Phased Antenna Research Articles

Design, fabrication and testing of UHV compatible high power RF devices for lower hybrid current drive system on SST-1 tokamak

Maintaining plasma current under steady state conditions is one of the most important pre-requisites for a tokamak-based reactor. Lower hybrid current drive (LHCD) system aims to drive tokamak plasma current by means of RF power. The LHCD system on SST-1 tokamak is based on two 500 kW, CW klystrons operating at 3.7 GHz. A waveguide transmission line transmits power from source to the antenna. A phased array waveguide antenna is used to couple power to the plasma. The antenna side of the transmission line is placed inside the tokamak vacuum vessel. The design and fabrication of this In-Vessel system has to satisfy the demands of high power RF as well as ultra high vacuum (UHV) compatibility. This paper describes some of the critical UHV compatible In-Vessel RF devices, their design, fabrication, and test results.

Experimental Evidence ofm=-1 Helicon Wave Cutoff in Plasmas

A phased multiple helical antenna selectively excites electromagnetic fields with the azimuthal mode numbers of m =+1 and -1 in a low-density plasma independently of magnetic field strength. Both the m =+1 and -1 helicon waves excited by the antenna are observed to propagate in the plasma at magnetic fields lower than 5 mT. The m =+1 helicon wave propagates even in higher magnetic fields where density peaks are not observed, but only the m =-1 helicon wave disappears. The cutoff behavior of the m =-1 helicon wave experimentally observed coincides with a theoretical expectation that this helicon wave is forbidden to propagate in a low-density region.

Direct infrared measurements of phased array aperture excitations

A thermographic imaging technique has been developed to measure electromagnetic (EM) fields. This technique is applied in this paper to measure the aperture plane fields of large phased array radar antennas and to determine the aperture (source) excitations of the array, i.e., the distribution of the radiated energy over the elements in the aperture plane of the array. These IR measurements, therefore, can be performed on-site at the remote location of the antenna in-the-field to produce a non-distorted image of the field excitations in the aperture plane of the array, which control the overall radiation characteristics of the array. In general, these images can be used for field diagnostics to evaluate the electrical characteristics of the elements of the array, i.e., the state (strength) of the aperture excitations and the condition of the switching circuits (phase shifters and attenuators), which control the radiation pattern of the antenna. The aperture field distribution can be compared to a standard “test pattern” to quickly determine the operational state of each individual element of the array. Individual attenuators and/or phase shifters that produce incorrect element intensities can be easily and quickly identified with this technique. Short-circuited elements at various positions in the array are used to simulate faults in the elements and to test the feasibility of the thermal technique to determine the operational state of the array. Therefore, the overall “state of health” of the array and the need for repair can be determined in-the-field using the IR measurement technique to avoid the expensive and time consuming alternative of dismantling the array and shipping it to a maintenance depot for testing, calibration, and repair on a standard, planar, near-field antenna test range. In this paper, the IR technique is tested in a controlled environment to determine the feasibility of using the IR images as an array diagnostic tool to measure the aperture excitations of large phased array radar antennas.

Direct infrared measurements of phased array near-field and far-field antenna patterns

A thermal imaging technique has been developed to measure electromagnetic (EM) fields. This technique is applied in this paper to measure the EM fields radiated by large phased array radar antennas and to determine the near-field distributions and the far-field antenna pattern. This thermal technique is based on infrared (IR) measurements of the heating patterns produced in a thin, lossy detector screen made from a carbon loaded polyimide film placed in the plane over which the field is to be measured. The temperature rise in the screen material is related to the intensity of the field incident on the screen. An experimental calibration table was developed at NIST/Boulder to convert the temperature rise at any point on the screen into an equivalent incident radiated field strength. This thermal imaging technique has the advantages of accuracy, simplicity, speed, and portability over existing hard-wired probe methods and produces a 2D picture of the near field or the far field. These IR measurements, therefore, can be performed on-site at the remote location of the antenna in-the-field to produce an image of the radiating field of the array, which can be used to determine the overall radiation characteristics of the array, i.e., the radiation pattern of the combined array elements (gain and beam-width) and the condition of the electronic switching circuits (phase shifters and attenuators). Therefore, the overall “state of health” of the array and the need for repair can be determined in-the-field using the IR technique to avoid the expensive and time consuming alternative of dismantling the array and shipping it to a maintenance depot for testing, calibration, and repair on a standard, planar, near-field antenna test range. In this paper, the IR technique is tested in a controlled environment to determine the feasibility of using the IR images as an array diagnostic tool i) to measure the radiated field of large phased array radar antennas (near-field or far-field patterns in a transverse plane parallel to the plane of the array), ii) to measure the transition of the field from the near field to the far field (in an axial plane perpendicular to the plane of the array), and iii) to test the switching of the array from a scan mode to a target tracking mode.

