Azimuthal Antenna Research Articles

Design Approach to Azimuthal Null Frequency Scanning Circular Sector Patch Antenna Under Triple-Mode Resonance

Design Approach to Azimuthal Null Frequency Scanning Circular Sector Patch Antenna Under Triple-Mode Resonance

Mode excitation and radiation characteristics of antennas in cylindrically stratified media

Guided modes excited by a current source in a multilayer cylindrical structure is thoroughly investigated. The guidance condition in the multilayer structure is formulated. Based on this, three antenna configurations in cylindrical stratified media that are of strong practical interest are rigorously investigated: viz. a monopole antenna, a conformal axially-directed slot antenna and an azimuthal slot antenna in a cylindrical layered medium. While previous treatments of such configurations did not investigate the mode-excitation problem of antennas in stratified cylindrical media and were limited in their applicability for a dipole antenna in a cylindrical medium or assumed the slot-current distribution for a slot antenna over a conducting cylinder only, the current investigation provides an in-depth and rigorous insight to the modes excited and power coupled to the excited modes for the antenna configurations and removes such restrictions. In addition to the efficient and rigorous investigation of the coupling problem for the three antenna structures, the radiation characteristics of the antenna configurations are also evaluated using the method of stationary phase, enabling a comprehensive treatment of the antenna structures from both the coupling and radiation aspects.

RF power absorption by plasma of low pressure low power inductive discharge located in the external magnetic field

Present paper is aimed to reveal experimentally and theoretically the influence of magnetic field strength, antenna shape, pressure, operating frequency and geometrical size of plasma sources on the ability of plasma to absorb the RF power characterized by the equivalent plasma resistance for the case of low pressure RF inductive discharge located in the external magnetic field. The distinguishing feature of the present paper is the consideration of the antennas that generate not only current but charge on the external surface of plasma sources. It is shown that in the limited plasma source two linked waves can be excited. In case of antennas generating only azimuthal current the waves can be attributed as helicon and TG waves. In the case of an antenna with the longitudinal current there is a surface charge on the side surface of the plasma source, which gives rise to a significant increase of the longitudinal and radial components of the RF electric field as compared with the case of the azimuthal antenna current.

The transponder data recorder: Implementation and first results

The secondary surveillance radar (SSR), that is an evolution of the military identification friend-or-foe systems, is widely used by air traffic control service providers to localize and identify co-operating aircraft equipped with a standard transponder [1]. The ground SSR installation transmits interrogations, from a rotating, narrow azimuthal beam antenna. The airborne transponders, once they have received an interrogation, transmit at a 1,090-MHz carrier a reply signal containing the requested data, i.e., identity (mode A reply) or fight level (mode C reply). Azimuth and range of the aircraft are measured by the interrogator, based on the delay of the reply and on the antenna pointing angle. The current SSR standard is based on the use of selective interrogations and replies and is called mode S: to reduce the interferences, the mode S protocol is based on a message format that includes the unique address of the aircraft. The airborne segment of the SSR is composed of the transponder and a pair of antennas on top and on the bottom of the fuselage. As the aircraft antennas are omnidirectional, many ground stations can receive the replies. This allowed the development of multilateration (MLAT) systems for the aircraft localization based on 1,090-MHz signals [2]. A typical MLAT system is composed of a distributed network of 1,090-MHz sensors, an interconnecting facility, and a central processor for the fusion of the sensor data. The data fusion relies on the estimation of the signals arrival time at the different stations and on hyperbolic localization. To identify the emitters, the processing is done with mode A replies or mode S replies that contain the emitter identity. A transponder with mode S capability can also transmit a particular downlink format message, called squitter, containing the aircraft's unique address and other information. The squitter signals are not elicited by the SSR interrogation, but they are spontaneously emitted at pseudoperiodical intervals. They are the basis of the automatic dependent surveillance-broadcast (ADS-B) concept [3]: the ADS-B OUT function periodically transmits information (identity, position, state, etc.) about the aircraft, and the ADS-B IN function receives the messages from nearby traffic. On the airborne side, the ADS-B OUT broadcasts data with onboard equipment using the squitter signals. The airborne ADS-B IN equipment provides to the pilot the traffic scenario, receiving the messages from ground and nearby aircraft ADS-B OUT. Moreover, the ADS-B IN can receive other ground services: the traffic information services-broadcast (TIS-B) and the fight information services-broadcast (FIS-B) [3]. Airport vehicles can be equipped with an ADS-B OUT, a simplified, nonflyable device transmitting 1,090-MHz squitter signals containing the identity and the position. As a matter of fact, there are various users of the 1,090-MHz channel: 1) SSR transponder replies (modes A, C, S); 2) ADS-B OUT messages; 3) MLAT systems with interrogation capability; and 4) TIS-B.

