Coaxial Loop Research Articles

Low‐phase noise 3 42 GHz signal generation using high‐order longitudinal modes of a coaxial loop cavity

AbstractThis article demonstrates a novel coaxial loop cavity microwave/mm‐wave generator. Working at high‐order longitudinal modes, the generator has the single‐sideband phase noise of about −75 and −105 dBc/Hz at 10 and 100 kHz offset over the frequency band of 3–42 GHz. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 2329–2331, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22754

Superconducting screening of flux linked between a SQUID pick-up coil and a coaxial cylindrical sample

An expression is derived for the screened magnetic flux linked between a semi-infinite source solenoid and a coaxial loop, for small axial separations. The screen is a coaxial, infinitely long, superconducting cylinder. For large axial separations the equations of Osterman and Williamson (1983) are appropriate. Both expressions are integrated to give the flux linked between two semi-infinite solenoids, and the instrument function for a second-order gradiometer is constructed by superposition. A comparison of the screened instrument function with experimental measurements yields deviations of less than 2% compared with 6% for the unscreened function.

THE USE OF TWO‐ELECTRODE AND SCHLUMBERGER FILTERS FOR COMPUTING RESISTIVITY AND EM SOUNDING CURVES*

AbstractDifferent sets of filter coefficients for the linear filter technique for the computations of resistivity and EM sounding curves are evaluated for several electrode and coil configurations. Instead of this procedure, the two‐electrode filter can be used for computations of Wenner, Schlumberger, and dipole—dipole apparent resistivity model curves by defining convolutional expressions which contain the new input functions in terms of the resistivity transform function. Similarly, the Schlumberger filter performs the computations of dipole—dipole apparent resistivity model curves. The Wenner, Schlumberger, and dipole—dipole filter functions are defined in terms of the two‐electrode filter using the new convolutional expressions. A relationship between the Schlumberger and dipole—dipole filter functions is given.The above arguments are adopted for the computations of EM sounding curves. It is shown that the EM filter for the horizontal coplanar loop system (which is identical to the two‐electrode filter) performs the computations of the mutual coupling ratios for perpendicular, vertical coplanar, and vertical coaxial loop systems. In the same way, the Schlumberger filter can be used to compute vertical coaxial sounding curves. The corresponding input functions are defined in terms of the EM kernel for all convolutional expressions presented.After these considerations, integral expressions of the mutual coupling ratios involving zero‐order Bessel function are derived. The mutual coupling ratio for the vertical coaxial loop system is given in the same form as the mutual coupling ratio for the vertical coplanar loop system.

Fundamental and Higher Order Mode Couplers on Superconducting RF Cavities for Electron Storage Rings

The design of a coaxial high power loop coupler for use on a 500 MHz superconducting (sc) accelerating cavity of spherical shape is described. Nonsuperconducting construction materials (copper and copper plated stainless steel) are used. Cooling is obtained by cold helium gas in counter flow. The coupler is located at the cavity equator and a choke provides an uncritical RF joint at a minimum current flow position. For the coupling required (Qext = 105-106) the loop does not protrude into the cavity. Test experiments up to 70 kW incident RF power with a matched copper cavity and up to 17.5 kW with a superconducting cavity (corresponding to an acceleration field Eacc = 3.6 MV/m) are described. Design and measurements with two types of compact higher order mode (hom) couplers, coupling to the magnetic and electric field respectively are described. Both couplers are manufactured from Nb and are low Q resonant devices with their centrefrequency matched to the relevant part of the cavities' higher order mode spectrum. All couplers have been operated with a sc cavity up to the design field level.

Computation of the Electromagnetic Response of a Layered Earth to a Vertical Coaxial Loop System

The electromagnetic (EM) mutual coupling ratio for a vertical coaxial loop system over a layered earth is expressed as a Hankel transformable integral. This expression facili- tates the computation, using an available digital linear filter, for the response of either a perpendicular or a vertical coplanar loop configuration, or a Schlumberger electrode configuration. Such computations may be carried out on a programmable pocket calculator.

Irradiation of prolate spheroidal models of humans and animals in the near field of a small loop antenna

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 < 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.

Evaluation of a class of integrals occurring in mode matching solutions for electrical and electromagnetic modelling

Many integrals that go to make up the matrix for mode matching solutions of electrical or electromagnetic problems may be evaluated in closed form. Examples where this is possible include the electrical problems of finding the potential about a point electrode that is in a layered ground that includes either a spherical or cylindrical conductor. The results given here greatly reduce the amount of computation that is required for the calculations proposed by Lee. The integrals that arise analogous to the electromagnetic scattering of a cylinder in a half space may also be evaluated. For the case of a spherical scatter it is only possible to evaluate the integrals for the case where the source is a large coaxial loop.

