Charles W. Harrison

Transients in wide-angle conical antennas

The response of an antenna to dc pulse excitation, both in reception and in transmission, can be determined by use of Fourier transforms provided the complex impedance and complex effective height of the antenna are known over the frequency range where the excitation has significant components. We have computed the time history of the response of a wide-angle conical antenna to dc pulse excitation and show the results herein for a matched source in transmission and a matched load in reception for several values of cone angle and antenna length. Under matched conditions, the faithful transmission of a pulse shape, requires a long antenna while faithful reception is not possible. Faithful reception can be achieved by a short antenna with a resistive load of high resistance or with a capacitive load, or by a long antenna with a series RC load. Since this work required transfer functions in transmission and reception, we have also presented here graphs of these quantities.

Monopole with inductive loading

This paper presents the results of a study undertaken to determine the electrical characteristics of a base-driven inductively loaded monopole, shorter than self-resonant length, oriented normal to an infinite perfectly conducting ground screen. The antenna may be tuned by a combination of two lumped coils of finite Q . One is in series with the generator at the base; the other is in series with the antenna conductor some distance above the feed point. In some installations the use of two loading inductors may be a convenience in that this arrangement permits final tuning adjustments to be made at the base of the antenna. Sufficient information is available to permit determination of the reactance of the lossy loading coils to achieve resonance, the voltage rise across the coils, the impedance presented to the generator, and the antenna efficiency in the absence of antenna ohmic and ground system losses. Simple ancillary studies, as proposed by an illustrative example, permit decisions to be made regarding the merit of a particular inductively loaded monopole of small dimensions in terms of the operating wavelength-whether the application is for VLF transmission from a shore radio station, or for transmission from a moving motor vehicle at shorter wavelengths. An interesting by-product of the present analysis is a formula for the impedance of a monopole grounded at its base, and driven some distance above the earth plane.

Folded dipoles and loops

In Section I the theory of linear arrays consisting of two or more closely spaced elements that are interconnected by lumped reactances is reviewed. Specific application is made to two-element end-loaded folded dipoles and monopoles constructed of conductors with different diameters, to series tuned three-wire folded dipoles and monopoles, and to a three-wire-line reactor and impedance transformer. In Section II the circular folded dipole or Halo antenna is treated.

The imperfectly conducting cylindrical transmitting antenna: Numerical results

The complex wave number, the distribution of current, the admittance, and the radiating efficiency of cylindrical antennas made of imperfect conductors are evaluated numerically from a previously derived theory[1]. The quantity 2\lambda r^{i}/\zeta_{0} (where r^{i} is the resistance per unit length, \lambda is the free-space wavelength, and \zeta_{0} = 377 ohms) is used as the parameter in a range that extends from zero to 200. Extensive graphs and tables are given.

On the attenuation of transient fields by imperfectly conducting spherical shells

Exact formulas for the electric and magnetic fields at any arbitrary point within a cavity region completely enclosed by a conducting spherical shell of arbitrary size are derived under the assumption that the exciting electromagnetic field is a linearly polarized, monochromatic, plane wave falling on the external surface of the shell. It is shown that the polarization of the electromagnetic field at the center of the cavity is the same as the polarization of the incident wave. From a knowledge of this steady-state solution, the time history of the electromagnetic field at the center of the cavity is calculated for the case where the incident wave is a Gaussian pulse. Numerical information on the effectiveness of the aluminum and copper shields under steady-state and transient conditions is provided for several pulse durations, shield sizes, and wall thicknesses.

Transient electromagnetic field propagation through infinite sheets, into spherical shells, and into hollow cylinders

Gaussian electromagnetic field pulses of several durations are propagated through infinite sheets into the interior of hollow cylinders and into the interior of spherical shells. The plates, spheres and cylinders are made of aluminum and contain no slots. The time history of the propagated pulses is computed. Finally, the time sequence of the electric field is calculated in the interior of a cylinder of finite length when connected at its ends by wires to a generator delivering a current pulse of Gaussian shape. The dimensions of the cavities are assumed to be sufficiently small so that resonances are not excited by the highest significant frequency contained in the shortest pulse considered. The numerical study is restricted to thin-walled aluminum shields 1/32 inch, 1/16 inch, 1/8 inch and 1/4 inch thick. The half-amplitude widths of the pulses employed lie in the range 14 \mu sec to 2400 \mu sec. It is shown that the resultant Gaussian pulse electric fields defined on the surface of the plates and cylinders are propagated with little diminution in amplitude. This is understandable due to the requirement that the tangential fields are continuous across the interfaces, and to the fact that skin effect is almost nonexistent at low frequencies. The incident (as contrasted to resultant) field pulse undergoes reflection at the boundary surface. Hence, the attenuation sustained by the incident field is great, since reflection is the chief mechanism of attenuation of fields at low frequencies. Thin spherical shells form effective magnetic shields. The electric field is small in the interior of thin-walled cylinders carrying extremely large transient currents.

Theory of the Annular Slot Antenna Based on Duality

The general theory of annular slot antennas is developed in a lucid manner. The treatment is novel in that no Greens functions or contour integrations arise. The principle of duality is employed. Topics discussed include formulation of the integral equation for the aperture field and derivations of the leading term in the near- and far-zone fields and effective height. The theory has application in calibrating near-zone field strength instrumentation and in determining the response of an unsheathed coaxial cable connector mounted flush with the skin of a missile to intense plane-wave electromagnetic field pulses.

Excitation of a Coaxial Line Through a Transverse Slot

A rocket with removed access plate is simulated by a section of coaxial transmission line with a transverse elliptical slot cut in its sheath. The internal circuit consists of two arbitrary impedances in series with the inner conductor at its ends. The object is to find the currents in these impedances when the cylinder is illuminated from the outside by an electromagneticfield that enters the aperture and excites the internal circuit. The problem is solved by application of the reciprocal theorem. The current in a dipole antenna is determined when this is inthe far field maintained by the slotted coaxial line when driven by a generator in series with one of the load impedances. The field in the aperture is replaced by equivalent electric and magnetic dipoles. The reciprocal theorem gives the current in the load impedance when the distantdipole is driven. A numerical example is given.

Electromagnetic field in the sea due to an omnidirectional VLF antenna

The physical and electrical properties of typical VLF transmitters are described, and new formulas are given for the electromagnetic field they generate on the surface of and at any depth in the sea. These apply to the spherical earth at frequencies in the range 14 ≤ ƒ ≤ 30 kHz. They are useful for distances up to 10,000 km. Special application is made to communication with submerged submarines equipped with horizontal antennas.

On the Excitation of a Coaxial Line by an Incident Field Propagating Through a Small Aperture in the Sheath

A coaxial line having a small aperture in the sheath and terminating in arbitrary impedances is considered to be illuminated by an incident plane wave. The excitation of currents in the termination impedances is shown to depend upon three independent factors. Simple analytical forms are given for equivalent voltage and current sources of the aperture excitation.