Shock Oscillations in a Supersonic Jet Exhibiting Antisymmetrical Screech

T HIS contribution follows a Technical Note published by the authors inAIAAJournal [1], inwhich a shock-trackingalgorithm was applied to a screeching underexpanded jet. It was shown that the shocks oscillate at the screech frequency, in a symmetrical or antisymmetrical manner, according to the screech mode. The temporal signals of twodiametrically opposed near-fieldmicrophones were also considered. At a nozzle pressure ratio (NPR) of 2.54, corresponding to a flapping mode B, both microphones showed a simultaneous modulation of the screech amplitude at a frequency of the order of 1=1000 of the screech frequency. A schlieren video was extracted from the complete recording for each one of the weak and strong screech spells. The tracking algorithm was applied on the resulting movies, and it was deduced that the shock oscillation amplitude increases with an increase in the screech level, which is in agreement with the analytical model proposed by Panda [2]. The picture of the flapping screech viewed by two diametrically opposed microphones appears, however, too simplistic, and a property of screech that remained unseen at that time is highlighted in the present work. Screech is a tonal component of shock-associated noise arising in incorrectly expanded supersonic jets. It was first studied by Powell [3], who already mentioned the existence of modes. Of particular importance for the present study is the flapping mode B, whose pressure signal in the near field is antisymmetrical about a plane. The modal behavior of screech was later extensively studied, especially by Powell et al. [4] as well as Ponton and Seiner [5], who noted that the plane of antisymmetry can slowly rotate or oscillate. This feature, referred to in the following as plane rotation, was investigated in some detail by the authors [6] using a near-field azimuthal microphone antenna comprising 18 transducers located every 20 deg. It was shown that the plane rotation was related to the simultaneous presence of two counter-rotating helices of slightly different frequencies in the screech azimuthal-mode content. It was also explained that the screech level is minimum near the plane of antisymmetry and maximum perpendicular to that direction. In light of [6], it is believed that the strong amplitude modulation in time of the microphone signals in the original study [1] was caused by the rotation of the plane of antisymmetry. The plane rotation was then interpreted as a time modulation of screech strength due to the small number of microphones used. The identification of the plane rotation in the acoustic field has suggested a possible rotation of the plane of antisymmetry of the shock motion. This question is considered in the present paper. The experimental facility is first presented. Then, results for two cases of flapping screech are discussed.

Compact Azimuthal Omnidirectional Dual-Polarized Antenna Using Highly Isolated Colocated Slots

An omnidirectional dual-polarized antenna with high port isolation is presented for 2.4-GHz wireless local area network (WLAN) applications. The omnidirectional patterns of both vertical and horizontal polarizations in the azimuthal plane are achieved by positioning two orthogonal slots cut onto the walls of a slender columnar cuboid. The overall volume of the proposed antenna is only 83 × 11 × 11mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> (0.664λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> × 0.088λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> × 0.088λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> ), with low mutual coupling between two radiating slots. A prototype of the proposed antenna is fabricated and tested. The measured results show that the 10-dB reflection coefficient bandwidths of dual polarizations cover the desired band of 2.4-2.48 GHz and the port isolation is lower than -33.5 dB. Stable gains are greater than 3.17 and 1.19 dBi for vertical and horizontal polarizations, respectively. The diversity performance is also evaluated, including the correlation coefficient, mean effective gain (MEG) ratio, and diversity gain.

