Showing papers on "Coaxial antenna published in 1967"

The Admittance of Cylindrical Antennas Driven From a Coaxial Line

Abstract: The situation in which the internal conductor of a coaxial transmission line projects through a ground-plane without any change in diameter is analyzed with reference to the infinite antenna and the parallel-plate antenna. An experimentally meaningful value of the admittance which takes into account the geometry of the antenna-to-coaxial line junction is obtained. This is achieved by utilizing a mathematical model in which an annular aperture or “gap” is represented by a frill of of magnetic current. The field distribution in the aperture is assumed to be of the same form as the TEM mode alone in the coaxial line. It is shown that the effect on the admittance of neglecting the TM modes in the line at the aperture, is very small. In the case of the parallel-plate antenna, the theoretical results are shown to agree remarkably well with experiment. It is also shown how the theory may be applied to the problem of finite length cylindrical antennas.

The near-zone magnetic field of a small circular-loop antenna

Abstract: An improved formula is derived for accurately computing the near-zone magnetic field of a s mall c irc ular transmitting loop an tenna. Such a field can serve as a reference standard for calibrating fi eld-strength meters e mploying small receiving loop anten nas in the frequency range 30 Hz to 30 MHz. This formula inc ludes correction terms for fr equency (d ue to the finite time of propagation), as well as corrections for the finite radii of both the transmitting and receiving loops. Other formulas appearing in the literature often fail to include such correc tions whic h can result in errors of up to 20 pe rcent and more in computing standard-fie ld values. The NBS formula is de rived by ex panding the integrand of the re tarded vector potentia l into an infinite se ri es of sphe rica l Ha nkel fun c tions of in creasing o rde r. The resulting series express ion is in e rror by less than 0.2 pe rcent , is rap idl y co nverging and si mple to use without recourse to a table of fun c tions or a compute r.

An experimental study of the antenna with nonreflecting resistive loading

Abstract: The current amplitude, input admittance, and radiation field pattern of a dipole antenna with a step-function internal impedance were measured in the UHF range and compared with theory. The zero-order theory gives accurate descriptions of the current distribution, the field pattern, the wide bandwidth property, and the existence of the traveling wave on the antenna. The agreement in the input admittance is not good, but it can be improved with the help of the variational principle. The field pattern of this resistive antenna when displayed as a 90\deg V antenna was found to be directional and insensitive to changes in frequency.

Cylindrical Antenna With Tapered Resistive Loading

Abstract: Current, input impedance and far field pattern of cylindrical antenna with tapered resistive loading

Omnidirectional loop antenna

Abstract: A SINGLE TURN LOOP ANTENNA HAVING A CIRCUMFERENCE OF LESS THAN ONE-HALF OF THE WAVELENGTH OF THE ELECTROMAGNETIC ENERGY TO BE TRANSMITTED OR RECEIVED, AND AN AXIAL DIMENSION FROM 1/2 TO 1/100 OF ITS CIRCUMFERENCE.

Dipole antenna in which one radiating element is formed by outer conductors of two distinct transmission lines having different characteristic impedances

Abstract: One half of a dipole antenna is formed by the extension of a center conductor of a coaxial transmission line beyond the point of termination of the outer conductor at the feed point of the dipole. The other half of the dipole is formed by the outer conductor between a broadband cable choke and the feed point. The part of the antenna between the choke and feed point comprises two coaxial lines, one having a characteristic impedance equal to that of the antenna feed line and the other having a characteristic impedance greater than that of the feed line.

The helicone—A circularly polarized antenna with low sidelobe level

Abstract: An axial mode helix in a conical horn produces a circularly polarized pencil beam of low sidelobe level over a 2-to-1 bandwidth. The gain of the combination or "helicone" is four times that of a simple helix of the same length and the sidelobes are 15 to 20 dB lower.

The Thick Tubular Transmitting Antenna

Abstract: The growing importance of electrically thick monopole antennas has suggested a re-examination of the model used in the derivation of the integral equation for the current in tubular antennas. It is shown that when such an antenna is driven from a coaxial line in the usual manner, which maintains a field along a narrow circumferential region on the outside surface of the tube (instead of along both the inner and outer surfaces), an additional term is required in the integral equation. This is negligible when the antenna is electrically thin, but of major importance when it is not. A quantitative comparison of conductances is provided over a wide range of thickness.