Adaptive blind estimation algorithm for OFDM-MIMO radio systems over multipath channels

This paper investigates the blind algorithm for channel estimation of Orthogonal Frequency Division Multiplexing-Multiple Input Multiple Output (OFDM-MIMO) wireless communication system using the subspace decomposition of the channel received complex baseband signals and proposes a new two-stage blind algorithm. Exploited the second-order cyclostationarity inherent in OFDM with cyclic prefix and the characteristics of the phased antenna, the practical HIPERLAN/2 standard based OFDM-MIMO simulator is established with the sufficient consideration of statistical correlations between the multiple antenna channels under wireless wideband multipath fading environment, and a new two-stage blind algorithm is formulated using rank reduced subspace channel matrix approximation and adaptive Constant Modulus (CM) criterion. Simulation results confirm the theoretical analysis and illustrate that the proposed algorithm is capable of tracking matrix channel variations with fast convergence rate and improving acceptable overall system performance over various common wireless and mobile communication links.

Research and Development on the Ion Cyclotron Range of Frequency Heating System in JT-60U

Research and developments on the ion cyclotron range of frequency (ICRF) heating system in the JT-60 upgrade (JT-60U) are presented. The developments and experiences on the operation of the ICRF heating system contribute to its upgrade and to future ICRF heating systems in ITER. The ICRF heating system for JT-60U started operation in January 1992. RF power up to 7 MW for 1.1 sec at 116 MHz has been coupled to a plasma as a result of the developments described in this paper. New high power tetrodes having pyrolitic graphite grids for higher dissipation of screen and control grids were tested in the ICRF amplifier, and 1.7 MW of the output power at 131 MHz for 5.4 seconds was achieved. This was the highest power level for fusion research above 110 MHz in 1990. A pair of phased loop antenna arrays (2 × 2) showed sufficiently high coupling resistance. To keep the impedance matching between the antenna and the transmission line, a frequency feedback control (FFC) system was developed, and its effectiveness was proved to couple high power RF continuously to the variable plasma. In ITER, enhancement of dielectric loss tangent of ceramics due to neutron irradiation will limit power injection capability of the antenna significantly. To solve this problem, an all-metal support (AMS) was developed in the JT-60U ICRF heating system as a substitute for a ceramic support of a central conductor of a coaxial antenna feeder in the ITER ICRF antenna.

Realization of true-time delay lines based on acoustooptics

A true-time optical delay line for short radiofrequency (RF) pulses using path length dispersion is proposed. It is an optical implementation of the linear phase-shift theorem of the Fourier transformation. Acoustooptic signal processing is used for conversion into the optical frequency domain and for spatial Fourier decomposition of the pulse. The processing of the pulse is obtained by differentially phase shifting the particular frequency components, followed by a heterodyne reconversion into the RF domain. The optical system is intended to be used for delaying, but also for shaping and filtering of RF pulses, mainly in phased array radar antennas. Theoretical analysis of the system principle is given together with experimental results, demonstrating 2-/spl mu/s time delay of 0.5-/spl mu/s-long pulses with maximum optical phase shift of 1.2/spl pi/. A detailed theoretical and experimental bandwidth analysis is carried out, pointing to the main technical problems and their solutions.

