Pierre Moukala Mpele

| Franck

Moukanda Mbango

Dominic B O Konditi

| Fabien Ndagijimana

Pierre Moulala Mpele
email: pm.mpele@gmail.com

A tri-band and miniaturized planar antenna based on countersink and defected ground structure techniques

Keywords: Heart-shaped antenna, countersink, monopole, miniature antenna, multiband

In this paper, a compact planar heart-shaped antenna based on countersink and partial ground plane techniques is proposed to operate as a multiband device for modern wireless systems. With a surface area of 28.5 mm 2 , the countersink technique has been developed to achieve high electromagnetic antenna performance. The countersink technique consists of a particular multilayer configuration that uses two distinct materials where one of them with a radiating element on its top is placed inside another. In this paper, a 508 µm thick Rogers RO4350B with a radiating patch designed on its top is thermally embedded into a 1.524 mm thick FR4 HTG-175. The slits are used to get multiband behavior, and their combination with the defected ground structure results in about 79.58% reduction in resonant frequency compared to the conventional antenna. A mathematical model is developed to predict the resonant frequency of the proposed antenna configuration. After modeling the heart-shaped antenna with an electromagnetic simulator, its prototype was manufactured and validated through experimental measurements. The countersink technique considerably improved the antenna's performance compared to the use of a single layer technology for the same antenna configuration. The return loss read from -10 dB shows that the proposed antenna covers multiple modern wireless applications, including LTE bands, 5G, WLAN, WiMAX, and Unlicensed National Information Infrastructure (UNII) radio bands. Furthermore, the antenna operates as narrowband and wideband simultaneously on its lower / middle and upper operating frequency bands. A close agreement is found between simulated and measured results across all the frequency bands.

INTRODUCTION

The fifth generation of mobile technologies (5G) is expected to connect people, objects, data, applications, transportation systems, and cities in smart grid communication environments [START_REF]5G -Fifth generation of mobile technologies[END_REF] . At the same time, it will integrate and coexist with various existing services such as GPS, GSM, UMTS, Bluetooth, WLAN, WiMAX, 4G / LTE and more other wireless applications. In this context, multiple antennas can be used to cover different applications mentioned above. However, integrating several antennas into a single device is becoming more challenging due to the space limitation of wireless mobile terminals [START_REF] Zhi | Miniature multiband antenna for WLAN and X-Band satellite communication applications[END_REF] . So, compact and multiband antenna design has become very demanding [START_REF] Bag | Circularly polarized quad-band monopole antenna of wireless communication system[END_REF] .

Owing to their advantages of being low profile, lightweight, easy and low cost of fabrication, ease of integration within an environment of microwave circuit, planar monopole antennas with various structures are very popular in multiband applications [START_REF] Bag | Circularly polarized quad-band monopole antenna of wireless communication system[END_REF] . Recently, multiple works on WLAN, WiMAX, 4G/LTE and 5G antennas using different methods have been reported in the literature. Material loading with very high permittivity dielectric, shorting and folding, reshaping or introducing slots, modification of the ground plane, and the use of metamaterials are among the techniques used for antenna miniaturization and multiband [START_REF] Sharawi | Microstrip patch antenna miniaturisation techniques: a review[END_REF] . In Reference [START_REF] Patel | Low Profile Multiband Meander Antenna for LTE/WiMAX/WLAN and INSAT-C Application[END_REF] , a low profile multiband meander antenna was presented for LTE/WiMAX/WLAN and INSAT-C applications. The antenna has a size of 40×40×1.6 mm 3 and exhibits narrow bandwidths (70 and 90 MHz) in its first and second frequency bands. In Reference [START_REF] Chetouah | A miniaturized printed monopole antenna for 5.2-5.8 GHz WLAN applications[END_REF] , the miniaturization of a 10 × 6 mm 2 rectangular patch antenna has been achieved with an inverted 'L' shaped ground plane for 5.2-5.8 GHz WLAN applications. A similar technique was used in Reference [START_REF] Ambh | Size Reduction of Rectangular Microstrip Patch Microstrip Antenna for GSM Application[END_REF] to reduce the antenna size up to 86.72%. In Reference 8 , a 30×20×0.8 mm 3 triband planar monopole antenna for WLAN and WiMAX operations has been designed and investigated. The multiband characteristic was achieved using a toothbrush-shaped patch, a meander line, and an inverted U-shaped patch, which resulted in three resonance frequencies.

