Page 231 - Maxwell House

Chapter 5 211

amount consisting of the radiated power plus the heat loss due to the dissipation energy
Σ
in the antenna. Generally, this factor is frequency-dependent and can be appraised as
ℑ( ()) Σ () 0
() = = ≅ (5.17)
()+ Σ () ()+ Σ () 2∆
Here 2∆ is the antenna BandPass (BP) and f 0 is the central frequency of BP. Certainly,
antennas with a high Q are narrowband and typically radiate badly. For example, for short
electric dipole ~(⁄∆) is in order of 10 when ∆⁄ = 0.01. BP of this dipole does not
6
3
exceed 10 . For this reason ground-to-submarine very slow communication that uses such
−6
0
ultra short SLF (from 30 Hz to 300 Hz), ELF (from 3 Hz to 3 kHz), and VLS (from 3 kHz to
30 kHz) antennas are so narrow banded.
It is possible to lessen with some restrictions the reactive part of antenna impedance () and
Σ
transform its active part () in such a way that the quality factor becomes relatively low
Σ
within a certain bandwidth. Circuit theory [34] demonstrates that the maximum obtainable
bandpass is primarily defined by the initial (i.e. before matching) quality of the antenna. The
idea of such circuit techniques is quite trivial: to reduce the antenna quality factor at some
frequencies at the expenses of increasing it outside this band. The latter is correct when the
matching network between an antenna and generator is purely reactive, i.e. it consists of such
reactive elements as ideal transformers, inductors, and capacitors. The inclusion of resistors in
matching network allows loosening the BP restriction at the expenses of the Ohmic loss growth.
The antenna radiation efficiency is defined as the ratio of the radiated power to the active
Σ
total power (see Figure 5.2.2) delivered to the antenna from the RF generator
Σ () Σ ()
() = ≅ (5.18)
() ()
Radiation efficiency can be measured in dB like the return loss
= −20 log (1 − ) [dB] (5.19)
10
Typically, it is required for transmitting antennas to have radiation efficiency above 90% or less
than 0.5 dB.

5.2.5 Near-field Zone vs. Far field Zone

Traditionally, the EM fields in the space around an antenna can be classified as belonging to
three zones: the near-field zone, the transition or Fresnel's zone, and the far field or Fraunhofer’s
zone. The boundary between them is based on some agreement and depends on what kind of
EM energy, reactive or active, prevails. Let us illustrate such a partition using the EM field
emitted by an elementary electric dipole as an example:

1. First, we need the expression for total fields. Putting (4.60) from Chapter 4 into (4.59)
( = 0 for simplicity) we obtain
2