Integral Number Of Half-wavelengths Research Articles

Determination of Acoustical Impedance of Absorbing Surfaces by Two-Microphone Transfer Function Techniques: Geometrical and Measurement Errors

In the second of a series of papers on the measurement of acoustical impedance of absorbing surfaces in-situ, geometrical and measurement errors for the two microphone transfer function technique are detailed. The effects of finite sample area on the impedance measured are considered, and experimental results show that for edge effects to be negligible, the minimum dimension of a sample should be two wavelengths. The method also becomes inaccurate when the transfer function tends to unity. This is shown to occur at low frequencies, but more importantly, at higher frequencies when an integral number of half wavelengths exists between the path length differences to the microphones. The results of the error analysis show that great care should be taken in determining the precise locations of the microphones. In particular, care should be taken in the alignment of the microphones with respect to the normal to the sample, since errors due to the alignment of the probe combine features of errors not only in the microphone positions, but also in the source location and the angle of sound incidence. In comparison with typical errors in the experimental geometry, errors due to the precision of typical transfer function estimates are very small.

A monopole antenna loaded with a modified folded dipole

A traveling-wave distribution of current can be produced on a linear antenna by inserting a resistance of suitable magnitude one-quarter wavelength from its end. In this study the resistor is replaced with a modified folded dipole which has a radiation resistance approximately equal to that of the matching resistor. Thus the input section has a traveling wave distribution up to the inserted antenna, as before, but now the power that was previously dissipated in the resistor is also radiated. Input impedance, current distribution, and radiation patterns of this antenna are computed, and input impedance and radiation patterns are also measured. Horizontally polarized patterns are similar to those of a horizontal dipole over a ground plane, and vertically polarized patterns in a plane orthogonal to the folded element are similar to those of a monopole over a ground plane. Coverage is also obtained in the zenith direction in the plane of the folded element, as long as it is not an integral number of half wavelengths above the ground plane. The peaks and nulls can be controlled by adjusting the monopole height accordingly and it is possible to achieve near hemispherical coverage.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Comments on 'The measurement of the properties of materials' by M.N. Afsar et al

It is pointed out that the references cited in a previous paper (ibid., vol.75, no.1 p.183-99, 1986) do not support the argument that the optimum thickness of the sample should be an integral number of half-wavelengths thick. >

Possibility of partial phase self-locking of retroreflector elements in a resonator

The mechanism of partial phase self-locking of elements of a retroreflector in a laser resonator is considered for the case of a random distribution of these elements. This mechanism involves generation of wavelengths for which the shift between the elements of a retroreflector is close to an integral number of half-wavelengths.

A Fabry-Perot solid etalon for teaching

Fabry-Perot (F-P) interferometry is an important topic in an optics course because the interferometer provides an example of multiple-beam interference and finds widespread use in laser physics laboratories, both as a device for tuning dye lasers and as a diagnostic instrument for displaying cavity modes. From an optical point of view, a F-P etalon is equivalent to a pair of plane parallel highly reflecting surfaces. Light waves which are incident upon the etalon suffer multiple reflections within the cavity formed by these surfaces and are transmitted with maximum amplitude if the length of the cavity is an integral number of half-wavelengths. In the adjustable etalon, parallelism is achieved by pressing the plates against a spacer, and in a typical work environment this adjustment needs to be made frequently. This type of etalon is in common use in teaching laboratories, even though many students (by university second-year level) do not have the skills which are required to maintain it in good adjustment. For this reason the authors have designed an experiment around the solid etalon, which has the advantage of intrinsic stability of shape.

Symmetries of the overmoded right-angle corner

Diffraction theory has been used to analyse a symmetrically truncated right-angle corner in a previous paper by Lewin and Crain. An inspection of the diffraction fields radiated by the conducting wedges which define this configuration has given insight into the symmetries which should be exhibited by this waveguide junction. It is emphasised here that, in a closed waveguide structure, all interaction terms must be taken into account to converge to a reliable solution. This has not been the case in solving open structures such as conducting strips in free space or open-ended waveguide horns. It is also seen that special care must be taken to account for fields in penumbra regions. This analysis reveals a symmetry of higher-mode generation by the corner when the height is an integral number of half wavelengths which could not otherwise be seen.