Moreover, the defected ground structure (DGS) is one of the most significant techniques for achieving antenna compactness in terms of size or resonant frequency [START_REF] Kumar | Compact broadband circularly polarized Hook-shaped microstrip antenna with DGS plane[END_REF] . With the DGS and parasitic patch technique's help, a compact broadband antenna for LTE/WLAN/WiMAX applications has been proposed in Reference [START_REF] Wu | A novel compact broadband antenna for LTE/WLAN/WiMAX applications[END_REF] . The antenna was designed on a 1.6 mm-thick square F4B substrate with a footprint of 30×30 mm 2 . In reference [START_REF] Verma | A compact multi-slots loaded gap coupled CP antenna with DGS for WLAN/WiMAX applications[END_REF] , a 42.5×42.5×1.6 mm 3 compact circularly polarized slotted gap coupled semi-circular patch antenna for WLAN/WiMAX bands and other wireless applications. The antenna bandwidth improvement with better impedance matching have been achieved by etching slots on the patch and in the ground plane. In Reference [START_REF] Chen | A Compact Printed Monopole Antenna for WiMAX/WLAN and UWB Applications[END_REF] , a triangular planar monopole antenna fed by fed by a microstrip gradient line has been proposed. With an overall size of 30×17×1.6 mm 3 , the desired performance has been achieved using a partial ground plane along with an inverted-F shape and a 90º rotated counter-clockwise M-shaped structure. In reference [START_REF] Sah | Design and development of enhanced bandwidth multi-frequency slotted antenna for 4G-LTE/WiMAX/WLAN and S/C/X-band applications[END_REF] , a multi-frequency antenna fabricated using FR-4 substrate with dimensions of 56×44×1.6 mm 3 is presented. An inverted T-shaped stub and two E-shaped stubs are used to generate multiple frequency bands, enabling the antenna to cover 4G LTE, WiMAX, WLAN, and S/C/X-band applications. A compact triband slotted printed monopole antenna for WLAN and WiMAX applications showing omnidirectional radiation pattern was presented in [START_REF] Ahmad | Compact triband slotted printed monopole antenna for WLAN and WiMAX applications[END_REF] . The physical size of the antenna is 27.5×20 mm 2 . The triple-band operations and the desired antenna performance have been achieved by using an inverted L slot, Pi-shaped slot and a truncated square patch at its left corner.

Miniaturization can be defined as lowering the antenna's resonance frequency without changing its size [START_REF] Rothwell | Antenna miniaturization: definitions, concepts, and a review with emphasis on metamaterials[END_REF] . At the same time, it is an art of compromise between radiation efficiency, gain, and bandwidth [START_REF] Ahmad | Compact triband slotted printed monopole antenna for WLAN and WiMAX applications[END_REF] . As planar antennas are miniaturized, various approaches and techniques are employed to maintain or enhance their performances. These techniques include the DGS, parasitic elements [START_REF] Asif | A compact multiband microstrip patch antenna with U-shaped parasitic elements[END_REF] , air gap, slits, slots, metamaterial/metasurface, shorting pin [START_REF] Faisal | Multi-Band Microstrip Antenna Design for Mobile Phone Applications[END_REF][START_REF] Akinola | Gain and Bandwidth Enhancement Techniques of Microstrip Antenna: A Technical Review[END_REF] , frequency-selective surface, electromagnetic band gap (EBG), meander line [START_REF] Agale | A review on design approach for performance enhancement techniques of microstrip patch antenna[END_REF] , and multilayered substrates. In most recent antenna designs, each technique's benefit is exploited and combined with others to get the desired results. For instance, in Reference 20 , the antenna impedance matching and return loss have been improved using the defected ground structure through two F-shaped slots etched in the ground plane. In Reference [START_REF] Darimireddy | Design of triple-layer double U-slot patch antenna for wireless applications[END_REF] , triple layers and double U-shaped slots have been used to achieve the multiband behavior and antenna bandwidth enhancement.

