Coaxial Choke Research Articles

Development of a direct mirror angle detector for ITER EC launchers

The new mirror angle detector applying a variable capacitor and several hundreds MHz RF wave has been developed. It is composed of a rotary capacitor consisting of a rotary and stationary electrode, a RF feeder and RF circuits. Reflected RF wave at the rotary capacitor supposed to connect the rotation axis of the mirror comes back to the RF feeder and the phase change of reflected RF due to the overlap condition between the two electrodes tells us the rotation angle of the mirror. Application of a co-axial choke stub to the support of the stationary electrode has been utilized. As a result, cooling water can be supplied to the stationary electrode through the center conductor of the choke stub that can solve the cooling issue of the electrodes since it acts as a notch filter to isolate the electrode, electrically. In order to verify the design principle of the new mirror angle detector for the EC launchers, the mock-up of the detector and RF circuits were fabricated and tested. It was obtained that phase difference of the reflected RF wave at the rotation angle change of 14° was 26°, which was well agreed with the design value of 25.7°. It is concluded that the new angle detector is applicable to the ITER EC launcher steering mirror.

Parasitic Current Reduction on Electrically Long Coaxial Cables Feeding Dipoles of a Collinear Array

The reduction of parasitic currents induced on the outer conductor of electrically long coaxial cables feeding the dipoles of a collinear array is addressed. Firstly, an experimental model is presented, and its appropriateness has been verified through validation with simulations data. The above model has been used to show the effectiveness of a low-cost technique to suppress the RF currents induced on the coaxial cables running inside the metallic mast of a collinear dipole array. The proposed solution consists of a dielectric-loaded coaxial choke, realized as a shorted quarter-wavelength coaxial cylinder mounted on the external conductor of the coaxial cable, and filled with Polytetrafluoroethylene (PTFE) to reduce its physical length. The PTFE-loaded choke performance is shown through both numerical simulations and measurements on prototypes, in the 200-400 MHz frequency range. Finally, a numerical parametric analysis is performed to get some design criteria for more complex multiple-choke arrangements.

Reverse switch-on dynistor switches of gigawatt-power microsecond pulses

A high-power (150 kA and 16 kV) small switch based on an assembly of reverse switch-on dynistors (RSDs) connected in series and a coaxial saturable-core choke, which creates conditions for their efficient switching, is described. An essential feature of this switch is a drastic reduction of the duration of the control action, as a result of which minimum dimensions and a low inductance of the saturable-core choke are ensured at a high (25 kA/μs) rise rate of the switched current. Increases in the control-current amplitude and rise rate that are required for maintaining the triggering charge at a constant level are attained thanks to the use of a fast-acting switch based on new semiconductor devices—deep-level dynistors—in the RSD-control circuit.

Is Magnetic Resonance Image Guidance the Key to Opening Chronic Total Occlusions?

Through progressive plaque growth or fibrotic organization of occlusive thrombus, atherosclerosis may result in chronic total occlusion (CTO) of a major arterial conduit. If CTO develops slowly, collateral pathways may supply sufficient perfusion to retain tissue viability despite occlusion of the major inflow conduit. However, such collateral-mediated perfusion rarely matches that provided by an open main conduit, particularly during the peak demand of regional muscular activity. Although strategies to limit regional oxygen demand (eg, β-blockers for the treatment of angina pectoris) or to enhance collateral function (eg, a walking program for superficial femoral occlusion, experimental angiogenesis) may be of some benefit, these approaches generally offer less complete return of normal physiology and relief of ischemic symptoms than does direct revascularization of the occluded vessel. With surgical bypass, the nature of the occlusion (its length, duration, tortuosity, calcification) is less important than the presence of a suitable distal anastomotic target. On the other hand, with catheter-based techniques, success depends heavily on these lesion characteristics. In fact, crossing such occluded segments with a guidewire remains the dominant barrier to catheter-based treatments of occluded arterial segments. Once the guidewire has been passed successfully into the distal true lumen, the process of dilating and stenting the segment is usually straightforward and durable, at least with the use of drug-eluting stents in coronary CTOs.1,2 Article p 1101 Devices designed to cross CTOs should have 3 key features: an ability to distinguish a true luminal path from one created within (dissection) or through (perforation) the vessel wall of the occluded segment, an ability to change direction (steer) to correct deviations from the desired path through the occlusion, and an ability to penetrate the frequently fibrotic and focally calcific substance of the occlusion through the use of either mechanical stiffness or an alternative energy modality (laser, …

A High-Voltage Reverse Switch-on Dynistor Switch with a Control Circuit Based on Magnetic-Compression Sections

A high-voltage switch of submicrosecond range based on an assembly of reverse switch-on dynistors (RSDs) connected in series and a coaxial saturable-core choke is described. Dynistors are triggered using a control circuit in the form of a high-voltage magnetic-compression section, in which the current-forming capacitor is charged from the power-circuit capacitor using an additional low-power saturable-core choke. The results of tests of a small-sized (200 × 200 × 55 mm) RSD switch with an operating voltage of 6 kV switching 1-kA current pulses with a 600-ns duration are presented. Low switching energy losses in dynistors make it possible to use devices with a 12-mm diameter of the structure in the RSD assembly. Their temperature was within 20°C at a switching frequency of 100 Hz.

