by producing distortions, and cross-polarization levels 27 . This higher level of cross-polarized radiation may also be due to the undesirable higher-order radiating modes that affect the far-field performance of the anticipated antenna. Moreover, as the studied antenna is prototyped on substrate material with higher thicknesses and large dielectric constant, this cross-polarized radiation becomes significantly prominent near the broadside direction which reflects the finding reported in Refs. [14][15][16]28 . This higher cross-polarized level and distortions can be suppressed using modified feeding arrangement, exploration of defected patch surface, use of the shorted patch, use of microwave absorbers, and metamaterials.

Conclusion
To fulfill the demand of wireless communication towards next-generation and to operate over multiple frequency bands, a planar monopole antenna is presented for portable communication devices. The anticipated antenna consists of an inverted F-shaped monopole patch and an F-shaped ground plane. The multi-resonant modes can easily be excited and controlled by different branches of the patch and vertical and lower branches of the ground strip. For S 11 ≤ − 6 dB, the fabricated antenna achieved eight operating bands and can cover sixteen well-established service bands. The measured peak gains at the eight operating bands are 2.98, 2.54, 3.74, 4.44, 3.40, 4.29, 6.32, and 5.32 dBi respectively. The designed octa-band antenna also achieved satisfactory radiation efficiency and exhibits nearly omnidirectional radiation patterns which make it very suitable for being used in portable communication devices such as tablets, laptops, etc. The main advantage of the anticipated antenna is to cover sixteen narrow service bands in the 2G, 3G, 3G, 5G sub-6 GHz, WiFi, WiMAX, and WLAN communication systems without any lumped elements and keeping sufficient empty spaces for other microwave circuitries.