Electrically Small Loop Antenna Standing on Compact Ground in Wireless Sensor Package

Abstract: This paper is about the optimization of a small circular loop antenna using a Genetic Algorithm (GA) to significantly enhance its radiation efficiency for operation at 38.22MHz. Traditional small loop antennas are known for their poor radiation characteristics due to extremely low radiation resistance and high conductor losses, rendering them unsuitable for practical RF communication without a complex matching network. This research applies a Genetic Algorithm implemented in MATLAB to optimize two critical design parameters: the loop radius and the wire radius (thickness). An analytical electromagnetic model was developed to compute radiation resistance, conductor loss resistance (accounting for skin effect and radiation efficiency. The objective function was designed to maximize efficiency by minimizing its negative value. Results from the GA were analysed using convergence plots and two-dimensional contour maps, revealing the efficiency distribution across the design space. The optimized antenna achieved a loop radius of 0.02458m and a wire radius of 0.0099m, leading to a dramatic improvement in radiation efficiency from 0.02% (traditional design) to 99.4%. Additionally, gain and bandwidth performance were significantly improved, making the antenna suitable for terrestrial RF applications. The study demonstrates that evolutionary optimization techniques like GA can overcome the inherent limitations of traditional small loop antennas, by intelligently navigating the design space, the Genetic Algorithm identified configurations that would be difficult to obtain through manual tuning. These findings highlight the practical potential of computational intelligence in modern antenna design particularly for compact efficient and broadband systems in wireless and communication technologies.

A critical need in the evaluation of an ultrawideband (UWB) radio system is the calculation of the energy link loss between the source at the transmit antenna and the receiver load. While the rigorous calculation of link loss in a wideband pulsed system requires a full transient electromagnetic solution for the transmit and receive antennas, we show in this paper that accurate approximations for link loss can be obtained for the special cases of electrically small dipole or loop antennas, with Gaussian or Gaussian doublet (monocycle) generator waveforms. We also consider the error involved with applying the much simpler narrowband Friis transmission formula. It is found that the use of the basic Friis formula can result in link loss errors of more than 60 dB for a UWB system having severely (impedance) mismatched antennas, but may give results correct to within a few dB for well-matched narrowband antennas, or if the formula is augmented with an impedance mismatch correction factor. It appears that the dominant limitation of the Friis formula, when applied to UWB systems, is the broadband effect of mismatch between the transmit/receive antennas and their source or load impedances. Numerical examples are presented for electrically short dipoles, resonant dipoles, and broadband lossy dipoles for both Gaussian and monocycle input pulse waveforms.

The authors propose the structure of a monopole loaded small loop antenna (MLLA), where the inductive element of the inverted L antenna (ILA) is replaced by a loop antenna. The monopole part is used as the matching element that lets the loop antenna resonate. MLLAs are found to work without additional circuits when antenna length almost matches the length that yields resonance as calculated by the ILA model. The MLLA radiation pattern duplicates that of a combined antenna model consisting of a small monopole antenna and a small loop antenna. When an MLLA-equipped device is carried, its sensitivity is superior to that of devices using either monopole or loop antennas. >

We propose a new calibration method for circular loop antennas. Generally, the standard field method is widely employed in loop antenna calibration, and a thermocouple is widely used as a current sensor on a transmitting loop antenna's element to calculate the magnetic field strength at an antenna under calibration. Therefore, it has certain disadvantages such as low accuracy of measurement, a fragile structure, and so on. However, the proposed method has the characteristics of the standard field method and reference antenna method, and it is simpler than these methods. In this study, a comparisons are made in simulations and experiments. The results show that the proposed method is simple and efficient for calibrating loop antennas.

