Magnetic Loop Antenna Research Articles

Wireless audio signal receiver device for a hearing instrument

A receiver device for receiving audio signals from a remote source has a magnetic loop antenna for receiving radio frequency signals carrying audio signals, a signal processing unit for reproducing audio signals from the radio frequency signals received by the antenna, an output interface which is capable of being mechanically connected to an input interface of a hearing instrument to be worn at a user's ear in order to supply the audio signals from the signal processing unit as input to the hearing instrument, and a housing enclosing the antenna and the signal processing unit. The antenna is designed as a printed board circuit with a loop-shaped conductor on an at least partially flexible insulating substrate. A first portion defines a first plane and a second portion defines a second plane, the first plane and second planes being oriented at an angle of 60° to 120° relative to each other.

Nonlinear electron magnetohydrodynamics physics. IV. Whistler instabilities

A very large low-frequency whistler mode is excited with magnetic loop antennas in a uniform laboratory plasma. The wave magnetic field exceeds the ambient field causing in one polarity a field reversal, and a magnetic topology resembling that of spheromaks in the other polarity. These propagating “whistler spheromaks” strongly accelerate the electrons and create non-Maxwellian distributions in their toroidal current ring. It is observed that the locally energized electrons in the current ring excite new electromagnetic instabilities and emit whistler modes with frequencies unrelated to the applied frequency. Emissions are also observed from electrons excited in X-type neutral lines around the antenna. The properties of the excited waves such as amplitudes, frequency spectra, field topologies, propagation, polarization, growth, and damping have been investigated. The waves remain linear (Bwave⪡B0) and convert a small part of the electron kinetic energy into wave magnetic energy (Bwave2∕2μ0⪡nkTe).

Nonlinear electron magnetohydrodynamics physics. II. Wave propagation and wave-wave interactions

The propagation of low-frequency whistler modes with wave magnetic field exceeding the ambient field is investigated experimentally. Such nonlinear waves are excited with magnetic loop antennas whose axial field is aligned with the background magnetic field and greatly exceeds its strength. The oscillatory antenna field excites propagating wave packets with field topologies alternating between whistler spheromaks and mirrors. The propagation speed of spheromaks is observed to decrease with amplitude while that of mirrors increases with amplitude. The field distribution varies with amplitude: Spheromaks contract axially while mirrors spread out compared to linear whistlers. Consequently, the peak magnetic field and current densities in spheromaks exceed that of mirrors. Wave-wave interactions of nonlinear whistler modes is also studied. Counterpropagating spheromaks collide inelastically and form a stationary field-reversed configuration. The radius of the toroidal current ring depends on current and can be larger than that of the loop antenna. A tilted field-reversed configuration precesses in the direction of the electron drift. The free magnetic energy is dissipated in the plasma volume and converted into electron heat.

Nonlinear electron magnetohydrodynamics physics. III. Electron energization

Wave-particle interactions of low-frequency whistler modes with wave magnetic fields exceeding the ambient field are investigated experimentally. These highly nonlinear modes are excited with magnetic loop antennas in a large magnetized afterglow plasma. While the nonlinear wave properties are described elsewhere, the present paper focuses on the modification of the electron distribution function by the whistler waves. When the electron current flows in regions of magnetic nulls, such as in spheromak and field-reversed configurations (FRCs), strong electron energization is observed. When the whistler modes are created by electron Hall currents, such as in whistler mirrors, no significant energization occurs. The electron temperature can be raised locally by an order of magnitude. Non-Maxwellian distributions with energetic tail electrons are observed. Electron energization to ≳10eV produces visible light emission whose time and space dependence is mapped. The light source travels with the subthermal speed of whistler spheromaks. When counterpropagating spheromaks collide, the resultant FRC produces strong local heating and light which dissipates its free magnetic energy.

Performance Evaluation of Built-In Small LF Antennas inside a Metal Case

This paper describes a method for evaluating the performance of a small magnetic core loop antenna used for radio controlled watches. Recently, amorphous metal core loop antennas are used as built-in small antennas inside a metal case. It is difficult to perform electromagnetic simulation for amorphous core loop antennas because of the complicated laminate structure. Therefore, we modeled the amorphous metal core loop antenna as an equivalent bulk structure having anisotropic permeability property that we can simulate. We analyzed the receiving sensitivity of the amorphous antenna by calculating the antenna factor. The receiving sensitivity degrades remarkably when an antenna is inside a metal case. We performed further simulation to investigate eddy current losses that cause deterioration.

