LA DERIVACIÓN NO APRECIATIVA EN LITERATURA

Introduction

The target of this research was to devise a technique which could achieve analogue beam steering up to wide angles while maintaining low side lobes. Side-lobe level (SLL) reduction is essential for wireless communications because it determines how resistant the system is to noise and interference arriving from outside its main lobe. It is a vital parameter for an antenna since it may also limit the beam steering range. Decreasing the SLL, reduces the interference which increases the data rate capacity of

the communication system. Many researchers have proposed ways of reducing the SLL. One technique involves introducing non-uniform spacing between the antenna elements with the aid of a genetic algorithm (GA) [124]. However, in most cases, GAs are limited to use with antenna arrays comprising a small number of elements. Furthermore, they show only limited SLL reduction. Researchers have also attempted to reduce the SLL by controlling the amplitude weighting associated with the different signals applied across the array aperture. The drawback of employing the amplitude weighting technique is that it tends to improve the SLL at the expense of the main beam directivity. The work presented in this paper, uses a technique that is fully analog, does not use any digital algorithms in its implementation and yields a reduction in the SLL while employing main beam gain stability throughout the steering range.

Conventional phased array antennas require one phase shifter per antenna element. Such components are expensive and have high power losses, especially at millimetre wave frequencies [100]. The proposed solution shows reduction in the SLL, compared to a conventional phased array antenna, which implies reduction in the number of element and therefore phase shifters used. It is known that increasing the number of elements within a conventional phased array antenna, increases the gain and directivity of the array antenna. It also increases the steering range of the array antenna since the SLL, for steered angles, decreases. When the main beam of the array antenna is at boresight, the SLL decreases when the number of elements increase from 2 to 8. For more than 8 elements within a conventional phased array antenna, the SLL at boresight is expected to be approximately 13.2 dB irrespective of the number of elements used. Furthermore, reduction in the number of phase shifters used implies reduction in the total power losses of the antenna. Additionally, SLL reduction allows steering at wider angles since SLL can limit the steering range of an antenna. Generally speaking, when tilting the main beam of an array away from boresight, the gain of the array reduces and the SLL increases. Furthermore, maintaining a stable gain over the steering range is also important and it has been an open challenge for beam-steerable antennas. The considered application scenario for this work is access for 5G cellular portable wireless devices that require wireless transmission at very high data rates. The antenna design presented here is a proof of concept antenna which currently operates at 4.65

3.1. Antenna Array Analysis and Parasitic Switching 51

GHz. The frequency 4.65 GHz was chosen and used as a stepping stone, in order to prove the concept. However, the proposed array antenna is later on re-designed for operation at 10.6 GHz and 26 GHz which will then make it suitable for use within 5G cellular portable devices; as its size will decrease considerably.

Antenna Structure and Design

For the purpose of simplicity we will first explain the operation of the single element of the proposed antenna. The single element consists of one radiating (or driven) disk and four parasitic disks. The design for the element is based on a design presented in [47] and [49] and it can be seen in Figure 3.1. The antenna is printed on a Rogers/duroid 5880 substrate with a dielectric constant (εr) of 2.2 and a height of 1.6 mm. The

difference between the antenna structure shown in [49] and the antenna shown here is the operating frequency; which changed the structure of the antenna in terms of size, position of parasitics, position of switches on the parasitics. Furthermore, this antennas structure is later on re-designed into a phased array antenna.

Figure 3.1: Simulated geometry of the single element of the proposed 4.6 GHz phased array antenna.