The size of the antenna's radiating element strongly affects its radiation efficiency and bandwidth [START_REF] Gustafsson | Tradeoff between Antenna Efficiency and Q-Factor[END_REF] . Wider the bandwidth, larger is the size of the antenna [START_REF] Rao | Design and analysis of multi substrate microstrip patch antenna[END_REF] . This trade-off relationship between antenna size and bandwidth can reduce by using multiple layer substrates of different materials [START_REF] Kim | Performance enhancements of a microstrip antenna with multiple layer substrates[END_REF] . In Reference [START_REF] Ineneji | Gain enhancement in microstrip patch antenna using the Multiple Substrate Layer method[END_REF] , a multilayer substrate configuration involving silicon/glass/silicon has been used to improve the far-field antenna parameter, including gain, directivity, and radiation efficiency. However, one of the drawbacks of the conventional multilayer configurations is the increased height, which is not desirable for conformal applications [START_REF] Zehforoosh | Antenna Design for Ultra Wideband Application Using a New Multilayer Structure[END_REF] .

In this paper, a particular multilayer configuration is employed to improve antenna performance. Contrary to the conventional multiple substrate configuration, this paper investigates a miniature triband planar monopole antenna configuration where a Rogers RO4350B is embedded into FR4 HTG-175, leading to the countersink technique. A comprehensive design procedure is provided, and a mathematical model is developed to predict the resonant frequency of the antenna structure. The proposed triband heart-shaped planar monopole antenna (HPMA) covers several wireless communications applications such as Weather Radar, WLAN, WiMAX, 4G Long-Term Evolution (LTE), UNII radio bands, and 5G applications with a comprehensive design procedure. The HPMA design details and process are first presented, while the simulated and measured results are discussed later before highlighting the advantages and disadvantages of the configuration.

| ANTENNA CONFIGURATION

The geometry of the proposed antenna is presented in Figure 1. The top surface consists of a radiating patch made of two circular parts and a 45° rotated square. This yields a heart-shaped patch geometry fed by a microstrip feed line. The square's edge has been selected to be twice the circular part radius (Figure 1(b)). The antenna has an overall size of 15×12.5×1.524 mm 3 . The structure is based on the countersink technique (CT) and the DGS, as shown in Figure 1. A single-sided Rogers RO4350B is thermally embedded into a double-sided FR4 HTG175 to provide a particular multilayer configuration. All the design parameters are listed in Table 1 below. The thickness of the copper cladding top and bottom used in the CT mentioned above is 35 μm. To reach the goal, current paths are created through slits etched on the radiating element illustrated in Figure 1(b), whereby every corresponding resonant paths' length Li is close to quarter-wavelength at its fundamental resonant frequency. 10.

283 ii La  , i=1,2,3 (1) 75 10
.

283 i i eff f a   (2)
A Rogers RO4350B is embedded into the FR4 to create the CT and which is the proposed antenna configuration. Unlike the traditional multilayer substrate, the proposed countersink technique can be seen as a particular case of multilayer configuration involving two inhomogeneous materials. Therefore, the two-layer dielectric can be modeled using each substrate layer's effective permittivity and simplified into a single equivalent homogeneous substrate with a thickness of heq and the dielectric constant εeff, as illustrated in Figure 2.

eq eff Q C V h       (3)
The dielectric constant εeff and the height heq of the equivalent substrate in equation ( 3) can be expressed by [START_REF] Ali | Using equivalent dielectric constant to simplify the analysis of patch microstrip antenna with multi-layer substrates[END_REF] .

