EEE6223 Antennas 2023-2024 PDF

EEE6223 Antennas 2023-2024 Microstrip Antennas One of the most popular antenna types that offers several advantages such low profile, low fabrication cost and easy integration to other microwave circuits. Microstrip ant...

EEE6223 Antennas 2023-2024 Microstrip Antennas One of the most popular antenna types that offers several advantages such low profile, low fabrication cost and easy integration to other microwave circuits. Microstrip antennas can be found in numerous daily life applications such as mobile handsets, laptops, biomedical devices, and they are also popular for wearable devices. They are generally used at frequencies above 1GHz, as the dimensions are considerably larger at lower frequencies. Typically, a microstrip antenna consists of a metal patch that is printed on a grounded dielectric substrate with a relative permittivity of εr, e.g. a printed circuit board (PCB), as illustrated below; Microstrip antenna W Microstrip Substrate line L y εr Ground plane h x If the metallisation is long and narrow, then a microstrip transmission line is achieved that can be used as an antenna feeder. On the other hand, if both sides of the patch are appreciable fractions of the effective wavelength, then a microstrip antenna is achieved. It should be noted that a microstrip antenna can be of circular, triangular, elliptical and many other shapes. However, only a rectangular shape will be considered in this course. The ground plane can be assumed PEC and should be sufficiently large to minimise the backward reflections and increase the gain in the upper half space. 1 EEE6223 Antennas 2023-2024 Dielectric substrate From the last diagram, it can also be noted that the microstrip antenna is placed at the interface between the dielectric substrate and free space. As a result, an effective dielectric constant is defined as 1 ε𝑟 + 1 ε𝑟 − 1 ℎ −2 ε𝑒𝑓𝑓 = + [1 + 12 ] (80) 2 2 𝑊 where h and W denote the substrate thickness and microstrip line width, respectively. Therefore. the upper half space can be approximated as a homogenous media with a dielectric constant of εeff, in which the antenna is located. As a result, the dimensions of the microstrip antenna are usually defined λ in terms of the effective wavelength λ𝑒𝑓𝑓 =. On the other hand, the √ε𝑒𝑓𝑓 λ substrate thickness, h, is defined in terms of the guided wavelength λ𝑔 = and √ ε𝑟 should be within a range of 0.02λg≤ h≤0.05λg. From the above it can be noted that there are three wavelength definitions to consider; λ in the upper half space, λeff at the interface between the substrate and free-space and λg inside the dielectric substrate. It should be noted that a very thin substrate means the microstrip antenna is placed in a close proximity to the ground plane, which results in a low radiation resistance that leads to lower radiation efficiency and narrower bandwidth, which represent the key limitations of microstrip antennas. On the other hand, electrically thicker substrates offer wider bandwidths albeit with increased antenna profile in conjunction with exciting stronger surface-waves through the dielectric substrate that represent power losses and accounted for by an additional loss resistance defined as Rsw, which needs to be taken into account in the radiation efficiency calculations. Ideally, all the input power is radiated as space- waves, however, and depending on the dielectric substrate thickness and permittivity, a fraction of the power travels within the substrate to form the 2 EEE6223 Antennas 2023-2024 undesirable surface-waves. In addition, the dielectric losses of the substrate have an additional impact on the radiation efficiency particularly at higher frequencies. In a real world, the dielectric constant is a complex quantity defined as 𝜀𝑟 = 𝜀𝑟′ − 𝜀𝑟′′ 𝑗𝜀𝑟′′ , where the dissipation factor is given by the loss tangent, tan 𝛿 =. For 𝜀𝑟′ most dielectric materials tanδ is exceedingly small and can be ignored at lower frequencies. However, at mmwave frequencies and higher, the dielectric losses can have a significant impact on the antenna’s efficiency and should be taken into consideration. Generally, substrates with lower dielectric constants of εr≈2-3 are preferred to minimise surface-wave losses. However, a popular low-cost dielectric substrate is known as FR4 with εr≈4 and relatively high dielectric losses that limit the usage at higher frequencies. In addition, substrates with higher dielectric constants provide physically smaller configurations and usually used for semiconductor devices and special antenna types such as on-chip antennas. Optimum performance can be achieved by using thicker substrates with lower permittivity as this provides wider bandwidth and higher efficiency at the expense of a larger profile. Feeding Techniques One of the advantages of microstrip antennas is the compatibility with several feeding options such as microstrip line feed, probe feed, aperture coupling and proximity coupling. The first two feeding methods involve direct connections between the feed and antenna. However, the other two methods are based on electromagnetic coupling between the feed and the antenna. In order to maintain symmetry, the microstrip antenna is usually feed along the centreline that has axes of (-0.5L≤x≤0.5L, y=0). 