Reconfigurable Microstrip Antennas
Reconfigurability in antennas is desired to eliminate the need to employ multiple antennas for diverse characteristics such as polarization, frequency and radiation patterns.17-21 Reconfigurability is achieved by modifying substrate properties or physical dimensions using switches or tunable materials. Substrate material properties directly affect the resonant frequency (inversely proportional to relative permittivity and permeability) and the operational bandwidth (directly proportional to thickness of the substrate) of microstrip antennas. Ferroelectric or ferrite material can vary permeability by application of a DC bias field, thus, controlling an antenna’s resonant frequency by varying its electrical dimensions.18
Conductor dimensions in microstrip antennas can control radiative properties (see Figure 10).19 Figure 10a shows a microstrip rectangular patch (MRP) with an outer ring connected to an inner one through diodes. By switching the diodes ‘on’ and ‘off,’ the MRP size is altered, varying its resonant frequency. Figure 10b shows reconfigurability in polarization by virtue of shorting posts placed diagonally inside the patch. Figure 10c shows a common aperture supporting two antennas, a receive antenna (total aperture) and a transmit antenna (inner circle) in a shared manner, demonstrating reconfigurability of two radiating modes.
Body-Wearable Antennas
Body-wearable antennas have immense potential in applications for body area networks (BANs), wireless area networks (WANs), health monitoring and diagnosis and body-to-body communication.22-30 Figure 11 illustrates one use case of a person communicating to a body-mounted sensor with that sensor communicating to a remote health provider for monitoring and to a satellite for remote data collection. The antenna is in close proximity to the body and is very challenging to design in terms of the impedance matching, specific absorption rate (SAR), size, cost, weight, volume and conformability. Mandal et al.29 describe a low profile, circularly polarized, “button antenna” for Wi-Fi and WLAN applications with a frequency selective structure (FSS) to suppress back radiation. Njogu and Sanz-Izquierdo30 demonstrate a fingernail-shaped antenna for on body communications.
Fig. 11 Body-wearable antenna use case.
MIMO Antennas
MIMO31-33 is a radio antenna technology using multiple antennas at the transmitter (Tx) and receiver (Rx) to provide multiple signal paths for carrying data (see Figure 12). MIMO can enable beamforming, transmit diversity and receive diversity. Diversity techniques protect against fading and improve coverage in an urban scenario. MIMO spatial multiplexing uses the same frequency to transmit different signals. Massive MIMO (M-MIMO)32-33 uses large numbers of phased antenna arrays instead of active terminals and time-division multiplexing. Energy is focused into small spatial regions for high radiated energy efficiency and throughput. M-MIMO is important for 5G applications.
Ultra-Wideband (UWB) Antennas
UWB antennas34-40 are used in low power and short-range applications due to the limitation of pulse forming networks. An UWB transmitter in the U. S. is defined as an intentional radiator that, at any point in time, has a bandwidth equal to or greater than 500 MHz or a fractional bandwidth (FBW) greater than 0.2. Antenna design is challenging. UWB antennas should have flat group delay over their bandwidth. The FBW of UWB is defined as
The U.S. Federal Communications Commission (FCC) has authorized unlicensed usage of UWB systems below the 960 MHz and in the 3.1 to 10.6 GHz band with very low effective isotropic radiated power (EIRP). The FCC power spectral density emission limit for UWB transmitters is −41.3 dBm/MHz. The challenges in design of UWB antennas are: achieving consistent gain, HPBW, polarization and phase over the UWB; realizing a small antenna size (low profile) to fit into commercial systems; and controlling cost.
Many antenna designs belonging to the family of traveling wave antennas like log-periodic dipole antennas (LPDAs), Vivaldi antennas and spiral antennas have been developed to achieve UWB; however, they have large profiles and are bulky, making them impractical for wireless communication systems. Over the last decade there has been much research on planar monopole antennas for UWB.36-37, 39 The ground plane of these antennas is kept small to attain a low profile. Hence, design activity should consider the antenna ground plane in system design and optimization.
Nunn et al.39 describe an UWB circular monopole antenna array for surface-based ice sounding in the UHF Band. It comprises 16 planar subarrays forming a 16 x 17 m Mills cross array that maximizes sensitivity and spatial selectivity in both cross- and along-track directions. An insulation foam separates the ground plane from monopole such that maximum radiation is directed broadside. Nie et al.40 report on a two-port coplanar waveguide (CPW) fed UWB antenna with high isolation between ports and a common ground. Designed for full hemispherical coverage, its isolation is greater than 31.4 dB with a diversity gain of 10 dB from 2.98 to 10 GHz. A 2.5 dB gain with efficiency better than 80 percent is also demonstrated.
Metamaterial Antennas
Metamaterials are electromagnetic structures with unusual properties that are not seen in nature:41, 42
- Double negative (DNG) materials
- Negative Refractive Index
- Left-Handed
- Single-negative (SNG) materials (E – , H – fields and wave vector do not follow the right-hand rule.)