New wavelength division multiplexing true-time-delay network for wideband phased array antennas

Phased array antennas (PAAs) offer many advantages including steering without physical movement, accurate beam pointing, increased scan flexibility in two dimensions, precise PAA element phase and amplitude control to obtain low sidelobes, and reduced power consumption and weight. The complexity associated with controlling the many thousand array elements, while handling the broad bandwidth required of a shared antenna, makes the marriage of photonics and microwave radar attractive. To satisfy the simultaneous requirements of wide bandwidth and large antenna scan angle, true-time-delay (TTD) steering techniques are required so that efficient elemental vector summation (in the receive mode) or distribution (in the transmit mode) can be obtained that is independent of frequency or angle. We describe a new lower complexity and cost photonic TTD system which we call WDM delay broadcasting TTD. This approach (1) encodes fixed delays with wavelength, (2) makes available all of the delays at each of the subarrays, and (3) selects the appropriate delay using an optical filter (with correction via the element electronic delay line). The results of this effort show that it is indeed possible to design and construct a photonically controlled TTD network that will exhibit the performance and cost benefits needed for widespread deployment in government and commercial phased array radar antenna systems.

Macroscopic Stability Control Experiments with Phased Antennas in the GAMMA10 Tandem Mirror

Validity of averaged minimum- B configuration on Magnetohydrodynamics (MHD) stability of the GAMMA10 tandem mirror are studied in the ion cyclotron range of frequency (ICRF)-heated, hot ion mode plasma. It has been observed that the plasma can not be sustained mostly owing to the flute interchange mode when the high power ICRF is applied for the central cell ion heating. As the ratio of the central cell beta to the anchor cell beta increases, the amplitude of the flute interchange mode increases. The anchor cell plasma is also sustained by the another ICRF source in the central cell. The flute type MHD instability in GAMMA10 is well controlled by the phased ICRF antennas in the central cell.

Optical diffraction effects in a photonic delay line

Accurate, low-noise, wideband phased array radar antennas depend on the use of true time delays between microwave elements. A photonic means of generating the required element-to-element phase shifts was developed by Toughlian and Zmuda for systems operating in the VHF frequency range, and extended by Monsay and Caccuitto for operation in the radar C band. The photonic beamformer is based on a heterodyne optical system, in which the reference beam acts as a probe of the acoustic wave in the acousto-optic (AO) cell. It is apparent that the benefits of this photonic system rely greatly on the ability of the probe beam to pick out just the right phase shift/time delay from the acoustic waveform in the AO cell. A basic limitation to system performance could come from intrinsic diffraction effects due to the finite beamwidth of the probe in the AO cell. In addition, the finite size of the photodetector in the system will have an effect on the final SNR. These effects have been studied in detail and their implications for design and performance of the photonic beamformer are discussed. © 1997 Society of Photo-Optical Instrumen- tation Engineers. (S0091-3286(97)01312-3)

Antenna Pattern Analysis and Measurements for Multiparameter Radars

Abstract Measuring the full polarization matrix using a research radar requires knowledge of the complex (i.e., amplitude and phase) antenna pattern to properly correct for bias errors in the measurements. However, generally only power patterns are available for analysis. Techniques are presented for obtaining bounds on the system differential reflectivity ZDR bias and the integrated cross-polarization ratio. A method for use of the phase shift between pulses to simultaneously estimate the differential phase and the radial velocity over the full Nyquist interval is described. NCAR's CP-2 10-cm antenna patterns with different horizontal and vertical polarization sidelobes were matched by rotating the feed support struts to be aligned on the diagonals so no preferential blockage occurred. The matched antenna patterns show much improved ZDR measurements in strong reflectivity gradients. The CP-2 3-cm antenna patterns were shown to be axially symmetric and typical of a well-designed, unblocked parabolic refle...

Independent null steering in the sum and difference patterns of antenna arrays using real weights

A method for steering nulls in the sum and difference patterns of a phased linear antenna array using only real weights is introduced. The proposed method decouples the weights so that a given jammer is associated with a given weight value. A change in the jammer's direction changes only its corresponding weight value without affecting the other nulls. The main beam is steered independently using a progressive phase shift. This allows the signal-to-jammer power ratio to be maximized with relative ease. The drawback of this method is that the number of jammers that can be nulled will be reduced to about a quarter of the number of array elements.

