Margaret Owens

Properties and applications of the large circular resonant dipole array

Large closed-curve antenna arrays have been a subject of research for many years and have been shown to have many interesting properties. The paper investigates some of the properties of such a dipole array when the closed curve is a circle. Recently, because of its unique horizontal field pattern, a 90-element circular array of this type has been proposed as a microwave beacon for the coastal navigation of ships and airplanes. In the design of these arrays, it is suggested that the array be rotated mechanically. The question arises: can the mechanical rotation be replaced by an electronic rotation? We show that electronic rotation is not possible for the 90-element circular array originally described, but is possible for a modified array. The subtle difference between these two arrays is clarified and a simple criterion is given for the general case. Also presented is the derivation of an asymptotic formula for the radiation pattern of a resonant circular array of N equal elements with only one element driven. Since the theory for such an array is complicated and involves numerous numerical difficulties, a simple asymptotic formula for the field pattern has advantages over other methods. The simple formula is shown to produce a vertical field pattern that is indistinguishable from its numerically calculated counterpart. Generalization to noncircular arrays is discussed briefly.

The Wave Antenna

The excitation of lateral waves along the boundary between two electrically different media like air and earth or sea or the oceanic crust and the sea is effectively achieved with horizontal antennas in the earth or sea close to the boundary with the air or the sea floor. These are usually insulated terminated antennas, often of the traveling-wave type, such as described in Chapter 17. An alternative to the antenna in the sea or earth is the antenna in the air—it is clearly impractical to insert an antenna in the rock of the sea floor. An example described in Chapter 4 is the vertical monopole erected on the surface of the earth for AM radio broadcasting (Section 4.2) or for communicating with submarines (Section 4.5). Although the vertical dipole is an effective generator of lateral waves, it is difficult at low frequencies to construct one that is not electrically very short and requires very large currents to provide an adequate electric moment. An alternative is the horizontal antenna discussed in Chapter 17 for subsurface use.

Historical and Technical Overview of Electromagnetic Surface Waves; Introduction to Lateral Waves

When an oscillating infinitesimal electric dipole with unit electric moment Ih = 1 ampere meter is located in air far from all material media, it generates an electromagnetic field in the form of outward-traveling spherical waves. At sufficiently large electrical distances from the source [kr ≫ 1, where k = ω(μ0e0)1/2 the wave number], its component electric and magnetic fields in the spherical coordinates r, Θ, Φ are

Antennas in Material Media Near Boundaries: The Terminated Insulated Antenna

{E_{\Theta }}(r,t) = - c{B_{\Theta }}(r,t) = - \frac{{\omega {\mu_0}}}{{4\pi r}}\,\sin \,\Theta \,\sin (\omega t - kr)

Antennas in Material Media Near Boundaries: The Bare Metal Dipole

(1.1.1) where Θ is measured from the positive z-axis and c = ω/k is the velocity of light. In (1.1.1), the argument of the sine function is, in general, ωt - kr + constant; the origin of the time variable t has been chosen such that this constant is absorbed in ωt. The EΘ and BΦ components are mutually perpendicular and both are perpendicular to the radial direction of propagation which is also the direction of the Poynting vector S(r, t) = 1 E(r, t) × B(r, t). The expanding spherical surfaces of constant phase axe defined by

The Horizontally Layered Half-Space

\omega t - kr = \omega \left( {t - \frac{r}{c}} \right) = {\text{constant}}