Multiply driven and loaded coaxial circular loop arrays

Characteristics of single- or double-loaded coaxial loop arrays with arbitrary circumferences are studied when the sources or loads are located diagonally on the loops. For finite gaps at the deriving points and finite lengths of the loads, integral equations for the loop currents are obtained. These integral equations are then reduced to a matrix one by expanding the currents and the kernel in Fourier series of the azimuthal coordinate. For the special case of loaded Yagi arrays numerical results for the gain and the input impedance are presented. These results reveal that a capacitive loading of the array generally broadens the gain bandwidth of the array over which the gain remains essentially constant. A resistive loading of the exciter, on the other hand, yields a broadband characteristic for the driving-point admittance.

An Experimental Test Loop to Assist in Designing Digital Microwave Radio Systems

An experimental method for the study and trial of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</tex> -hop digital microwave radio systems is described. The method is particularly useful for those systems where the different disturbances accumulate and do not occur on each hop on an independent basis. The method simulates a digital transmission system by generating an information sequence of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</tex> bits at a rate of f <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</inf> bits/s and circulating the sequence <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</tex> times through one regenerator and a coaxial cable loop having a time delay <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\tau &gt; m/f_{b}</tex> . The circulating loop can be employed to investigate the influence on noise and error rate in digital radio systems of factors such as: cochannel and intersymbol interference; heavy rain attenuation on one or two repeater sections; additive noise; gain and phase nonlinearities; fading; changes in components and operating conditions within the regenerator; and combinations of these disturbances occurring simultaneously.

Properties of coaxial Yagi loop arrays

The resonant properties of circular loops as elements of a coaxial loop array are studied. It is shown that array characteristics can be obtained accurately by retaining only the resonant modes or the few modes around them. Effects of the various array parameters on the array characteristics are also studied. It is found that the reflector, the exciter, and the wire cross section have a small effect on the forward gain and radiation patterns, but the directors have a large effect. All parameters are found to have a large effect on the backward gain and the input admittance.

A Loop Yagi-Uda Array for VHF Television Reception

A Yagi-Uda array using a coaxial loop as a feeding element and wire loops as directors and reflector has been designed for multichannel TV reception. Optimum performance has been obtained by adjusting the parameters, viz. the loop diameter, and spacing of the directors and reflectors.

On calculating transient electromagnetic fields of a small current-carrying loop over a homogeneous earth

The basic theory of airborne EM surveying, in the time domain, is considered. Rather than resorting to tediousdouble numerical integration, a more direct approach is adopted. This method, valid in the quasi-static regime, is illustrated for a homogeneous flat earth. The results exhibit a number of clear-cut features that are relevant to remote sensing. For example, a vertical co-axial loop system has a desirable transient response from the standpoint of yielding conductivity data without requiring accurate information on the height of the device above the ground.

Multiloop antenna arrays for high-frequency shipboard direction finding

The application of multiloop arrays to high-frequency direction finding in the complex reradiation environments of shipboard sites is analyzed in detail. Theoretical results show that the bearing error and blurring are reduced and multivalued calibration curves are eliminated as the number of loops in the array is increased. The coaxial spaced loop is shown to produce a 2 to 1 reduction in bearing error with respect to the simple loop and eliminates multivalued calibration curves for thin reradiators. The use of combination arrays such as <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(n- 1)</tex> or <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(n- 3)</tex> loops are shown to produce unambiguous direction finding patterns. The DF performance characteristics of a coaxial spaced loop plus simple loop array are demonstrated for a shipboard site. Comparison of simple loop and three loop array calibration curves verify the performance characteristics predicted by the theory.

A phase-comparison method of measuring the direction of arrival of ionospheric radio waves

The most convenient research method of measuring completely the direction of arrival of waves reflected at the ionosphere appears to be that in which the phase differences of the signals received in an assembly of aerials are measured. If two pairs of similar aerials erected on lines perpendicular to each other are used, two independent phase angles may be obtained from which both angle of elevation and azimuth may be deduced.The apparatus described uses spaced coaxial loop aerials at a separation of 100 m. The signals from the aerials in a pair are amplified by means of matched receivers. The phase difference between the output signals from these receivers is displayed direct on a cathode-ray tube as the angle of inclination of the trace. With pulsed signals emitted from a suitable transmitter and with corresponding timing equipment in the receiver, the individual rays making up the total ionospheric signal may be separated from each other. The apparatus covers the frequency band 4–15 Mc/s, and the r.m.s. error of phase measurement is about 1°. Site errors, however, set a more severe limit to the accuracy of the directional measurements than do instrumental errors, and in practice it is found that, for example, over an oblique path corresponding to a range of 700 km, bearings can be measured with an accuracy of about 1° while the angle of elevation can be measured with an accuracy better than about 1½° so long as it exceeds 30°.These limitations mean that angles of elevation of E-layer reflections cannot be measured accurately at long range; it is possible, however, to obtain measurements of useful accuracy of the angle of elevation of F-layer reflections at ranges up to 1000 km or more. Bearings can be measured accurately at all ranges and for all reflections. The apparatus has so far been used principally for the study of F-layer reflections.