A 915-MHz antenna for microwave thermal ablation treatment: physical design, computer modeling and experimental measurement.

A 915-MHz antenna design that produces specific absorption rate distributions with preferential power deposition in tissues surrounding and including the distal end of the catheter antenna is described. The design features minimal reflected microwave current from the antenna flowing up the transmission line. This cap-choke antenna consists of an annular cap and a coaxial choke which matches the antenna to the coaxial transmission line. The design minimizes heating of the coaxial cable and its performance is not affected by the depth of insertion of the antenna into tissue. The paper provides a comparison of results obtained from computer modeling and experimental measurements made in tissue equivalent phantom materials. There is excellent agreement between numerical modeling and experimental measurement. The cap-choke, matched-dipole type antenna is suitable for intracavitary microwave thermal ablation therapy.

Endourethral MRI.

Although high-resolution MRI with phased array pelvic, endorectal, and endovaginal coils has dramatically enhanced the ability to visualize abnormalities of the female urethra and periurethral tissues, controversy still remains about the anatomy of this region. This study introduces an endourethral approach for ultra-high-resolution MRI of the female urethra and the periurethral tissues. To this end, two different radiofrequency (RF) receiver coil designs for an endourethral insertion have been developed: a single-loop coil and a phased array/quadrature coil. Both designs feature a flexible coil circuit, small loss tuning and matching directly at the coil, active decoupling, and the integration of a lambda/4 coaxial choke to decrease unbalanced currents and limit potential RF heating effects. Effective reduction of the mutual inductance between the two coils of the phased array design was achieved by introducing a metallic "paddle" to steer the flux between the coils. The performance of the coils has been evaluated in female human cadaver studies and in an in vivo pig experiment. The novel endourethral approach enabled a dramatic increase of the signal-to-noise ratio (SNR) at the region of interest (ROI). High-resolution MR images of the female urethra have been acquired with a spatial resolution down to 78 x 78 microm. Histologic correlation was achieved for the MR images generated. The achieved high local SNR and resulting high spatial resolution will add valuable information to the discussion of female urethral anatomy. Magn Reson Med 45:138-146, 2001.

Reduction of resonant RF heating in intravascular catheters using coaxial chokes.

The incorporation of RF coils into the tips of intravascular devices has been shown to enable the localization of catheters and guidewires under MR guidance. Furthermore, such coils can be used for endoluminal imaging. The long cable required to connect the coil with the scanner input inadvertently acts as a dipole antenna which picks up RF energy from the body coil during transmit. Currents are induced on the cable which can lead to localized heating of surrounding tissue. Cables of various lengths were measured to determine if a resonance in the heating as a function of cable length could be found. Coaxial chokes with a length of lambda/4 were added to coaxial cables to reduce the amplitude of the currents induced on the cable shield. A 0.7-mm diameter triaxial cable, small enough to fit into a standard intravascular device, was developed and measured both with and without a coaxial choke. It is demonstrated that resonant heating does occur and that it can be significantly reduced by avoiding a resonant length of cable and by including coaxial chokes on the cable.

The cap-choke catheter antenna for microwave ablation treatment.

A matched-dipole type catheter antenna is described for intracavitary and/or transluminal microwave treatment of diseases that may be responsive to thermal ablation therapy. This cap-choke antenna consists of an annular cap and coaxial choke design that can yield SAR distributions in and can produce heating of tissue surrounding the distal end of the catheter antenna. The cap-choke antenna is simple in construction, provides the desired specific absorption rate (SAR) distribution, and is efficient, i.e., the measured power reflection coefficient is 2% in tissue equivalent phantom modeling materials.

A new microwave applicator with integrated cooling system for intracavitary hyperthermia of vaginal carcinoma.

An improved design of a previously described intracavitary microwave hyperthermia applicator is presented. The applicator consists of a coaxial choke antenna designed to be positioned into a perspex obturator. The antenna can be fitted in the obturator in three defined positions depending on the specific clinical situation: the selected median, paramedian or lateral position can each provide differently directed heating patterns. This feature combined with the additional axial variability of the antenna position within the obturator can lead to a highly targeted heating of tumours and a reduced risk of unwanted heating of normal tissues. Various phantom studies were conducted using both liquid and solid phantoms. The saline phantom was used to check the typical action of the choke of the antenna where it was found that the antenna choke is efficiently working resulting in a heating pattern which is dependent of the insertion depth of the antenna. The solid phantom was used to measure the typical specific absorption rate (SAR) distribution of each antenna/obturator configuration.