1 2 1 n eff eq n effn h h               (4) 2 12 eq n n h h h h     (5)
Expanding equation ( 4) and using equation ( 5) yields

  1 2 1 2 2 1 1 2 eff eff eff eff eff hh hh       (6)
where the effective permittivity for each layer can be approximated as

1 1 1 2 r eff     (7) 2 2 1 2 r eff     (8)
Using the Equations ( 2), ( 6)-( 8), the three resonant frequencies of the triband are 2.74 GHz, 3.62 GHz, and 4.43 GHz.

To analyze the developed mathematical model for the proposed antenna structure, Maxwell's equations through an electromagnetic software were used to evaluate and compare the resonant frequencies obtained analytically from Equation ( 2), which mainly depends on the different current path lengths of each heart-shaped branch. To further characterize the antenna design, a parametric study of the circular part radius "a" of the radiating element is carried out. To facilitate the parametric analysis, every path's circular part radius is expressed in terms of the primary radius "a" and given by

  0.4 2 i a a i   , i=1,2,3. ( 9
)
It can be observed from the plots in Figure 3 that by increasing "a", the resonant frequencies shift downward. Similarly, the resonant frequencies of the presented antenna increase when the size of the antenna reduces. Band I (Using Eqs. ( 2), ( 6)-( 8)) Band I (Simulation) Band II (Using Eqs. ( 2), ( 6)-( 8)) Band II (Simulation) Band III (Using Eqs. ( 2), ( 6)-( 8)) Band III (Simulation)

Figure 3 Parametric analysis of the proposed antenna Figure 3 illustrates that by varying the antenna structure dimensions and using Equations ( 2), ( 6)-( 8), the analytical and simulated results agree for the three frequency bands. This validates the developed mathematical model for the proposed heart-shaped planar monopole antenna.

| RESULTS AND DISCUSSION

| COUNTERSINK TECHNIQUE EFFECT

Two triband heart-shaped planar monopole antennas with and without the embedded substrate are analyzed to evaluate the effect countersink technique on the antenna performance. The return loss simulation results through an electromagnetic simulator are displayed in Figure 4 below.

Figure 4 Simulated return loss comparison

Figure 4 demonstrates that the proposed antenna is well much across all the frequency bands compared to the same antenna configuration without CT. The other performance parameters for the two antenna configurations are presented in Table 2. From Table 2, it can be observed that except in (Band III), embedding the Rogers RO4350B into FR4 and employing slits along with DGS yield better antenna performance results in the first (Band I) and second (Band II) operating frequency bands. It is observed an offset of about 100 MHz between the resonant frequencies for the two antenna configurations. This shift increases by 100 MHz at each resonance frequency within each band. Moreover, the triband HPMA with CT achieves a better matching level through the return loss with a slight difference of resonant frequency for the third band (Band III). It is also noticed that apart from the first and the second frequency bands, the triband HPMA without CT yields wider bandwidth and radiation efficiency in the third band. However, parameters like antenna gain and radiation efficiency are better when the countersink

Frequency [GHz]

Triband HPMA without CT Triband HPMA with CT technique is used. Finally, the HPMA using the countersink technique seems to be a promising way to improve an antenna's performance regarding the results obtained by electromagnetic simulations.

| ANTENNA RETURN LOSS

From the results detailed above, an HPMA prototype in the countersink technique has been manufactured. Figure 5 displays the HPMA prototype fabricated using the countersink technique with the two insulators: RO4350B thermally embedded into FR4 HTG175. The simulated and measured return loss curves for the HPMA are presented in Figure 6. From the measured frequency response, it can be observed that for |S11| ≤ -10 dB, the antenna can operate over the bands (2.695 -2.89 GHz), (3.51 -3.77 GHz), and (4.92 -5.46 GHz). The first band is suitable for weather and airport surveillance radars operating in the 2.7-2.9 GHz range; the second band coverage includes LTE bands 42/43/48, 5G, WiMAX applications, and in the third band, 5 GHz WLAN (5.15 -5.35 GHz) and UNII Low UNII -2A/2B (5.15 -5.47 GHz) applications are covered.