Gain measurements of vertically polarized antennas over imperfect ground

Abstract: A technique is described for measuring the absolute gain of vertically polarized HF antennas. A horizontal half-wave dipole, located near the antenna being measured (test antenna), is used as a reference in a comparison type of gain measurement. A rotatable, linearly polarized, transmitting antenna is mounted on an airplane and used to illuminate the test-reference antenna location. With the airborne antenna vertical, the response of the test antenna is measured. The airborne antenna is quickly rotated to a horizontal position and the response of the reference dipole is measured. When the airplane is in the main lobe of the test and reference antennas, these two measurements yield the gain of the test antenna relative to that of the reference dipole. The gain of a horizontal half-wave dipole is presented graphically, versus main lobe elevation angle, for various heights above imperfect ground. Mutual impedance variations were considered in the computations for these graphs. Model antenna measurements were made at 400 MHz to verify the principle of the technique. On-site HF measurements could give gain figures accurate to within a few tenths of a dB of their true value.

Long‐Periodic Dipole Antenna

Abstract: A 12-element log-periodic dipole antenna is investigated with the three-term theory described and verified experimentally in a companion paper (Cheong and King, 1967). The general frequency-independent behaviour for the distributions of current, the driving-point admittances, and the power for all elements over a wide range of frequencies. The driving-point admittance, field pattern, beam width, and minor-lobe level are determined for the antenna over the same frequency range. The director and reflector action of the elements is pointed out. It is concluded that a new and powerful tool is available for the complete and accurate analysis of log-periodic dipole antennas.

Compact steerable antenna array

Abstract: A COMPACT, STEERABLE, BOARD BANDWIDTH ANTENNA ARRAY WHICH IS CAPABLE OF PROVIDING MULTIPLE INDEPENDENT OUTPUTS WHICH ARE UTILIZABLE BY A PLURALITY OF INDEPENDENT RECEIVING SYSTEMS. THE ARRAY INCLUDES A PLURALITY OF RELATIVELY CLOSELY SPACED (I.E., LESS THAN ONE WAVELENGTH) ANTENNA ELEMENTS WHICH ARE COUPLED TOGETHER INTO GROUPS OF ANTENNA ELEMENTS, EACH GROUP PROVIDING A RADIATION PATTERN WHICH IS SUBSTANTIALLY INVARIANT WITH WAVELENGTH ABOVE A PREDETERMINED VALUE OF WAVELENGTH. THE OUTPUTS OF THESE GROUPS ARE THEN SELECTIVELY COMBINED TO PROVIDE A DIRECTIVE RADIATION PATTERN.

Circularly polarized broadcast antenna

Abstract: A circularly polarized broadcast antenna comprising a pair of conducting arms having portions forming an interrupted loop polarized in a first plane and terminating in ends diverging in different directions from the first plane to form a dipole.

Pattern of a linear antenna erected over a tapered ground screen

Abstract: A theory is presented for the radiation from a thin vertical antenna located above and at the center of a sector ground system. The formulation is carried out for a general variation of the surface...

Measurements of dipole antenna impedance in an isotropic laboratory plasma

Abstract: The impedances of relatively long dipoles in an isotropic laboratory plasma have been measured at 9.2 GHz ( X band). A helium plasma was generated by discharging a capacitor between electrodes in a cylindrical container ten free-space wavelengths in diameter. Impedance measurements were made in the decaying afterglow plasma at electron densities both above and below the critical electron density ( 10^{12} cm-3). The dipole antennas were fed from miniature solid-jacketed coaxial cables attached to a waveguide on which the voltage reflection coefficient was measured. The effect of plasma on the impedance of the dipole antenna was derived theoretically by treating the dipole as a dissipative transmission line. The theory described in a qualitative fashion the observed impedance variation with electron density for all the dipoles investigated. The measurements indicated certain antenna impedance properties, due to the plasma, which were not accounted for by the simple theory.

Broadband choke for antenna structure

Abstract: Broadband chokes and absorbers to reduce spurious radiation patterns of antenna array caused by support structures

Direction finding antenna assembly

Abstract: A direction finding antenna assembly including a horn antenna structure and associated microwave circuitry capable of operating to instantaneously and continuously determine the direction from which a signal is being received. The assembly may also include in the same structure a collinear antenna array.

An experimental study of imperfectly conducting dipoles

Abstract: Input admittances of dipole antennas with moderately high internal impedance were measured in the UHF range with the antenna lengths varying from one-tenth wavelength to such a value that the antenna behaved as if infinitely long. The measured results are compared with the three-term theory of King and Wu[1] and with the theoretical values obtained by Shen and Wu[3] for an infinitely long antenna.