Numerical and experimental demonstration of a coordinate transformation‐based azimuthal directive emission

AbstractThis article deals with the modeling, practical implementation, and characterization of an azimuthal directive antenna around 10 GHz. The design of the antenna is based on transformation optics concept by transforming the radiation of a plane source into an azimuthal radiation. This coordinate transformation procedure is achieved by modifying the electromagnetic properties of the space around the plane source. Metamaterials presenting electric and magnetic resonances are used to produce the effective material parameters necessary for the transformation. S11 parameter and direct far‐field measurements are performed on a fabricated prototype to experimentally demonstrate the narrow beam profile and the beam deflection. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:2536–2540, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27122

Experimental study of flight effects on screech in underexpanded jets

Flight effects on screech from an underexpanded supersonic jet have been experimentally investigated in a free jet facility. Screech frequency prediction is addressed, and some conclusions about the convection velocity under flight conditions are drawn. An azimuthal near field acoustic antenna is used to investigate the modal behaviour of screech with forward flight effects. Several mode switchings are identified as the flight velocity is increased but none can be related to a change in the screech azimuthal mode content. Screech is enhanced by flight at high fully expanded Mach number Mj. This conclusion, established from acoustic far field measurements, is supported by the analysis of schlieren visualizations.

Investigation of dipole antenna arrays over a grid reflector

The paper is devoted to research in characteristics of small-size axially symmetric radial and azimuthal dipole antenna arrays placed above a small-size grid reflector. To diminish the dipole dimensions and to lower the level of backward radiation of the antenna arrays, it is suggested to use lumped inductive elements connected between the radiating arms of the dipoles.

Hot-Electron Production and Wave Structure in a Helicon Plasma Source

Energetic wave-trapped electrons of $\ensuremath{\sim}20\mathrm{eV}$, moving at the 13.56 MHz helicon wave phase velocity, are directly measured with a 3 ns response-time retarding-potential analyzer. The rf axial wavelength is measured with $B$-dot probes scanned axially. Changes in the axial magnetic field or the rf power cause order of magnitude changes in the energetic electron current. The current correlates with either a transition from partially propagating to nearly pure standing waves or quantized jumps in the number of half wavelengths between the two azimuthal antenna straps.

An antenna tracking method for land‐mobile satellite communications system

AbstractA new satellite‐tracking method is proposed for a directional antenna installed on a car for L‐band land‐mobile satellite communication. This method uses both a fiber optic gyroscope (FOG) and a magnetic compass (MC) with an open‐loop control algorithm, which com pensates for the disadvantages of individual sensors. Since FOG can detect the angular velocity precisely but cannot detect the absolute azimuthal direction, this type of sensor has inherent errors due to accumulating the angular velocities necessary to obtain the azimuthal direction. Cumulative errors caused mainly by a disagreement between the FOG axis for measuring rotation and the vertical axis during turning curves can be compensat ed by MC. the disadvantage of MC is that errors due to magnetic disturbance are caused by magnetized bridges and trucks. In the proposed method, the occurrence of a magnetic disturbance is detected from a statistical analysis of MC acquired data and FOG is used to control the antenna direction when a car turns either right or left. According to an analysis of the three‐dimensional motions of cars, it was determined that only azimuthal antenna tracking can provide the precise control required to obtain good tracking performance, even when using an 91 L‐band directional antenna with an antenna gain of ap proximately 13 dBi.

94 GHz integrated horn monopulse antennas

A monolithic azimuthal monopulse antenna for 94-GHz applications has been developed. The structure consists of a single dipole suspended in one plane of an integrated horn cavity to obtain the sum pattern, and an antiparallel pair of dipoles suspended in a different plane of the same horn cavity to achieve the difference pattern. Pattern measurements of microwave models and on the millimeter-wave antennas show good agreement with theory and exhibit symmetry with a sharp -30-dB null at broadside for the difference antenna. Microwave model measurements show input impedances close to 50 Omega , with greater than -25-dB isolation between sum and difference antennas across a 10% bandwidth.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Rain Cell Size Statistics Derived from Radar Observations at Wallops Island, Virginia