Figure 6 Return loss of the proposed antenna

At -10 dB of the return loss parameter, the HPMA guarantees 195 MHz of bandwidth. Furthermore, the experimental results demonstrate that the use of slits and the partial ground plane yielded significant antenna miniaturization. The antenna size reduction has been evaluated using two approaches. The first approach compares the active patch area of a typical microstrip antenna resonating at the same frequency with the proposed antenna active patch area as given in Equation (10).

Simulation Measurement             00 0 (%) 100 rr r r r p r f f f f rr ff A f A f M Af          ( 10
)
where Ar is the active patch area of a typical microstrip antenna (rectangular patch, circular patch, elliptical patch) resonating at the same resonant frequency f0 with the proposed antenna having an active patch area of Ap.

A typical rectangular microstrip antenna resonating at 2.83 GHz has a minimum active patch area of 801.78 mm 2 (24.9 mm × 32.2 mm). In contrast, a standard circular microstrip patch antenna resonating at the same frequency (2.83 GHz) has a radius of 14.34 mm with a minimum active patch area of 646 mm 2 . Comparing the proposed HPMA to the typical rectangular and circular microstrip patch antennas mentioned above through the Equation ( 10) yields a significant antenna's active patch size reduction of about 96.4 % and 95.6 %, respectively.

The proposed design structure is simulated with and without slits and a partial ground plane to quantify the antenna miniaturization in the second and final approach. The results are compared in terms of the lower resonant frequency, as shown in Figure 7. As it can be observed, the conventional heart-shaped antenna resonates at 13.42 GHz, while the proposed antenna's lower resonant frequency is 2.74 GHz.

Figure 7 Comparison between the conventional antenna and the proposed

This clearly illustrates that truncating the ground plane and etching slits on the radiating element after embedding the rogers RO4350B into FR4 yields a 79.58% reduction in resonant frequency, which can be computed using the following formula:

01 0 (%) 100 rr r ff M f      ( 11
)
where fr0 is the lower resonant frequency of the conventional planar heart-shaped antenna and fr1, the lower resonant frequency of the proposed antenna design based on countersink technique and partial ground plane along with open-ended multiple branches through slits.

The simulated and experimental results presented in Figure 8 indicate that the HPMA has good matching with a Voltage Standing Wave Ratio (VSWR) values less than 1.56 at all its respective resonant frequencies. This points out a good matching between the feed network and the antenna structure. Simulation Measurement

| RADIATION PATTERN OF THE HPMA

The antenna radiated field consists of near-field and far-field regions from the antenna. For this work, the farfield radiation pattern measurement has been conducted in an anechoic chamber in the presence of a horn receiving antenna as a reference device. The results are presented in Figure 10. The antenna exhibits an omnidirectional radiation pattern in the elevation plane, and a deviating quasi-bidirectional pattern in the azimuth plane in the entire operating frequency range with maximum radiation in the azimuth plane found at ϕ=0º. This satisfies the radiation performance characteristics of monopole antennas.

| GAIN OF THE HPMA

The antenna gain is one of the essential parameters that predicts its effective transmission or reception ability. The antenna gain is determined from the Two -Antenna Method 29 based on the Friis' formula given below

where d is the distance between both antennas, Gt and Gref the gain of the transmitting and receiving antenna, f0 the operating frequency, and S21 the transmission coefficient between both antennas. Using two identical antennas and applying the Friis principle, Equation (12) becomes in decibel as: By placing the transmitting and receiving antennas at a distance d=135 mm from each other and using Equation ( 13), the measured results show that the proposed antenna achieves a maximum gain of 0.18 dBi at 3.78 GHz, whereas 0.56 dBi maximum gain is found at 3.76 GHz from the simulation.