The effective antenna noise figure Fa for a vertical loop antenna and its application to extremely low frequency/very low frequency atmospheric noise

Expressions tabulated for the effective antenna noise figure Fa usually assume an electric field antenna, since most measurements of radio noise are made on the electric field of the noise. Furthermore, the International Radio Consultative Committee (CCIR) noise model predictions for Fa are made only for electrically short grounded vertical monopoles over a perfect ground. However, at frequencies lower than those traditionally used for communications, i.e., at extremely low frequencies (ELF; frequencies in the range 3 Hz to 3 kHz) and very low frequencies (VLF; frequencies in the range 3–30 kHz), it is common for magnetic field loop antennas to be used, and the tabulated expressions for Fa do not apply. This communication reports the derivation of an expression for Fa as measured by a small vertical magnetic loop antenna and its subsequent application to ELF/VLF radio noise measurements made at a variety of locations around the world. There is good agreement between the measured Fa values and estimates of maximum and minimum Fa values for the ELF/VLF range published by Spaulding and Hagn in 1978, but an improved fit to the measurements can be obtained by making moderate adjustments to the maximum and minimum values at both the low (10–100 Hz) and high (8–32 kHz) frequency limits.

Simulation and Design of a Very Small Magnetic Core Loop Antenna for an LF Receiver

SUMMARY In this paper, we evaluated the characteristics of the magnetic core loop antenna that is used to receive long wave radio signals for time standards. To evaluate the receiving sensitivity of the antenna, we calculated the antenna factor of the magnetic core loop antenna by combining a magnetic field simulation and a circuit simulation. The simulation results are in good agreement with the results obtained from the experiments. We then investigated the optimization of the antenna shape, and showed the re

Comment on “The magnetic response of the ionosphere to pulsed HF heating” by K. Papadopoulos, T. Wallace, G. M. Milikh, W. Peter, and M. McCarrick

Comment on “The magnetic response of the ionosphere to pulsed HF heating” by K. Papadopoulos, T. Wallace, G. M. Milikh, W. Peter, and M. McCarrick

AN INVESTIGATION OF MAGNETIC ANTENNAS FOR GROUND PENETRATING RADAR - Abstract

For ground penetrating radar (GPR), smaller antennas would provide considerable practical advantages. Some of which are: portability; ease of use; and higher spatial sampling. A theoretical comparison of the fundamental limits of a small electric field antenna and a small magnetic field antenna shows that the minimum Q constraints are identical. Furthermore, it is shown that only the small magnetic loop antenna can be constructed to approach, arbitrarily closely, the fundamental minimum Q limit. This is achieved with the addition of a high permeability material which reduces energy stored in the magnetic fields. This is of special interest to some GPR applications. For example, applications requiring synthetic aperture data collection would benefit from the increased spatial sampling offered by electrically smaller antennas. Low frequency applications may also benefit, in terms of reduced antenna dimensions, by the use of electrically small antennas. Under these circumstances, a magnetic type antenna should be considered in preference to the typical electric field antenna. Numerical modeling data supports this assertion.

Quasi‐optical antenna‐mixer array composed of a magnetic loop antenna operated at 10‐GHz band

AbstractThe quasi‐optical antenna‐mixer is a receiving system that spatially receives RF signals and local oscillator (LO) signals and at the same time converts the RF signals to intermediate (IF) frequency signals. The line transmission loss, which causes a problem at millimeter wave frequencies, can be suppressed. In this paper, test fabrication and measurement of a quasi‐optical antenna‐mixer array receiving RF and LO signals with a magnetic loop antenna are carried out at 10 GHz and quasi‐optical mixing is shown to be possible. A two‐dimensional array antenna using this element is proposed. Its receiving characteristics are theoretically predicted. By changing the illumination angle or frequency of the LO, it is found that the receiving direction of the RF can be scanned. In addition, the suppression of RF and LO signals appearing at the output end is demonstrated. An experiment with a two‐dimensional array consisting of eight elements of quasi‐optical antenna‐mixers is carried out and the validity of the proposal is confirmed. © 2003 Wiley Periodicals, Inc. Electron Comm Jpn Pt 1, 87(4): 45–53, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecja.10114