Results of an investigation are given describing two-di-two-dimensional rain size statistics employing radar and disdrometer data bases spanning more than 5 years and all seasons. Radar data were obtained employing the SPANDAR radar located at the NASA Wallops Flight Facility, Wallops Island, Virginia. These data corresponded to approximately 100 low-elevation azimuthal antenna scans encompassing 17 rain days. For each of these rain days, disdrometer data were also obtained. Analysis of the regression relations relating the rain rates to the disdrometer data enabled the determination of least square fit radar reflectivity factors. The individual disdrometer results along with the radar data were employed with a contouring program. More than 22 000 contours were generated, where each isopleth belonged to predefined rain rate intervals. Computing the areas of each of these contours, statistics were generated relating the contour diameters and rain rate categories. Two types of contour have been analyzed. One pertained to the above-described isopleth regardless of the rain rate levels interior to it. Another type corresponded to those isopleths in which the rain rates interior to it were equal or greater than the isopleth values. These isopleths were referred to as and cell contours, respectively. An abundance of total and contours were observed belonging to all rain rate categories. In particular, a dominant number of smallarea contours were observed belonging to the lower rain rate levels. The results showed that the average and median equi-circle contour diameters were 2.4 and 1.9 km, respectively.

Ground Backscatter Observed With High Resolution Oblique Sounders

The amplitude of ground backscatter at high frequency (HF) recorded with oblique sounding equipment that utilizes azimuthal antenna beamwidths of about 6° and effective pulse lengths of about 50 μsec is characterized by sharp spikes that rise 10 to 20 db above the diffuse background clutter and have a range extent as short as the pulse length. These spikes suggest that the backscattering efficiency of the ground is highly nonuniform as a result of the occasional presence of a localized surface feature with a backscatter cross section one or two orders of magnitude larger than any object in the surrounding countryside. When short‐pulse ground backscatter echoes are displayed as intensity modulated range versus azimuth records, these sharp spikes produce discrete echoes with an azimuthal extent approximately equal to the effective beamwidth of the antenna. The sources of many of these discrete echoes have been identified with specific topographic and geographic features, including cities and mountains in the western United States and islands in the Caribbean Sea.

Radiation characteristics of a circumferential leaky-wave antenna on a conducting cylinder

The residue series representation of the fields for an azimuthal leaky-wave antenna on a conducting cylinder is studied. It is shown that the zeroth term corresponds to the case of an omnidirectional antenna, it being understood that omnidirectional refers to the azimuthal plane only. For each term of the residue series, it is shown that two regions exist for the radiation fields in the elevation plane. Within one region, the fields exhibit the characteristic of real rays, while in the other, the fields are that of diffracted rays. If the radius of the cylinder is allowed to increase to infinity, such that the cylindrical surface approaches a plane surface, only the region in which the real rays exist remains. The existence of the two regions in the cylindrical case is, therefore, attributed to the curvature of the surface. It is shown that the boundary between these two regions forms a hyperboloid of one sheet and that, in the far field, it approaches a conical surface about the cylindrical axis. An omnidirectional antenna was designed and constructed. The radiation patterns show good agreement with theoretical predictions.

A Statistical Theory of Mobile-Radio Reception

The statistical characteristics of the fields and signals in the reception of radio frequencies by a moving vehicle are deduced from a scattering propagation model. The model assumes that the field incident on the receiver antenna is composed of randomly phased azimuthal plane waves of arbitrary azimuth angles. Amplitude and phase distributions and spatial correlations of fields and signals are deduced, and a simple direct relationship is established between the signal amplitude spectrum and the product of the incident plane waves' angular distribution and the azimuthal antenna gain. The coherence of two mobile-radio signals of different frequencies is shown to depend on the statistical distribution of the relative time delays in the arrival of the component waves, and the coherent bandwidth is shown to be the inverse of the spread in time delays. Wherever possible theoretical predictions are compared with the experimental results. There is sufficient agreement to indicate the validity of the approach. Agreement improves if allowance is made for the nonstationary character of mobile-radio signals.