Smart antennas have capability of adaptive beam-forming. For WLAN applications the smart antennas can improve the received signal-to-interference ratio by placing a null in the direction of interference and the maxima towards the desired signal source [1] . Electronically steerable parasitic array radiator antennas (ESPAR) provide a simple and cost effective beam steering solution. ESPAR antennas are typically not low profile and miniaturization is required for using such antennas in compact devices. In this paper, we propose a miniaturized ESPAR antenna with a broad bandwidth and a single/dual lobe radiation pattern. The design of the miniaturized seven element ESPAR antenna consists of a circular array of six parasitic elements around a central active element. Our design is three times smaller in height compared to a conventional ESPAR antenna [2] and supports single/dual lobe radiation patterns and dual band coverage.

Previously designed miniaturized/slim ESPAR antennas have resulted in lower gains [3] [4] . The proposed miniaturized ESPAR antenna is superior to miniaturized ESPAR antennas in [3] [4] in terms of gain and bandwidth. Miniaturized ESPAR antennas discussed in [3] [4] [5] [6] have limited bandwidth and support single lobe radiation pattern. The proposed design supports reconfigurable single and dual lobe radiation patterns, its dual-band coverage includes the frequency ranges 2.3 GHz - 2.53 GHz and 2.9 GHz - 3.7 GHz. Dual lobe antennas can play an important role in increasing network capacity without using additional antennas by providing beam diversity which is effective in addressing obstruction. As a result, such antennas can considerably enhance the reliability of the wireless system in non-line-of-sight links.

In the proposed model six parasitic monopoles are mounted on a skirted ground plane. The electric field of a seven element ESPAR antenna can be calculated by using the vector effective length equivalent weight vector method [7] .

E ( θ , ϕ ) = − j Z 0 2 λ e − j k r r ∑ m = 0 6   l e m ( θ ) i m a m ( θ , ϕ ) (1)

where Z 0 is the characteristic impedance, a m is the steering vector that is expressed as [7] :

a m ( θ , ϕ ) = e x p j k ( sin θ cos ϕ x m + sin θ sin ϕ y m + cos θ z m )

l e m is the vector effective length and is expressed as:

l e m = l e m 0 ( 1 − α m x m ) (2)

i m is the port current that is expressed as:

[ i 0 i 1 i 2 i 3 i 4 i 5 i 6 ] = ( [ Z m n ] + d i a g [ Z x , j X 1 , j X 2 , j X 3 , ⋯ , j X 6 ] ) − 1 [ v s 0 0 0 0 0 0 ] (3)

Here, X₁, X₂, X₃, X₄, X₅ and X₆ are reactances loaded on each parasitic element. By varying these reactances the port current varies and the antenna beam is steered. Therefore, the antenna gain can be expressed as:

D ( θ , ϕ ) = π Z 0 sin θ | ∑ m = 0 M l m i m | Z s = 0 a m ( θ , ϕ ) | 2 λ 2 | v s | 2 R e ( 1 Z i n ) (4)

The proposed design has a gain of 6.3 dBi, which is 2.3 dB higher than comparable size slim/miniaturized ESPAR antennas [3] . The miniaturized ESPAR antenna designed in [3] has a peak gain of 4 dBi. In comparison, the proposed design has 2.3 dB higher gain, and the antenna has the capability of single/dual beam forming along with dual band operation. The key differentiators of our design are:

· The height of the proposed design is three times lower than conventional ESPAR antennas.

· Capable of forming up to two reconfigurable beams.

· Supports dual band operation (2.3 - 2.53 GHz and 2.9 - 3.7 GHz).

· Antenna beam can be steered by a simple switching mechanism.

In typical seven element ESPAR antennas there is an array of six parasitic monopole elements around a central active element and each monopole is λ/4 in height [8] [9] . The simulated model of the designed antenna is shown in Figure 1. The antenna consists of a conical active element and folded monopole passive elements. The folded monopoles consist of three arms, where each arm is λ/12 in length and all three arms of the folded monopole collectively form a length of λ/4 [10] . Folded monopoles are used to reduce the antenna height. The central active element is made conical in order to improve the bandwidth, the upper radius of the central conical element is 1.4 mm and the lower radius is 1 mm. The Length of the active element is set to λ/5. It has a larger height than the surrounding parasitic monopoles, which is what allows for the dual band nature of the antenna.

The height of the ground plane is λ/5 and it is skirted. The radius of the ground plane in the proposed antenna is λ/4, whereas in conventional ESPAR antennas the radius of the ground plane is λ/2. Table 1 shows the size comparison between the proposed miniaturized dual-band ESPAR antenna and a conventional ESPAR antenna.