Magnetostatic modes in axially magnetized polycrystalline YIG rods at 9.96 GHz

A large number of magnetostatic mode resonances have been observed in axially magnetized cylindrical rods at 9.96 GHz. The data can be satisfactorily explained on the basis of magnetostatic mode theory of volume and surface modes for rods in air and surface modes for rods encased in metal sleeves. The resonances were observed by inserting 0.075-inch diameter polycrystalline YIG rods through holes in a shorted TE <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</inf> rectangular waveguide section one half wavelength from the short so that the portion of the rod inside the waveguide was in a region of uniform microwave field, and measuring reflected microwave power as a function of static magnetic field strength with the field applied parallel to the rod axis. With the rod supported by 1-inch long metal sleeves on both sides of the waveguide, the spectrum consists of a series of weak resonances from 1800 to 2600 Oe, a main peak at 2725 Oe, and a series of resonances, decreasing in intensity with increasing field, above the main peak. By removing the sleeves and supporting the rod with dielectric inserts in the waveguide wall, the weak resonances were eliminated from the spectra. The positions of the observed peaks were found to agree with predictions from magnetostatic mode theory. The resonances on the low field side of the main peak correspond to surface modes excited in the sleeve-enclosed portions of the rod with an integral number of half wavelengths along the sleeve length. The main resonance corresponds to the excitation of a surface mode excited in the portion of the rod which is in the waveguide. The resonances on the high field side of the main peak correspond to volume modes excited in the portion of the rod which is in the waveguide.

On Radiation from Antennas

This paper presents some theoretical remarks and experimental data relating to applications of the transmission-line theory to antennas. It is emphasized that the voltage, the current, and the charge are affected by radiation in different ways, a fact which should be considered in any adaptation of line equations to antennas. It is shown experimentally and theoretically that in an antenna of length equal to an integral number of half wavelengths, which is energized at a current antinode, the effect of radiation on the current and the charge (but not on the voltage) can roughly be represented by adding to the resistance of the wires another fairly simple term.

The electric and magnetic fields of a linear radiator carrying a progressive wave

The vector- and scalar-potential method of calculating electric and magnetic fields is applied to a straight conductor carrying a progressive wave of current. It is shown that unless the conductor is assumed to be terminated by charges which satisfy the condition of electrical continuity at the ends, the calculated field has an anomalous and impossible character. Such charges need not be included in the case of closed circuits of conductors carrying progressive waves. The mutual cancellation in such cases of the anomalous features associated with each linear element is confirmed by analysis of a particular case. It is pointed out that a linear progressive wave radiator has only one axis of symmetry, i.e. the radiator itself, differing in this respect from a standingwave radiator, which is symmetrical about the equatorial plane. The consequent asymmetry of the field from a progressive wave radiator is confirmed by analysis.The field system associated with a progressive wave of current is regarded as a more fundamental conception than that due to a standing wave, since any standing-wave system of currents is resolvable into positive and negative travelling waves. This feature is illustrated by the synthesis of the travelling wave fields into the known form for a dipole standing-wave radiator.The radiation resistance of a progressive-wave radiator is calculated by Pistolkors&apos;s method, and is shown to be equal to that for a standing-wave radiator when the length of radiator is an integral number of half wavelengths.

The Acoustic Resonator Interferometer: I. The Acoustic System and its Equivalent Electric Network

The steady state of motion of a fluid between two infinite plane parallel boundaries is found for the case in which one of the boundaries is given a prescribed periodic motion normal to its surface, the other boundary being infinitely rigid or being assigned a coefficient of reflection. The excess pressure at any point in the fluid is found, being of particular interest at the boundary of the source where it has a term in phase with the velocity of the source and one in phase with its displacement. These terms pass through cyclical values as the distance between the source and reflector is increased, the first passing through sharp maxima, the second changing rapidly from negative to positive values at reflector distances of an integral number of half wave-lengths in the fluid. Application is made to the case where the source is the surface of a piezoelectric plate maintained in forced vibration. The equivalent electric network of the plate and coupled fluid column is found to be the same as that for the plate alone, with modified resistance and capacity coefficients, making possible consideration of the theory of the acoustic resonator interferometer in conjunction with driving and measuring circuits.