The main radiator is positioned in the middle of the structure and it is excited by a 50 Ω coaxial connector. The ratio of the inner and outer diameter of the coaxial

Table 3.1: Antenna Dimensions Parameter Dimension (mm) Symbol Representation Substrate Width 66.36 Ws Substrate Length 66.36 Ls

Radius - Driven Element 11.85 R_d

Radius - Parasitic 11.85 Rp

Spacing between Parasitics 0.4λ0 Sp

Position of the Switch 16.2 Xs

Position of the Feed 7.5 X_f

Spacing between Driven Element and Parasitic

0.04λ0 Dp

Spacing between Antenna Elements in the Array Antenna

λ0 Dx

connector, and its position, are very crucial for the impedance matching between the coaxial connector and the input impedance of the antenna [121]. The ratio of the outer over the inner diameter of the coaxial connector in this design is equal to 3.42. The point (0,0) corresponds to the centre of the driven circular patch. The co-ordinate of the feeding via is (0, -2.85) mm on the driven circular patch. Each parasitic incorporates a switched via (SW1, SW2, SW3, SW4) that connects the disk to the ground plane. For the purpose of this study the switches were hard wired meaning that ON and OFF state switches are represented by copper vias and vacuum, respectively. The behavior of the parasitics is controlled by the activation or de-activation of these switches. The single driven element excites four parasitic elements by means of mutual coupling. The centre of the switches is placed at (0, 4.35) mm. The main radiator and parasitic disks have the same radius, namely 11.85 mm. By switching ON and OFF different pairs of switches, it is possible to switch the beam between 2 discrete directions in the elevation plane, 2 discrete directions in the azimuth plane and 4 discrete directions in

3.1. Antenna Array Analysis and Parasitic Switching 53

both azimuth and elevation plane. The current distributions on Figure 3.2 shows three different modes of operation of the antenna, when different pairs, or single, switches are ON and OFF.

The operation of the antenna is very similar to that of a Yagi-Uda antenna, which con- sists of a radiating element, as well as a reflector and a series of directors. Meanwhile, all of the other parasitics become reflectors. The performance of such an antenna is strongly dependent on the radius of the radiator, the position of the switches (vias), the position of the feed and the distance between the parasitic elements and the driven element [47]. Hence, some optimisation and parametric analysis on the antenna struc- ture has been done. The parametric analysis ensured that the antenna is matched with a tolerable return loss (| S11|< 10dB at the resonance) and with a tolerable SLL

(< –10dB) when the main beam is tilted away from boresight.

Figure 3.2: Current distribution of single element for three modes of operation; (i) when SW1 and SW4 ON, (ii) SW2 and SW3 ON, (iii) SW1 ON

The aforementioned antenna, was arrayed with a spacing of λ₂g between the elements, where λg is the guided wavelength and it can be calculated by [12]:

λg= C

f√ε_eff (3.1)

Where εeff is the effective relative dielectric constant of the microstrip substrate. The

spacing between the elements of the array is critical in determining the performance of the antenna. An interelement spacing of at least λ₂g is necessary to reduce mutual coupling. Grating lobes can be avoided (side lobes with approximately the same lobe magnitude with the main beam lobe magnitude that exist in the visible space of the

Figure 3.3: Simulated geometry of the proposed phased array antenna.

antenna’s radiation), the interelement spacing between tge radiating elements should be less than one wavelength. Furthermore, to avoid side lobes (any other lobe apart from the main beam lobe), the spacing should be equal or less than λ₂g. In order to satisfy all three conditions, the spacing was chosen to be λ₂g. The spacing for the design 1 array antenna array design is measured between the edges of each element (edge of the parasitics) as seen in Figure 3.3. This spacing was used only for the design 1 array antenna at 4.6 GHz. The spacing was later changed and taken from centre-to-centre of the driven elements, shown in [12]. This showed an improvement in the performance of the array antenna in terms of SLL at steered angles which also increases the steering range of the antenna as will be shown later on in this work. Furthermore, for the work presented later in this work the free space wavelength was used instead of the guided wavelength which will again be explained in later chapters of this thesis. The proposed array antenna consists of a linear array of four elements and it can be seen in Figure 3.3. Each element incorporates four parasitics and hence four switches that can be turned ON or OFF. The total size of the four element array antenna is 5.19 λ by 1.38 λ (338.2 mm by 90 mm). The array antenna operates at a frequency of 4.665 GHz. It is matched at the resonance with a return loss of | S11|< 10 dB. The proposed

array antenna can perform analogue beam-steering by turning ON and OFF different combinations of switches on the elements of the array, and by applying different phase shifts between consecutive feeding ports.