An Investigation of Magnetic Antennas for Ground Penetrating Radar

For ground penetrating radar (GPR), smaller antennas would provide considerable practical advantages. Some of which are: portability; ease of use; and higher spatial sampling. A theoretical comparison of the fundamental limits of a small electric field antenna and a small magnetic field antenna shows that the minimum Q constraints are identical. Furthermore, it is shown that only the small magnetic loop antenna can be constructed to approach, arbitrarily closely, the fundamental minimum Q limit. This is achieved with the addition of a high permeability material which reduces energy stored in the magnetic fields. This is of special interest to some GPR applications. For example, applications requiring synthetic aperture data collection would benefit from the increased spatial sampling offered by electrically smaller antennas. Low frequency applications may also benefit, in terms of reduced antenna dimensions, by the use of electrically small antennas. Under these circumstances, a magnetic type antenna should be considered in preference to the typical electric field antenna. Numerical modeling data supports this assertion.

Worldwide monitoring of VLF‐LF propagation and atmospheric noise

A joint effort is underway between The Johns Hopkins University Applied Physics Laboratory and the Space and Naval Warfare (SPAWAR) Systems Center, San Diego, to deploy monitoring equipment capable of characterizing worldwide VLF‐LF radio wave propagation and atmospheric noise levels. The monitoring equipment consists of a PC‐based spectrum analyzer and orthogonal ferrite core magnetic loop antennas. The analyzer performs continuous measurements of the radio spectrum from 12 to 62 kHz and records time histories of VLF‐LF signals (equivalent vertical electric field strength), noise amplitude probability distribution, noise impulsiveness, and average noise field strength. Data are downloaded via the Internet to a central database server. The Internet connection also provides for system reconfiguration and clock synchronization. Data collected by the monitoring network will be used to improve communication coverage forecasts and to analyze transient and long‐term propagation effects. This paper provides an overview of the monitoring network and samples of data collected by it.

Whistlers in Thermally Generated Ducts with Enhanced Plasma Density: Excitation and Propagation

A study is made of the excitation and guided propagation of whistler waves along magnetic-field-aligned cylindrical ducts with enhanced plasma density. The ducts have been created in the large plasma device as a result of the thermal-diffusion-driven redistribution of plasma due to electron heating in the quasistatic field of a current loop having a radius commensurate with the electron heat-conduction length in the radial direction. The whistler waves are excited by a comparatively small magnetic loop antenna immersed in the duct. Detailed measurements of the excited field and the density distribution are reported. It is concluded that thermally generated ducts observed in the experiments can guide whistler-mode waves launched from the magnetic antenna. With the use of the full wave formulation, the total source-excited field is calculated and compared with the experimental data. Excellent agreement is found between the measured and calculated wave patterns. The results are relevant to both the basic properties of whistlers and to applications such as transmitting systems using artificial near-antenna ducts in space plasmas.

Vortices and Flux Ropes in Electron MHD Plasmas I

Laboratory experiments are reviewed which demonstrate the existence and properties of three-dimensional vortices in Electron MHD (EMHD) plasmas. In this parameter regime the electrons form a magnetized fluid which is charge-neutralized by unmagnetized ions. The observed vortices are time-varying flows in the electron fluid which produce currents and magnetic fields, the latter superimposed on a uniform dc magnetic field B0. The topology of the time-varying flows and fields can be described by linked toroidal and poloidal vector fields with amplitude distributions ranging from spherical to cylindrical shape. Vortices can be excited with pulsed currents to electrodes, pulsed currents in magnetic loop antennas, and heat pulses. The vortices propagate in the whistler mode along the mean field B0. In the presence of dissipation, magnetic self-helicity and energy decay at the same rate. Reversal of B0 or propagation direction changes the sign of the helicity. Helicity injection produces directional emission of vortices. Reflection of a vortex violates helicity conservation and field-line tying. Part I of two companion papers reviews the linear vortex properties while the companion Part II describes nonlinear EMHD phenomena and instabilities.

Generation of dc Magnetic Fields by Rectifying Nonlinear Whistlers

A magnetic loop antenna, immersed in a laboratory plasma, excites whistlers whose wave magnetic field exceeds the ambient dc field and forms magnetic null points. The propagation becomes amplitude and polarity dependent. A time-average dc magnetic field is generated with an applied ac magnetic field of zero mean.