In this paper, we propose two reference calibration methods (RCMs) for small circular loop antennas. Generally, the standard field method is widely employed in loop antenna calibration at many national metrology institutes and calibration laboratories. In this method, a thermocouple is employed as a current sensor on an antenna element of a transmitting loop antenna to calculate the magnetic field strength at an antenna under calibration. This method has certain disadvantages: The standard transmitting antenna is complicated, cumbersome, and fragile. On the other hand, a calibration method is necessary to be reasonably traceable to the standard of the magnetic antenna factor. Our proposed methods can establish and keep traceability to it. We also compare the two proposed RCMs and show that they are efficient in calibrating the loop antennas.

Analysis of the near‐field irradiation of prolate spheroidal models of humans and animals by a small coaxial loop antenna is described. The near fields of the antenna are known exactly and hence are used to identify the suitable field parameters involved in the near‐field absorption in the spheroidal model. An integral equation is formulated in terms of the transverse dyadic Green's function, and the fields radiated by the current loop are expanded in terms of the vector spherical harmonics. The extended boundary condition method is then employed to solve the integral equation. The power distribution and the average specific absorption rate (SAR) are calculated and plotted, for different human and animal models, as a function of the separation distance from the loop. It is shown that for distances less than 5λ the average SAR values oscillate about the far‐field value. In particular, for d/λ < 0.4 an increase in the average SAR values was generally observed. It is also shown that in spite of the complicated nature of the near fields the absorption characteristics can still be explained in terms of the incident radiation. Furthermore, from the calculated SAR distributions at different frequencies it is shown that at all frequencies, excessive heating occurs at the surface of the spheroid while a limited absorption occurs in the central region around the major axis. This result is of particular importance in hyperthermia, where extensive efforts are being directed toward achieving deep‐tissue heating by a coaxial coil carrying RF power at about 27 MHz.

Superconductive small loop antennas are investigated. The antenna radiator is a five turn spiral coil made of sintered 50%Ag–50%YBCO ceramics. The inner and outer diameters of the coil are 90 mm and 120 mm, respectively. Two serial capacitors are joined parallel to the coil and used for impedance matching between a radiator and a feed line. The antenna resonates at about 25 MHz and its gain at 77 K is 4.15 dB higher than a metal antenna at 300 K. This result shows that very small loop (diameter < λ/100) antennas with high efficiency can be realized by using superconductive radiators.

The electromagnetic properties of electrically small loop antennas for two different applications are discussed: near-field and far-field RFID systems. In the first case miniature loop antennas are designed for the near field wireless biological and medical sensing application. The magnetic field in the near-field zone is used for coupling between the loops. Using regularly placed capacitors between segments of the loop provides a homogeneous current distribution along the electrically small loop resulted in the improved magnetic coupling. In another case the loop antenna with the improved radiation characteristics is designed for far distance RFID reader antenna. The antenna consists of a combination of the noninteracting loop and dipole. Such a combination shows a cardioid-type radiation pattern and doubled antenna gain. Results of simulations and measurements of test structures of the loop antennas are presented.

The paper presents the method for measuring parameters (gain, antenna factor, impedance and radiation pattern) of small antennas in GTEM cell, which is a novel method and environment for antenna measurements. In order toinvestigate the suitability of GTEM cell for this kind ofmeasurement, the measurement results for a biconical dipole, microstrip patch antennas and small loop antenna were compared with those obtained by calibration inside fully absorber lined anechoic site, two-antenna measurements and FEKO simulations. The measurements were carried out over the wide frequency range. Measurement setup was limited to small antennas that fit into the usable test volume of the GTEM cell. Different sizes that satisfy this restriction were examined. The GTEM measurement results showed considerable agreementwith compared results.

In this paper, the electromagnetic radiations of a PCB planar inductor are researched based on the antenna theory. For the far-field radiation, each turn in the spiral windings of a PCB planar inductor can be viewed as a loop antenna. According to the antenna theory, it is a small loop antenna in the frequency range of 30MHz to 300MHz. By this assumption, the radiation power of a PCB planar inductor is estimated and found to be negligible. Furthermore, by comparing the radiation resistance of a small loop antenna with it of a short electric dipole, show that the radiation power of a turn in the spiral windings of a PCB planar inductor is far smaller than a copper track with the same dimensions in the printed circuit board. The near-field coupling is discussed based on a simple resonant electric circuit with a PCB planar inductor. The 3D plot of near-field radiation intensity is given, by which illustrate that the near-field coupling is only occur in the area where the inductor is located.