Comments on "A design of AM/FM mobile telephone triband antenna" [with reply

Ploussios has read the paper by Egashira et al. (see ibid., vol.42, p.538, 1994), in which they describe the design of multiple band antennas using "double sleeves." He claims that the design of multiple band antennas employing multiple coaxial sleeves of the type described in the paper has been done before and is patented by Ploussios. The patent, 4509056, entitled "Multifrequency antenna employing tuned sleeve chokes," describes the use of multiple coaxial chokes in monopole and dipole type elements. The chokes (sleeves) are arranged in series and parallel, resulting in operation over selected frequency bands that are not harmonically related. The patent includes a description of two- and three-band monopole and dipole designs. Egashira et al. reply to the Comment. They conclude that it is their view that the triband antenna of the AM/FM and mobile telephone (with two bands of mobile station band and base one) cannot be designed using the ideas in Dr. Ploussios' patent.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A catheter antenna for percutaneous microwave therapy

AbstractA flexible catheter antenna with a curved tip is described for transcutaneous microwave treatment of cardiac tachyarrhythmias. In tissue‐equivalent phantom modeling materials, the enlarged inner conductor cap and coaxial choke design yields SAR distributions that accentuate heating of tissue toward the distal end of the catheter antenna. The power reflection coefficient is less than 4%.

A theoretical model for input impedance of interstitial microwave antennas with choke

Purpose : Two important characteristics for interstitial microwave antennas used in clinical hyperthermia are: (1) a good impedance match to minimize reflected power; and (2) a good power deposition pattern which is independent of insertion depth. A major problem of the miniature coaxial dipole antennas used for interstitial hyperthermia is the fact that the impedance and power deposition patterns of these antennas change with insertion depth. One possible solution is the addition of a coaxial choke. A theoretical model for calculating the input impedance of interstitial microwave antennas having a coaxial choke is presented, which may serve as the first step in the design of such antennas. Methods and Materials : A theoretical model for calculating the input impedance of coaxial microwave antennas with and without a choke is presented using insulated antenna theory. The theoretical model was used to calculate the input impedance of several prototype antennas having various choke and feedline dimensions, and comparison was made with experimentally measured impedance measurements in tissue-equivalent phantom. Results : The choke section of the antenna is not ideal if conventional plastic insulation is used as the choke dielectric, because the desired radiating length of the antenna is significantly shorter than the quarter-wavelength in the choke dielectric. Impedance calculations based on the theoretical model correlate reasonably well with experimentally measured impedance. Based on these calculations, the effect of parameters such as choke layer thickness and choke dielectric constant are discussed for a 915 MHz antenna with choke. Conclusion : The theoretical model can serve as a design aid for optimizing choked microwave antenna designs, as well as predicting the impedance match of a given antenna design at a given insertion depth. The model allows the effect of some variables not accessible experimentally such as termination impedance to be studied, which may also be useful in the understanding of these antennas. Calculations are easily performed on a desktop computer.

Applications of coaxial chokes to a-c bridge circuits

Applications of coaxial chokes to a-c bridge circuits

AC Bridge Methods for the Measurement of Three-Terminal Amittances

To avoid uncertainties due to finite lead impedances, a direct admittance is best defined as the transfer admittance of a two-terminal-pair network. The network may contain both voltage and current transformers. Unwanted loop currents may be suppressed by coaxial chokes and the equivalent circuit reduced to that of a three-terminal admittance. Two direct admittances may be compared with a voltage or current ratio, and two such ratios may be combined to give a general form of four-arm bridge. This general bridge network may be balanced in a way which leads to simple relations independent of the ground admittances between the four direct admittances. At least two separate balance conditions must be satisfied, but it is usually best to make three balances since the error in the main balance is dependent on the product of the errors in the other two. There is no restriction on the type of network which may be used to couple the source and the detector to the bridge, provided the ground balance conditions are satisfied. Networks may be chosen to provide for filtering, impedance matching, and the elimination of transformers. Examples are given of four-arm bridge networks which use adjustable decade ratio-transformers as the main balance controls and fixed ratio-transformers for multiplication or inversion.

Broad-Band Coaxial Choked Coupling Design

Equations and curves are presented to predict the frequency bandwidth of coaxial choke couplings in terms of the choke parameters. Choke couplings discussed are those applicable to rotary joints and dc isolation units.