Input impedance approximations for small dipole and loop antennas near ground

This paper presents the study of comparison of input impedance and total gain of Side Fed Bifilar Helix Antenna (SFBHA) with the height variation (in terms of wavelength) from Perfect Electric Conducting ground plane (PEC) which is fixed in size. The simulated results show that a 10λ×10λ size metallic ground plane has reasonable accuracy approximating the infinite PEC ground plane. The input impedance and Voltage Standing Wave Ratio (VSWR) variations are noticeable between the heights of 0.05λ to 0.35λ both in actual measurements and simulations. The observed variations above the height of 0.35λ are negligibly small. The simulated results of input impedance and VSWR are very similar to measured results. The simulated results and the measured results show the total gain pattern of SFBHA remains stable by varying the height of SFBHA above PEC. In this paper the performance comparison shows that electromagnetic performance of SFBHA till 0.35λ is better than Horizontal Dipole antenna (HD), Vertical rectangular Loop antenna (VL) and also another orientation of rectangular loop antenna that is Horizontal and Perpendicular Loop (H&PL).

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.

This paper first examines and corrects Storer’s theory of circular loop antennas, and then investigates theoretically the proposed Ω-shaped loop-dipole antenna. Special emphasis is given to the analysis of the effect of the higher-order terms of the Fourier series as a remainder on the current distribution, input impedance, and radiation patterns of the thin-wire circular loop antenna. It is found that the remainder must be included in the calculation of the current distribution and input impedance but can be neglected to simplify the calculation of far-field radiation. Then, a theoretical model is proposed to analyze the radiation fields, directivity, and input impedance of the proposed Ω-shaped loop-dipole antenna based on the loop and dipole antennas’ theories. Analytical expressions for the radiation fields, directivity, and input impedance of the Ω-shaped loop-dipole antenna are derived. The theoretical and numerical results are compared for the Ω-shaped loop-dipole antennas of practical interests. They are in good agreement.

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.

It is shown that when a small bare circular loop antenna is buried in the ground it will respond to a vertically ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</tex> field) polarized ground-wave or sky-wave source with a directional azimuthal pattern. The response to a horizontally polarized skywave source is greatly enhanced by burying the antenna and is also shown to be far from omnidirectional in azimuth. These results depend upon the presence of a relatively small departure from a constant current distribution on the loop when it is regarded as a driven element. The current distribution formulas of Chen and King permit easy analytical estimates of these effects.

A 3D location determination method using only four elements antenna is proposed. A group of frequencies is received from the transmitter (on mobile phone) or returned signals (Radar). The matrix pencil method is used to identify the length of each path. Depending on the geometrical distribution of the four elements, the phase shifts are determined for each path. Knowing the phase shift between the elements helps to find the direction of arrival for each path. Small dipole or small circular loop antennas are used in the numerical examples to clarify this method in the presence of noise.

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. >

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.

Implantable devices for wireless biotelemetry applications in the Medical Implant Communication Service (MICS) band ask for miniature, low-profile, broadband and insensitive to environmental changes antennas. Fundamental design techniques, if applied properly, can lead to optimum results. In this work, we discuss the appropriate approach for designing extremely small planar dipole antennas. As demonstrated, modifications on antenna structure driven by current vector alignment and conductor coupling factors affect antenna effective dimensions. Also, metallization proves to offer greater antenna performance compared to impedance matching with lumped elements. Finally, the quality factor of two small dipole antennas is evaluated.

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.

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 &lt; 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.

In this article an experimental methodology for predicting far-field radio-interference from the pulse conducted of positive corona current measured in a rod-plane configuration is presented. From the pulsed conducted corona current, the fundamental frequency is determined. The source of the corona discharge (rod-plane structure) is considered as a small dipole antenna of length equal to the gap of the rod-plane. Applying the basic propagation expressions and the far-field expression for a small dipole antenna, the radio-interference is predicted. The radio-interference (electric field) is measured by a loop antenna according to CISPR-16 standard. One of main result in this work is the determination of the fundamental frequency of the corona discharge for different gaps of a rod-plane configuration frequency where the energy is concentrated. Using this parameter, the far-field electric is predicted. The emissions radiated measurements at a distance of 4 m are above of the EMC standard limits.

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.

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.