Page 229 - Maxwell House

Chapter 5 209

5 describes the unavoidable Ohmic loss in antenna while is
in Figure 5.2.2 . The term Σ
defined by the energy stored in reactive fields near the antenna. The equivalent circuit in Figure
5.2.2 illustrates how naturally the radiation phenomena can be included into an ordinary
engineering circuit analysis and synthesis. For example, the circuit tells us that the power from

generator to load can be maximized when () = () + () ≅ () (assuming
Σ Σ
low Ohmic loss in antenna) and () = − () or
Σ

() = () (5.11)
∗

Note that the matching condition (5.11) guarantees that only up to 50% power can be delivered
from the generator to the load while the internal resistor (i.e. the generator) absorbs another

half. We can put up with such low or even lower efficiency in the case of receiving antennas
when the driving factor is not efficiency but the ratio between received signal and unavoidable
noise signals. The latter is due to some external (i.e. natural as lightning or manmade as jammer,
car engine, etc.) or receiver internal noise sources. Meanwhile, maximum efficiency is a
mandatory requirement when an antenna is connected to transmitter. Subsequently, the special
matching transmitter-antenna network minimizing power loss can be synthesized for the given
or measured generator () and antenna () impedances using the standard circuit theory

tools. It is worth noting that sometimes such task can be a fairly challenging due to irregular
dependency of antenna impedance over frequency (many antennas are resonant devices). The
further discussion of this topic is beyond of the scope of this course. The reader interested in
more details should consult the classical circuit theory books [34].

5.2.3 Return Loss
The knowledge of antenna impedance brings into play the reflection coefficient defined

in Chapter 3 by (3.87) for single mode propagation. Recall that the term ‘single mode’ means
waves delivered and departed (i.e. reflected) must have the same EM field structure. The
antenna impedance, given or measured, can be obtained from (3.88)

2
1−| |
= 1+| | 2

−2| |cos
� (5.12)
2| |sin
=

1+| | 2 −2| |cos
and
−
= (5.13)

−
In general, the wave coming back from an antenna input to the generator is absorbed there.
Therefore, this portion of energy is customarily called return loss and must be included in a
reflection or mismatch loss budget and commonly measured in decibels (dB)
= −20 log | | [dB] (5.14)

10
Since power is proportional to the square of the magnitude, the reflection loss is defined as

5 Public Domain Image, source: http://www.intechopen.com/books/radar-technology/wideband-
antennas-for-modern-radar-systems