The simulated antenna gain is displayed in Figure 12 below. At the first resonant frequency (2.74 GHz), the simulated gain is about -0.7 dB. For the medium band, the simulated antenna gain is about 0.4 dB at 3.62 GHz, and at 4.43 GHz, the simulated gain is -2.1 dB for the higher band of operation. When miniaturizing an antenna, its gain, radiation efficiency, bandwidth, and polarization purity are affected [START_REF] Skrivervik | PCS antenna design: The challenge of miniaturization[END_REF] . Moreover, an Electrically Small Antenna 31 can have a negative gain, even lower than -10 dB, without accounting for impedance mismatch loss [START_REF] Gain | [END_REF] . This follows directly from the fact that using DGS in the HPMA is responsible for low radiated power that yielded low antenna gains and efficiencies. Table 3 summarizes the simulated and experimental results for the HPMA. It can be observed that the antenna performs as a narrowband in its lower and middle operating frequency bands and exhibits wideband behavior in its upper bands with a minimum bandwidth of 500 MHz. The proposed antennas find their application in mobile phones and consumer electronics products whose efficiency range is 20% -70% [START_REF]Antenna Efficiency[END_REF] . Table 4 compares the performance of the proposed antenna to the other existing antennas in the literature. As it can be read from Table 3, the HPMA can be used in more than three applications. Also, with a physical size of 15×12.5×1.524 mm 3 and an electrical size of 0.127λ0×0.105λ0×0.013λ0 at its lowest operating frequency (2.53GHz), this HPMA is the smallest antenna device compared to the other triband designs.

| CONCLUSION

In this paper, a new configuration of a miniaturized triband heart-shaped planar monopole antenna (HPMA) has been investigated. The antenna design is based on the countersink technique (CT) to improve certain specific electromagnetic performance parameters with microstrip technology. A mathematical model has been developed to predict the resonant frequencies of the proposed antenna structure configuration. Furthermore, the HPMA, proposed with further details, guarantees a minimum bandwidth of 195 MHz and achieves wideband operation in its upper band. The embedded substrate plays a key role in improving antenna performance without increasing the antenna surface area or the structure volume. In contrast, the defected ground structure and slits resulted in a 79.58% reduction in resonant frequency while preserving the fidelity of other performance parameters within an acceptable range for the targeted wireless applications. The measurement results demonstrate that the proposed HPMA achieves a minimum return loss of -32.65 dB and a maximum of -13.25 dB. The countersink technique complexity makes the structure challenging to manufacture, yielding high fabrication costs. Its measured and computer-simulated results well-agreed in the operating frequency range. The proposed antenna configuration covers WLAN, WiMAX, LTE, 5G, UNII, Weather radars, and Airport surveillance radars applications. Besides its simple design methodology, the HPMA is compact and meets the requirement of 2:1 VSWR, which are the key features that make it a potential candidate for handheld wireless devices.

Figure 1

 1 Proposed Antenna geometry. (a) top view; (b) heart-shaped configuration; (c) bottom view; (d) side view

Figure 2

 2 Figure 2 Simplification of the two-layer substrate into a single layer through parallel plate capacitor. (a) model; (b) circuit By using the parallel plate capacitor model presented, the equivalent capacitance C per unit area can be computed in terms of the electric charge Q per unit area and the electric potential difference V between two plates27

  circular part radius a [mm]

Figure 5

 5 Figure 5 Photograph of the fabricated antenna prototype using countersink technique. (a, b, c) to view; (d) bottom view

Figure 8 Figure 9

 89 Figure 8 Antenna simulated and measured VSWR

Figure 10

 10 Figure 10 Antenna measured radiation pattern. (a) 2.83 GHz; (b) 3.67 GHz; (c) 5.22 GHz

Figure 11 .

