Page 227 - Maxwell House

Chapter 5 207

and Direct Broadcast Satellites (DBS) mainly use dual polarization to increase their
channel capacity (see statement 2 above). Unfortunately, CP is more affected by
atmospheric conditions than LP for high rainfall rates (greater than 12.5 mm/h) and low
angles of arrival.

The reader may find much more detailed information about polarization in specialized literature
[31, 32].

5.2 ANTENNA PARAMETERS
5.2.1 Introduction

Good understanding of the essence of antenna parameters is a vital aspect of the engineering
experience that makes you be prepared better for quite challenging modern antenna
development. Sometimes such understanding is more important than an in-depth knowledge of
antenna theory itself because current engineering practice has shifted to “off-the-shelf”
approach. Consequently, an engineer spends more time increasingly looking through antenna
catalogs and manuals than on antenna development itself.
5.2.2 Radiation Resistance and Lumped Equivalent Circuit of Antenna

Looking back at expression (4.72) from Chapter 4 we can detect surprising parallels with (3.11).
Both describe the power defined by energy conversion from one form into another. It is clear
that the RF generator connected to a dipole (see Figure 4.2.1b) delivers and supports the
current . Then is the power radiated by the dipole, i.e. roughly speaking, the power lost
Σ
by dipole forever in an unbounded medium. If so, the expression in (4.72) can be rewritten as
2
1 ∆
= ( 2 2 2 (5.8)
)
) = 40 � � (
Σ Σ
2
Here is customarily called radiation resistance, caused by the radiation EM energy from a
Σ
dipole antenna, and equal to
2 2
∆
= 80 � � ≅ 800 � � [Ω] (5.9)
∆
2
Σ

The imaginary or quadrature part of the reactive power ℑ( ) concentrated near a dipole ( ≪

) can be derived in a similar manner and obtained from (5.20) below
2
1 2 5 ∆ 2
ℑ( ) = ( ) ≅ − � � � � ( )

Σ
2 � (5.10)
2
10 ∆
≅ − � � � �
Σ