Page 246 - Maxwell House

226 ANTENNA BASICS

The equity (5.59) follows from the equations established in Section 5.2.10 of this chapter.

5.2.15 Antenna Power Handling

It is defined as the maximum input power the antenna can handle while working properly. Most
antennas are devices with a somewhat complex structure that include metal and dielectric
materials. Antenna elements overheated at high power may change their parameters or
completely collapse. Electrical arcing, multipaction and Corona are additional courses of
failure. The prediction of such electrical breakdowns is a tough engineering task and beyond
the scope of this course. The reader may find more information in specialized literature [35].

5.3 SYSTEM REQUIREMENTS AND ANTENNA GAIN

5.3.1 Introduction

For the sake of the subsequent discussion, we will divide all antennas into three categories
depending on their application:

1. Antennas in line-of-sight communication systems that includes ground and satellite
broadcasting, GPS, wireless and other similar systems.
2. Monostatic radar antennas where the same antenna transmits EM waves in the direction to
a target or targets and then receives and processes the scattered EM waves.
3. Bistatic radar antennas [2] where transmitting and receiving antennas are separated in
space.

As usual, in engineering practice, there are no single solutions entirely satisfying all
requirements. Despite this, some standard guidelines for antenna gain selection can be
formulated. A detail discussion of this topic is far beyond the scope of this course. We will,
therefore, touch on it in quite general terms. Let us start with communication antennas and
analyze the line of sight communication links as shown schematically in Figure 5.1.7.

5.3.2 Path Loss (Friis Transmission Formula) and EIRP

Eventually, the simplest line of sight communication includes two antennas; one transmits a
signal, and another receives it. To simplify the following consideration, assume that both
antennas are located in free space with no obstructions nearby. Accept that the RF generator
delivers to the transmit antenna (see the equivalent antenna circuit in 5.2.2) the power =

( < ). Supposing that this antenna is directional, we can calculate the power

density at the distance in far field zone where the receiver antenna is located

t
= � � (5.60)

4 2
Here is the transmit antenna gain and the factor within the parentheses is the power density

created at the receiving point by the isotropic radiator. Evidently, the product called Effective
(Equivalent) Isotropic Radiated Power or EIRP = is the amount of power that must

brought to an isotropic antenna to produce the equal signal power density at the receiving point.
EIRP is one of the critical parameters of the communication link and ordinarily counted in