Wide-Range Antennas

Boris Levin

• Publisher: CRC Press
• Year: 2019
• Print ISBN: 9781138487444
• eBook ISBN: 9781351043229
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Biology
• Index: Biological Sciences

Expanding the range of antenna frequency is the main objective of this book. Solutions proposed are based on the development of new theoretical methods for analyzing and synthesizing antennas. The book shows that concentrated capacitive loads connected along linear and V-antennas provide a high level of matching with a cable over a wide frequency range and improves directional characteristics of antennas, i.e. increases the communication distance.

New theoretical methods are proposed for analysis and synthesis of antennas under consideration: 1) method of calculating directional characteristics of radiators with a given current distribution, and 2) method of electrostatic analogy for calculating mutual and total fields of complex multi-element radiating structures. These methods allow us to obtain optimal directional characteristics for director-type antennas (arrays of Yagi-Uda) and log-periodic antennas with concentrated capacitances and show that use of capacitors makes it possible to extend the frequency range of the director antennas and to decrease dimensions of the log-periodic antennas

Multi-element (flat and three-dimensional) self-complementary antennas with different variants of connecting generator poles and cable wires to antenna elements are proposed, which improves the matching with a cable. Characteristics of flat structures are compared with characteristics of volume structures: conical, parabolic, and located on a pyramid edges.

The book describes new versions of transparent antennas, antennas for cellular communication, multi-tier and multi-radiator antennas, and much more.

PART 1

WIDE-RANGE ANTENNAS

1

Radiators with Distributed Loads

1.1 Radiators with non-zero (impedance) boundary conditions—Constant surface impedance

As stated in the Introduction, applying concentrated capacitive loads (capacitors) played a fundamental role in creating wide-band antenna. Such an antenna is a radiator with loads which are installed along an axis of a linear antenna and form an in-phase current with a given amplitude distribution law. An antenna with surface impedance in the form of a metal rod with a ferrite shell was a first studied radiator of this type [1]. It is the radiator with distributed loads whose study showed that using a ferrite shell allows to change in the range of antenna operating frequencies.

Accordingly, as a first step, it is expedient to consider radiators with distributed loads. From the foregoing it follows that the antenna in the form of a metal rod covered by a layer of magneto dielectric (Fig. 1.1) can serve as an example of such a radiator. Let the rod length be L, the rod radius be a₁ and the outer radius of the shell be a. The boundary conditions on the shell surface have a form

$\frac{E_z(a,z)+K(z)}{H\phi (a,z)}{|}_{-L\le z\le L}=Z(z),$

(1.1)

where E_z(a, z) is the vertical component of an electric field, H_φ(a, z) is the azimuthal component of a magnetic field on the antenna surface, K(z) is the exciting (external) electromotive force (emf) and Z(z) is the surface impedance, which in general case depends on coordinate z (it is assumed that the antenna axis coincides with z axis of a cylindrical coordinate system—see Fig. 1.1). The boundary conditions of this type are valid if the field structure in one of the media (for example, in the magneto-dielectric shell) is known and does not depend on the structure of the field in another medium (in surrounding space). Radiators, on whose surface the boundary conditions (1.1) are fulfilled and the surface impedance changes substantially the current distribution along the antenna, are called radiators with non-zero (impedance) boundary conditions, or simply impedance radiators (it means in the first approximation).

Fig. 1.1: Antenna with distributed surface impedance.

In accordance with the equivalence theorem, when considering electromagnetic fields in a free space surrounding a radiator, an antenna should be replaced by a field at the boundaries. Although in this case, it is possible to operate only with fields, but for clarity and simplicity of reasoning, it is expedient mentally to put a metal coat on the antenna surface. The surface density j_s of a total linear electric current J(z) along the metal coat is related to the strength H of the magnetic field by an expression

$[e_{\rho },H]=j_s,$

(1.2)

where e_ρ is a unit vector directed along the axis ρ. Then

$H_{\phi }(a,z)-j_z(z)=J(z)/(2\pi a).$

(1.3)

Substituting in (1.1) the magnetic field H_φ(a, z) and the tangential component of the electric field, we obtain an equation for the current in an impedance radiator, generalizing Leontovich’s integral equation f...

Table of contents

• Cover
• Title Page
• Copyright Page
• Table of Contents
• Introduction
• PART 1: WIDE-RANGE ANTENNAS
• PART 2: MULTI-FREQUENCY ANTENNAS
• References
• Index