- Backward wave (energy flow is anti-parallel to wave vector)
DNG materials cause the phase velocity and power flow to be anti-parallel; thus, cut-off waveguide realization is not hypothetically feasible. The challenge is the fundamental limitation with respect to an antenna’s quality factor and its electrical size. Resonances in these materials make it possible to design smaller antennas (see Figure 13).42 Although thin and measuring only a few millimeters these multiband antennas double the range and enhance reliability and battery life of mobile phones, Wi-Fi routers and wireless modems.
Connected Array Antennas
Wheeler43 introduced the concept of a continuous current sheet (CCS) that radiates a transverse electromagnetic (TEM) wave (see Figure 14a). The radiation resistance, R, of the current sheet with a ground plane separated by λ/4 is 120π Ω at
broadside and leads to a hypothetically infinite bandwidth. The CCS absorbs an incident plane wave without any reflection. For an E-plane scan with an incident angle the boundary resistance is proportional to cos due to the cosine projection of the E-field on the aperture; and, for H-plane scan, the boundary resistance is proportional to cos - 1. In reality, however, an infinite and uniform current sheet is not feasible. Munk44 was the first to demonstrate it practically utilizing interconnected antenna elements with a 5:1 bandwidth dipole array called a ‘current sheet array’ (CSA) (see Figure 14b). TCDAs are being researched to yield more efficient forms of CSAs.3, 38, 45
Windscreen Antennas
The automobile industry is introducing multiple on-board sensors for safety and entertainment.46 Figure 15 shows a car supporting various antennas connected to its body (f1, f2, f3, f4, etc.). Cars require antennas for music (f3), file sharing (f2), navigation (f1) and anti-collision radar (f4) systems. They require GPS antennas for navigation and antennas to support various sensors such as rain and sleep sensors. The challenges are to achieve a small form factor while mitigate the effects of mutual interference between the multiple sensors, as well as the effect of the vehicle’s body on performance. The design process is not simple and requires hybridization of various low frequency design techniques such as the method of moments (MoM) and FEM with high frequency design techniques such as the uniform theory of diffraction (UTD) and shooting bouncing ray (SBR).47, 48
Fractal Antennas
A fractal is a fragmented geometrical shape that looks the same, independent of size scaling. A fractal shaped metal element can be used as an antenna over a very large band of frequencies demonstrating self-similarity and scaling independence. Mandelbrot coined the term in 1983,49 but the origin dates back to von Koch in 1904.50 He showed that it is possible to devise a curve without a tangent, anywhere. The most popular fractal shapes used for antennas are the fractal carpet, Sierpinski’s gasket, Cantor’s comb, von Koch’s snowflake, the Mandelbrot set and the Lorenz attractor. One significant property of all these fractals is their irregular nature. Figure 16 shows some typical fractal antennas.
Fig. 16 Typical examples of fractal antennas.A few recent publications,52, 53 demonstrate new types of fractal antennas such as the flower fractal and fern fractal leaf inspired Vivaldi antennas. Mondal et al.52 describe a flower fractal based circularly polarized folded microstrip patch antenna with 49 percent miniaturization, 110 degree 3 dB beamwidth and 120 degree AR beamwidth. A nature inspired fern fractal leaf structure demonstrates an impedance bandwidth of 19.7 GHz and a gain of 10 dBi.53
Smart Antennas
The smart antenna is a misnomer. It is actually a smart system combined with an antenna structure. It is used for applications such as direction of arrival (DoA) estimation, adaptive beamforming and adaptive null formation.54, 55 Figure 17 is an analogy comparing a smart antenna system with a blinded man and two speakers. The blinded man can “tune” his ears to hear one speaker while ignoring the other one. Similarly, a smart antenna system can suppress interference from one direction and enhance reception of a signal from the desired source. MIMO is one example of a smart antenna system; it is not a separate type of antenna but an intelligent system of existing antenna types.
DGS Antennas
In practice, an antenna must be mounted on some structure, which may be an aircraft, ship or a stationary structure like a cell phone tower. Conventionally, to facilitate analysis, antenna design assumes an infinite ground plane or a finite ground plane with good planarity. Mounting structures affect antenna performance because they do not satisfy the precise conditions assumed in design. Figure 18 is a sketch showing the effect of the platform on an antenna pattern. The black and the blue curves show a hypothetical antenna pattern without and with the effect of the airborne platform, respectively.
DGSs or EBG structures derived from photonic band gap (PBG) structures offer a convenient solution. DGSs are artificial periodic structures exhibiting characteristics similar to band stop filters. They prevent certain bands of frequencies to pass through. They can be used to realize antenna ground planes locally beneath antennas on mounting platforms to minimize platform interaction.56-58 Obelleiro et al.56 present a detailed study of array antennas mounted on aircraft, ships and other vehicular platforms. They consider the effect of mutual coupling between elements and the platform on antenna performance using MoM analysis, with a degradation in sidelobe levels of up to 15 dB. Kumar et al.57 improve polarization purity by incorporating a DGS integrated with a microstrip antenna. A 12 dB improvement in isolation between co- and cross-polarization patterns is demonstrated. Bell et al.58 achieve profile reduction by using DGS structures. DGSs are important in controlling mutual coupling, enhancing polarization purity, enabling miniaturization and mitigating the effects of non-ideal ground planes in antennas designed for practical applications.