High resolution long range ISAR imaging systems

The requirements for high resolution, long range ISAR imaging systems capable of recognising targets have been outlined. Systems meeting these requirements and operating at mm-wavelengths have been proposed. The systems consist of N phased locked transmitters feeding as many antennas in phase. Quasi-optical power addition at the target occurs. In the receive mode all antennas are connected in phase. These systems can at least double the range obtained by utilising a system having one transmitter/antenna combination. Several realisation options have been studied including the annular synthesis antenna systems which offer some advantages over more conventional realisations.

Comparison of power deposition by in-phase 433 MHz and phase-modulated 915 MHz interstitial antenna array hyperthermia systems

The interstitial microwave antenna array hyperthermia (IMAAH) system produces a pattern of specific absorption rate (SAR) that is nonuniform within a 2-cm square array when driven in phase at 915 MHz. It was found that phase modulation makes the time-averaged SAR pattern significantly more uniform in planes perpendicular to the antennas. To drive antennas in phase at 433 MHz similarly improves SAR uniformity when the antennas are of resonance length. >

Optical feed for a phased array microwave antenna

Microwave signals for all array elements in a phared array microwave antenna are simultaneously generated by spatial sampling of a moving sinusoidal interference pattern formed between two frequency-offset injection-locked laser diodes. The offset and radar frequency of 3.2 GHz were generated by a Bragg cell. A nearly distortionless antenna beam pattern and 180° steering are demonstrated in a linear seven-element array.

A network model of a feedthrough phased array lens of printed dipole elements

An easy-to-use simplified network representation for the internal multiple reflection model of feedthrough phased array lens antenna elements is presented. This model makes use of the readily known active radiation impedances of an infinite array rather than the self and mutual impedances between array elements. In particular, this model is applied to a printed dipole lens array case, for which the computation of the radiation impedance is described. Numerical results for this lens array are given to show the aperture distribution variations due to the lens internal multiple reflection, and the gain falloff as a function of scan angle.

Area coverage adaptive nulling from geosynchronous satellites: Phased arrays versus multiple-beam antennas

Antenna design trade-offs associated with providing an area coverage communication link via a geosynchronous satellite, where the geographical area to be serviced by the associated adaptive antenna system is characterized by a narrow-cone angle directed at an a priori known region on the earth's surface, are considered. Interference sources within and just surrounding the coverage area are adaptively nulled; interference sources located far from the coverage area are minimized by designing the antenna system to have low sidelobes outside the coverage area. This minimizes the number of adaptive channels which must be processed, with subsequent decrease in satellite weight and power requirements. The antenna design trade-off is between the phased array (PA) and the multiple-beam antenna (MBA). Three specific performance factors are considered in the MBA/PA design trade-off: susceptibility to out-of-coverage-area interference sources which use up the adaptive degrees of freedom allocated to the desired coverage area; effects of reduced link margin for low-gain thinned arrays; and nulling bandwidth restrictions imposed on spread spectrum communication systems. Three classes of PA's are considered: filled arrays, high gain thinned arrays, and low-gain thinned arrays.

Electromagnetic wave diffraction on a two-dimensionally periodic lattice of semiinfinite dielectric rods

1) Numerical study of the convergence of approximate solutions of the diffraction problem and comparison of the numerical results obtained with exact solutions for a number of special cases indicate the high efficiency of the algorithm developed. 2) The results obtained may be used for engineering calculations of a wide class of devices containing periodic dielectric structures. In particular, in combination with the results of [15], the solution constructed can be used to develop algorithms for calculation of phased waveguide antenna lattices, matched to free space by dielectric rods [9].

Some Comments on Broadcast Antennas

A constant phase and current antenna is suggested as having more desirable characteristics than sinusoidal distribution. It is shown that the maximum nonfading range is a function of the antenna height, frequency, and ground conductivity. A modified antenna system is suggested which combines a high angle suppressor with a vertical constant phase and current antenna. It is shown that the modified system cancels the high angle lobe of radiation and increases the nonfading range.