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Communication Systems
Published in Arun G. Phadke, Handbook of Electrical Engineering Calculations, 2018
Low-gain antennas, such as dipoles, slots, and some traveling wave antennas such as the Yagi and Helix antennas, can have effective areas much larger than their physical area. For example, a halfwave dipole antenna has a gain of 1.64, or 2.1 dB, broadside to the dipole. Using Eq. (4.86), the effective area of the dipole is 1.64λ2/4π = 0.13λ2. A halfwave dipole is 0.5λ high and typically less than 0.1 λ in width. Thus the physical cross-sectional area of the dipole is no more than 0.05λ2, but its effective area is 0.13λ2. Yagi antennas, consisting of an end fire array of directors which are equivalent to short-circuited halfwave dipoles, driven by a dipole or folded dipole, can have a gain of 11 dB or more, giving an effective area greater than a square wavelength, yet the cross-sectional area is no greater than that of a halfwave dipole.
EM behavior when the wavelength is about the same size as the object
Published in James R. Nagel, Cynthia M. Furse, Douglas A. Christensen, Carl H. Durney, Basic Introduction to Bioelectromagnetics, 2018
James R. Nagel, Cynthia M. Furse, Douglas A. Christensen, Carl H. Durney
Antennas are classified into several groups: wire antennas, aperture antennas, array antennas, reflector antennas, and lens antennas. Wire antennas are various combinations of wires or rods. Some commonly used ones are shown in the upper part of Figure 3.50. A dipole antenna consists of two segments of rod or wire, with a transmission line connected between them. The length of a dipole antenna is typically one-half of a wavelength. A folded dipole, as the name indicates, is a dipole with an additional connection between the ends. Loop antennas may be circular, square, or other shapes.
Introduction
Published in Sibley Martin, Modern Telecommunications, 2018
Before we discuss the various bands and what they are used for, it is instructive to look at the wavelength as quoted in the final column of Table 1.1. As noted previously, a dipole antenna should have a length of λ/2. If we were to use the extremely low frequency (ELF) band, the antenna size would be five million metres long for 30 Hz. This is clearly impractical. Another problem is that if we were to transmit speech with a range of 300 Hz to 3.4 kHz, we would take up the whole of the ultra low frequency (ULF) band and some of the very low frequency (VLF) band and the antenna length would have to vary as well. Clearly, we can’t use very low frequencies for broadcasting audio. Now check what happens as the frequency increases. The antenna size reduces to manageable values and the bandwidth (the difference between the upper and lower frequencies) goes up. Taking the high-frequency (HF) band as an example, the bandwidth is 27 MHz and we can fit many broadcast stations in such a bandwidth.
Optimal Design of Linear Antenna Arrays of Dipole Elements Using Flower Pollination Algorithm
Published in IETE Journal of Research, 2019
Gopi Ram, Rajib Kar, Durbadal Mandal, Sakti Prasad Ghoshal
The dipole antenna is one of the utmost influential and generally used radio frequency (RF) antennas. Dipole antenna is generally used independently and also can be incorporated in many other RF antenna designs where it forms the driven element for the antenna. Dipole antenna is constructed with two thin dipole elements that are symmetrically fed at the centre by a balanced two-wire transmission line [1]. Dipole antennas are of various types like Hartzian dipole, half-wave dipole, small dipole [2] etc. To match with the line impedance, the radiation resistance of the dipole antenna should be of 73 ohms. In this paper, linear array of dipole element is considered. The radiation properties of the antenna arrays can be modified by their geometrical configurations or by the variable parameters of the array factors [1–9].
Control of carbon nanotube cantilever vibrator for nano-antenna applications
Published in Cogent Engineering, 2019
Ali Jasim Ghaffoori, Wameedh Riyadh Abdul-Adheem
Several devices, including nano-antennas, have been widely applied in the field of radio communication. To achieve maximum transmission efficiency, the length of the dipole antenna must only be half of the wavelength at which the transmission is carried out (Nikolayev, Zhadobov, Karban, & Sauleau, 2018). A mechanical oscillator can collect radiation over a relatively large area and be applied to develop the circuitry of retrievers that use CNT. This idea is constantly evolving and has been applied in creating unique design methods for controlling the CNT cantilever vibrator of VHF nano-antennas (Lokman et al., 2017).
Improvements in radio-frequency transmission for ultra-high field magnetic resonance imaging through a bilateral monopole antenna
Published in Electromagnetics, 2018
Han-Joong Kim, Phil Heo, Sang-Doc Han, Donghyuk Kim, Hyunwoo Song, Kyoung-Nam Kim
The signal-to-noise ratio (SNR) for a loop coil is in a proportional relationship with the static magnetic field (B0) strength (Cho et al. 2008; Hayes and Axel 1985). Better SNR can be obtained as the field strength of magnetic resonance imaging (MRI) increases, but inhomogeneous RF magnetic fields (B1) also appear (Hoult 2000; Vaidya et al. 2016). Inhomogeneous B1 under ultra-high field (UHF) MRI are mainly caused by two factors: constructive or destructive radio-frequency (RF) interference and the twisted B1 pattern created by the transmission field (|B1+|) and reception field (|B1–|), which is in turn due to the B1 field being affected by the B0 strength, different values of dielectric properties (i.e., relative permittivity (εr) and electrical conductivity (σ)), and subject geometry. In particular, B1 twisting is generated by the phase differentiation between the |B1+| and |B1–| generated by the current of the loop coil and the magnetic field induced by the conduction current in the sample. Using higher B0 strength has advantages such as higher SNR but also poses a challenging task due to development of an inhomogeneous field pattern. Many studies on several types of traveling-wave antenna as RF coils for UHF MRI are proceeding due to the resulting homogeneous MR images and deeper RF penetration than conventional RF coils (Brunner et al. 2009; Vaugha et al. 2009; Raaijimakers et al. 2011; Raaijimakers et al. 2016). Among many traveling-wave antennas, the dipole is known to be the simplest and most basic type of antenna. The dipole antenna has a self-resonance frequency when the length of the dipole is slightly less than λ/2, and the input impedance is about 73 Ω (Balanis 1982). However, this is calculated in the free-space condition, so when a dielectric sample approaches the antenna, the dielectric loading effect shifts the input impedance, altering the length of the dipole antenna at the targeted frequency. At 7 T, the half-wave dipole antenna is about 500-mm long, which is too large for human head images; this is being studied in order to reduce the length of the half-wave dipole antenna (Oh et al. 2017; Olaode, Palmer, and Joines 2012; Raaijmakers et al. 2011). Another application that aims to reduce the length of the existing half-wave dipole antenna is the quarter-wavelength monopole antenna (Hong et al. 2014). The monopole antenna has a self-resonance frequency at the half-length of the half-wavelength dipole antenna, but has the disadvantage of being limited to a focused RF signal near the ground region. In this letter, a novel monopole antenna structure in which two monopole antennas are overlapped is proposed to improve |B1+| uniformity at 7 T. The proposed bilateral monopole antenna is optimized by performing an electromagnetic (EM) simulation with the finite-difference time-domain method, and improvements achieved using the optimized bilateral monopole antenna structure are discussed.