Pulsed currents carried by whistlers. V. Detailed new results of magnetic antenna excitation

A low frequency, oblique whistler wave packet is excited from a single current pulse applied to a magnetic loop antenna. The magnetic field is mapped in three dimensions. The dominant angle of radiation is determined by the antenna dimensions, not by the resonance cone. Topological properties of the inductive and space charge electric fields and space charge density confirm an earlier physical model. Transverse currents are dominated by Hall currents, while no net current flows in the parallel direction. Electron-ion collisions damp both the energy and the helicity of the wave packet. Landau damping is negligible. The radiation resistance of the loop is a few tenths of an Ohm for the observed frequency range. The loop injects zero net helicity. Rather, oppositely traveling wave packets carry equal amounts of opposite signed helicity.

The Polar plasma wave instrument

The Plasma Wave Instrument on the Polar spacecraft is designed to provide measurements of plasma waves in the Earth's polar regions over the frequency range from 0.1 Hz to 800 kHz. Three orthogonal electric dipole antennas are used to detect electric fields, two in the spin plane and one aligned along the spacecraft spin axis. A magnetic loop antenna and a triaxial magnetic search coil antenna are used to detect magnetic fields. Signals from these antennas are processed by five receiver systems: a wideband receiver, a high-frequency waveform receiver, a low-frequency waveform receiver, two multichannel analyzers; and a pair of sweep frequency receivers. Compared to previous plasma wave instruments, the Polar plasma wave instrument has several new capabilities. These include (1) an expanded frequency range to improve coverage of both low- and high-frequency wave phenomena, (2) the ability to simultaneously capture signals from six orthogonal electric and magnetic field sensors, and (3) a digital wideband receiver with up to 8-bit resolution and sample rates as high as 249k samples s−1.

A magnetic current loop array in a reflector antenna

A magnetic current loop antenna array is designed, implemented, and measured. Radiation pattern, input impedance, and efficiency of the array are presented. The array is intended as a feed in a reflector antenna. Using a 360 mm solid dish, the overall gain of the reflector antenna is 24.6 dB at 9 GHz. The tolerance in placing the feed at the focal point of the dish is high. The present feed is low cost, self-supportive, robust, and easy to manufacture. It is an ideal substitute for the horn in a TVRO or VSAT antenna.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Thermal magnetic fluctuations of whistlers in a Maxwellian plasma

Thermal fluctuations have been measured with a magnetic loop antenna inside a large afterglow plasma in the regime of whistler waves (f≲fce≂30 MHz≪fpe≂3000 MHz; Ar, 2×10−4 Torr, 1 m diam×2.5 m length). The magnetic fluctuations B̃(ω) exhibit a 1/f-like spectrum for whistlers (f&amp;lt;fce), no resonant enhancement at the electron cyclotron frequency fce, and a flat spectrum in the evanescent regime (fce&amp;lt;f≪fpe). Thus the observed fluctuations are neither described by blackbody radiation laws (B̃∝ω) nor by cyclotron emission (lines at nfce), but resemble the decaying Alfvénic fluctuation spectrum calculated by Cable and Tajima [Phys. Rev. A 46, 3413 (1992)].

Pulsed currents carried by whistlers. Part I: Excitation by magnetic antennas

Time-varying plasma currents associated with low-frequency whistlers have been investigated experimentally. Pulsed currents are induced in the uniform, boundary-free interior of a large laboratory plasma by means of insulated magnetic antennas. The time-varying magnetic field is measured in three dimensions and the current density is calculated from R∇×B(r,t)=μ0J, where J includes the displacement current density. Typical fields B(r,t) and J(r,t) induced by a magnetic loop antenna show three-dimensional helices due to linked toroidal and solenoidal field topologies. Constant amplitude and phase surfaces assume conical shapes since the propagation speed along B0 is higher than oblique to B0. The wave vector is highly oblique to B0 while the energy flow is mainly along B0. The electric field in the wave packet contains both inductive and space-charge contributions, the latter arising from the different dynamics of electrons and ions as explained by physical arguments. The dominant electric field in a whistler packet is a radial space-charge field. Neither the field topology nor the propagation characteristics are sensitive to the induced magnetic field amplitude up to Bwave≲B0. The results are relevant to both the basic properties of whistlers and to applications such as large loop antennas and electrodynamic tethers in space plasmas.