3 EEE6223 Antennas 2023-2024 A probe feed can be achieved by extending the inner connector of a coaxial couple so that it passes through the dielectric substrate and soldered to the antenna. The required input impedance of the microstrip antenna can be achieved by adjusting the position of the probe-antenna connections point. However, the W probe has its own inductance that Microstrip may shift the antenna’s resonance antenna frequency. In addition, for a thicker εr L h substrate, a longer probe is required Groundplane which can cause radiation that Coaxial connector Coaxial probe feed interfere with the desired antenna radiation. Another feeding method involves a microstrip feeding line that is in the same plane, which provides a simple Microstrip antenna planar structure. In addition, the feeding line is directly connected to the antenna and is usually used with inset. The dimensions of the Groundplane inset can be optimised to achieve the required matching. Inset microstrip line feed Proximity coupling involves the Microstrip Microstrip antenna addition of a second substrate with a line relative permittivity of εr2. The microstrip antenna is printed on top of that substrate, while the strip ~ εr2 εr1 feeding line is placed at the interface between the top substrate and the Proximity coupled feed 4 EEE6223 Antennas 2023-2024 lower grounded substrate. Two dielectric substates provides an extra degree of freedom in the design. However, the structure is more complex to fabricate. The top and side views of the aperture fed microstrip antenna are illustrated below, where again two substrates have been used that are separated by a ground plane in which a slot has been created to facilitates the electromagnetic coupling between the antenna and the source. The strength of coupling depends on the slot shape, size and position. However, in most cases the slot is positioned below the centre of the microstrip antenna. The existence of the ground plane between the antenna and feed network reduces the interaction between them and eliminates any spurious feed radiation that might interfere with the antenna radiation. Matching can be achieved by using a stub that is defined as the length of the microstrip line that extends beyond the slot. Slot Microstrip antenna Microstrip antenna Slot εr2 εr1 εr2 Microstrip line Groundplane Microstrip line εr1 Groundplane Aperture coupled feed Radiation Mechanism The microstrip antenna can be analysed either using transmission line method, cavity method or full-wave simulations. Since cavity method is more complex, the transmission line approach will be followed. In the diagram below, the 3D microstrip antenna view is duplicated as a reference. From the side view it can be noted that the feed line has been omitted since the focus is on the antenna’s electromagnetic radiation. 5 EEE6223 Antennas 2023-2024 In addition, the illustrated antenna-ground plane combination can be considered as a transmission line. Therefore, when the length L≈0.5λeff , a maximum electric field exists at the edges and minimum at the centre of the antenna. However, fringing electric fields can be noted near the microstrip antenna’s left and right edges that are exposed to free space and, hence, cause the desired radiation. As can be noted from the side-view, the fringing fields extend along the width of the antenna, W, and their x components have equal magnitudes since they are (a) 3D view Microstrip antenna εr E E h Ground plane L (b) Side view L W ∆L ∆L (c) Top view 6 EEE6223 Antennas 2023-2024 separated by approximately half wavelength and they are in phase to satisfy the boundary conditions. Furthermore, the fringing fields can be represented as two slots to the left and right of the microstrip antenna, each with a width of ∆L as demonstrated in the above diagram, where 𝑊 (𝜀𝑒𝑓𝑓 + 0.3) ( + 0.264) ∆𝐿 = 0.412ℎ ℎ 𝑊 (81) (𝜀𝑒𝑓𝑓 − 0.258) ( + 0.8) ℎ The impact of the fringing electric fields increases the effective length of the antenna, i.e. Leff=L+ 2∆L. Therefore, the antenna will resonate at a lower frequency due to the extended length. The lowest resonance frequency can be 𝑐 𝑐 determined when Leff ≈0.5λeff, i.e. 𝑓𝑟 = , which is less than 𝑓𝑟 = , 2𝐿𝑒𝑓𝑓 √𝜀𝑒𝑓𝑓 2𝐿 √𝜀𝑒𝑓𝑓 where c is the speed of light in free space. Finally, it should be noted that the field distribution between the microstrip antenna and ground plane corresponds to the fundamental TM10 mode. Design Procedure For a given antenna configuration, a typical design problem involves calculating the required antenna dimensions L and W for a given h, εr, and fr. It should be noted that commercial substrates are available with certain dielectric constants and thicknesses combinations. Therefore, for a practical design, choosing the appropriate h and εr combination is recommended for a given fr and this can be acquired from the data sheets provided by suppliers. Although 3D printing technology has been used widely to fabricate microstrip antennas, there are certain materials that are compatible with this technology and antenna designers need to be aware of this in advance. The next step is to determine the antenna width, W, that provides the maximum radiation efficiency as 7 EEE6223 Antennas 2023-2024 𝑐 2 𝑊= √ (82) 2𝑓𝑟 𝜀𝑟 + 1 The effective permittivity, εeff, and ∆L need to be calculated next as explained earlier. Finally, the actual microstrip antenna length can be determined as L= Leff- 𝑐 2∆L, where 𝐿𝑒𝑓𝑓 =. 2𝑓𝑟 √𝜀𝑒𝑓𝑓 Radiation Pattern and Input Impedance Based on (68), the radiated fields from a y-directed slot, with a length of W, can be expressed as 𝑉𝑜 𝑒 −𝑗𝛽𝑟 𝐸𝜃 = 𝑗𝛽 𝑊 cos 𝜙𝑓(𝜃, 𝜙) 𝐴𝐹 (83a) 2𝜋𝑟 𝑉𝑜 𝑒 −𝑗𝛽𝑟 𝐸𝜙 = −𝑗𝛽 𝑊 cos 𝜃 sin 𝜙 𝑓(𝜃, 𝜙) 𝐴𝐹 (83b) 2𝜋𝑟 where 𝜋𝑊 sin ( sin 𝜃 sin 𝜙) 𝑓(𝜃, 𝜙) = 𝜆 𝜋𝑊 ( sin 𝜃 sin 𝜙) 𝜆 and AF is the array factor given by 1 𝑂𝑛𝑒 𝑠𝑙𝑜𝑡 𝐴𝐹 = { β𝐿eff 2cos ( sin θ cos ϕ) 𝑇𝑤𝑜 𝑠𝑙𝑜𝑡𝑠 2 (83) is valid for the two radiating slots at the sides of the microstrip antenna provide βh