Theoretical and experimental analysis of a newly proposed loop antenna for handsets is presented. A simple model, using a small rectangular and L-shaped loop antenna mounted on a ground plane, which represents a rectangular shielding plate used in the handset, is analyzed in order to confirm the effectiveness of using the balance-fed antenna system.

Approximate expressions for the input impedances of small vertical and horizontal dipole and loop antennas above a conducting ground plane are developed. The formulas give increasingly accurate impedance estimates as [Formula: see text], where N is the refractive index of the ground. A comparison with the exact solution for a specific case, [Formula: see text] and varying antenna heights, is given.

—This paper presents the test results of an electrically small loop antenna sensor towards detecting the radiated magnetic field due to partial discharge (PD) activities. The loop antenna prototype was printed on an FR-4 substrate with a thickness of 1.6 mm, featuring a compact size of 20 mm × 20 mm × 1.7 mm. The proposed sensor, initially simulated with Ansys HFSS software, has a resonant frequency of about 100 MHz. Its detection capability for three types of partial discharges (i.e., corona, surface and internal PDs) has been evaluated. Laboratory experiments showed that the evaluated loop spiral antenna is appropriate for the detection of the three types of PDs. Indeed, the results show a peak-to-peak voltage equal to 22.87 mV, 74.8 mV, and 79.72 mV, respectively, for corona, surface, and internal PDs, with a frequency spectrum showing a maximum at 42.19 MHz, 16.41 MHz, and 35.1 MHz, respectively. Lastly, the performance of the proposed small loop sensor was compared to that of the commercial HFT sensor. The results show that the two sensors present comparative sensitivity for the three types of PD.

An exact expression for the fields produced by an electrically small loop antenna is obtained in terms of a series involving a new set of functions, namely en(σ,γ)=(1/2π)∫02πcos(nΦ)sin[σ(1−2γcosΦ)1/2]dΦ and fn(σ,γ)=(1/2π)×∫02πcos(nΦ)cos[σ(1−2γcosΦ)1/2]dΦ, where − ∞&amp;lt;σ&amp;lt;∞ and 0≤γ≤1/2. Series expansion, approximate expressions for γ2σ≪1 and recursion relations for the functions are derived. These functions permit the exact evaluation of the fields in the Fresnel and near-field regions of the loop antenna. In the case that the distance to the point of observation is of the order of a wavelength or greater, but greater than the loop radius, the field expression is given in terms of the function q1(σ,γ) = f1(σ,γ) + ie1(σ,γ). Numerical results describing the behavior of the functions f1(σ,γ) and e1(σ,γ) are given. Field expressions for the electrically large loop antenna are also derived.

The fundamental properties of carbon nanotube (CN) loop antennas have been determined using numerical modeling in both transmitting and receiving modes. The variation of the antenna impedance, reflection coefficient, radiation efficiency, Q-factor, and extinction cross section with varying loop radii was determined numerically using method-of-moment simulations incorporating the quantum mechanical conductivity of carbon nanotubes. Contrary to metallic loop antennas (well known for their inductive input impedance), the carbon nanotube loops with a large circumference show capacitive reactance over the entire frequency range. Therefore, one cannot make CN loop to act as an inductive radiator or even a simple inductor (when ka → 0) if the circumference is beyond ~56 μm. However, the carbon nanotube loops with circumference smaller than ~56 μm exhibit properties similar to ordinary loop antennas by going through antiresonance and then resonance regions. Despite the dispersive conductivity model, carbon nanotube loop antennas possess normalized dispersion relation, which is linearly related to the frequency.