Monopole antennas are generally sensitive to inductive and capacitive changes. In conventional ESPAR antennas, varactors are used to produce variable capacitive effects for beam steering [9] . By varying the capacitive loading of the parasitic monopole elements, the effective length is changed and the elements tend to act as reflectors or directors. In the proposed design instead of varying the capacitive loading, we open or short the parasitic monopoles thereby achieving a similar effect in a more cost effective manner. In the high frequency structure simulator (HFSS) simulations the active element is loaded with a lumped port excitation. Further, the parasitic elements are electrically shortened by using perfect electric boundaries.

The simulated antenna model has a return loss of 15.2 dB at 2.45 GHz and 43 dB at 3.25 GHz. Figure 2 shows the return loss plot of the designed antenna. Considering S₁₁ = −10 dB as our reference point antenna bandwidth, it is 220 MHz for band 1 since S₁₁ is below −10 dB from 2.31 GHz to 2.53 GHz. Similarly, the antenna bandwidth is 800 MHz for band 2 as S₁₁ is below −10 dB from 2.9 GHz to 3.7 GHz.

Parasitic monopoles are placed at an angle of 60˚ from each other. The element along the x-axis is denoted as element 1 and all other elements are marked counterclockwise with respect to element 1, as shown in Figure 1. The beam is formed between two open elements as open elements act as directors and short elements act as reflectors. When element 1 and 2 are open and all other elements are shorted, the beam is formed between elements 1 and 2. Similarly, when element 2 and 3 are open and the remaining elements are shorted the beam is formed between these two elements, and so on. Mechanical shorts were used for opening and shorting the parasitic elements, but these would be replaced with solid-state components in a practical implementation. The beam control switching configurations are detailed in Table 2. The antenna main beam is steered between 0 - 360 degrees in steps of 60˚ each. Figure 3 shows 2D radiation pattern of the designed antenna at −90˚, −30˚ and +30˚ respectively. Figure 4 shows 2D radiation pattern of the designed antenna at 90˚, 150˚ and −150˚ respectively.

To achieve a single lobe radiation pattern two adjacent monopoles are to be opened, whereas to produce a dual lobe beam pattern two oppositely placed monopoles are opened. 2D single lobe patterns of the designed antenna are shown in Figure 3 and Figure 4. Dual lobe radiation patterns are shown in Figure 5.

The monopoles and central elements are made from brass and the ground plane is made up of aluminum. The monopoles are mounted on a ground plane by

using SMA connectors. Figure 6 and Figure 7 show front view and side view of fabricated antenna. RF shorts are used for opening/shorting of the antenna monopoles for beam steering.

The designed antenna was tested in an anechoic chamber and the measured results aligned well with HFSS simulated results. However, the simulated S₁₁ value was better as compared to measured one because the antenna elements were considered perfect electric conductor in the HFSS antenna simulations. Figure 8 shows S₁₁ plot when element 1, 2, 3 and 4 are shortened and Figure 9 shows S₁₁ plot when element 2, 3, 4 and 5 are shortened.

The radiation pattern of the designed antenna was measured in an anechoic chamber and the measured results show complete 360˚ beam steering in azimuth plane. Figure 10, and Figure 11 show the measured radiation patterns of the designed antenna at 90˚, 165˚ and 195˚, respectively. These measured results are aligned well with simulated results.

This paper presented the design of a miniaturized dual-band and dual-lobe

ESPAR antenna with enhanced bandwidth for WLAN applications. The height of the proposed antenna is three times lower than that of conventional ESPAR antennas. The proposed antenna offers dual bandwidth characteristics and covers the bands from 2.3 - 2.53 GHz and 2.9 GHz - 3.7 GHz. The proposed antenna is capable of replacing standard ESPAR antennas as it has a more compact structure, dual-band coverage, and both single and dual lobe radiation pattern

options. The switching mechanism used for beam steering is cost effective and easy to implement while allowing sufficient angular resolution in beam steering. The achieved peak gain was 6.3 dBi, and it is anticipated that similar structures can be implemented at mm-Wave frequencies. Future prospects of the proposed design include gain enhancement for applications requiring high gain. Gain can be increased by phase combining two or more circular monopole arrays.

The authors declare no conflicts of interest regarding the publication of this paper.

Kausar, S., Kausar, A., Mehrpouyan, H., Hadi, M. and Tariq, S. (2023) Miniaturized Electronically Steerable Parasitic Array Antenna. Open Journal of Antennas and Propagation, 11, 1-10. https://doi.org/10.4236/ojapr.2023.111001