 11 Figure 11. Simulated efficiency versus frequency of the proposed antenna

Figure 12

 12 Figure 12 Simulated gain. (a) 2.74 GHz; (b) 3.62 GHz; (c) 4.43 GHz

Table 1

 1 Antenna design parameters

	Parameter	Value (mm)	Parameter	Value (mm)
	LS	15	a3	1
	WS	12.5	p	4.422
	L	6.242	t	0.035
	W	6.828	Lg	2.5
	a	2	Wf	0.5
	a1	1.67	LRO	6.085
	a2	1.25	WRO	7
	h1	1.524	h2	0.508

Table 2

 2 Simulation results comparison for two HPMA configurations

Antenna parameter Antenna configuration Triband HPMA without CT Triband HPMA with CT

		Band I	Band II	Band III	Band I	Band II	Band III
	Resonant frequency (GHz)	2.56	3.43	4.23	2.74	3.62	4.43
	Return Loss (dB)	-13.63	-20.79	-33.96	-15.41	-27.46	-27.59
	Bandwidth (MHz) (S11≤ -10 dB)	236	388.6	989.7	347	439.4	786.3
	Radiation efficiency (%)	41.16	52.47	60.55	57.8	71.26	40.16
	Gain (dB)	-2.1	-0.6	-0.2	-0.7	0.4	-2.1
	VSWR	1.528	1.201	1.041	1.398	1.095	1.091

Table 3

 3 Simulation and experimental results comparison for the HPMA

		Results	Band I	Band II	Band III
		fr (GHz)	2.74	3.62	4.43
		S11 (dB)	-15.41	-27.46	-27.59
	Simulation	Bandwidth (MHz)	347	439.4	786.3
		Efficiency (%)	57.8	71.26	40.16
		Gain (dB)	-0.7	0.4	-2.1
		fr (GHz)	2.83	3.66	5.21
	Measurement	S11 (dB)	-22.84	-32.65	-13.25
		Bandwidth (MHz)	195	260	530
		Gain (dBi)	-0.77	-0.84	-2.48

Table 4

 4 Comparison with previously reported antennas

	Ref.	Electrical size	Physical	size	Patch area	Substrate Method	Application
			(mm)		(mm 2 )
	34	0.176λ0×0.144	18×22×1		195.75	FR4	Multiple	WLAN,
		λ0×0.008λ0					branches and	WiMAX, X-
							DGS	band
								application
	35	0.150λ0×0.150	15×15×1.6		105	FR4	Slots/Slits	WLAN,
		λ0×0.016λ0					and DGS	WiMAX, ITU
	36	0.220λ0×	34×29×0.8		238	FR4	Slits and	WLAN,
		0.258×0.006λ0					DGS	WiMAX
	37	0.193λ0×0.204	17×18×0.8		100.53	FR4	Slots and	WLAN,
		λ0×0.009λ0					DGS	WiMAX
	38	0.235λ0×0.267	30×34×1.6		520	FR4	DGS	WLAN,
		λ0×0.013λ0						WiMAX
	39	0.417λ0×0.292	50×35×1.6		93.5	FR4	DGS, L-and	WLAN,
		λ0×0.0133λ0					U-shaped	WiMAX
							strip-loaded
							Open-ended	WLAN,
							multiple	WiMAX, LTE,
	HPMA	0.127λ0×0.105	15×12.5×1.524	28.5	FR4+RO	branches,	5G, UNII,
		λ0×0.013λ0				4350B	DGS, and	Weather
							countersink	radars, Airport
							technique	surveillance
								radars

ACKNOWLEDGEMENTS

Grateful acknowledgement is made to the African Union Commission for sponsoring the research through the Pan African University Institute for Basic Sciences Technology and Innovation (PAUSTI). Also, the authors give special thanks to Pheline Laboratory for great help during the validation process of the antenna prototypes.