0195136047.pdf

678 COMMUNICATION SYSTEMS

G=

4 πAe

λ^2

(15.1.17)

whereλis the wavelength being transmitted (or received), andAeis theeffective apertureor

effective areaof the antenna. Power amplifiers are needed to overcome the radio-transmission

loss just as in a transmission-line system. Similar considerations hold foropticalradiation, in

which the electromagnetic wave takes the form of a coherent light beam.

Radio transmission is inherently abandpassprocess with a limited bandwidthBnominally

centered at the carrier frequencyfc. Thefractional bandwidth B/fcis a key design factor with

a general range of 1/ 100 ≤B/fc ≤ 1 /10. It is obvious then that large signal bandwidths

require high carrier frequencies to satisfyfc≥ 10 B. You can reason why television signals

are transmitted atfcof 100 MHz, whereas AM radio signals are transmitted atfcof 1 MHz.

Since optical communication systems offer tremendous bandwidth potential on the order of 10^12

Hz, and a corresponding high information rate, they have become topics of current research

interest.

Antennas do not radiate power equally in all directions in space. Theradiation intensity

patterndescribes the power intensity (which is power per unit solid angle, expressed in units of

watts per steradian) in any spatial direction. Conceptually it is convenient to define anisotropic

antennaas a lossless antenna that radiates its power uniformly in all directions. Although an

isotropic antenna cannot be realized in practice, it serves as a reference for comparison with real

antennas. The radiation intensity for such an antenna, with input powerP, is a constant in any

direction, given byP/ 4 π. Thepower gain Gof a realistic antenna is a measure of the maximum

radiation intensity of the antenna as compared with the intensity that would result from an isotropic

antenna, with the same power input.Gis then expressed as

G=

maximum radiation intensity

radiation intensity of isotropic source (with the same power input)

=

4 π(maximum radiation intensity)

P

(15.1.18)

Referring to Figure 15.1.5, when a powerPt/Ltis applied to the transmitting antenna, let us find

the signal powerSravailable to the receiver from the receiving antenna. An isotropic transmitting

antenna would cause a radiation power density (power per unit area of a sphere) of

Power density=

Pt

4 πR^2 Lt

(15.1.19)

For a practical antenna that has power gainGtand lossLtarelative to an isotropic antenna, Equation

(15.1.19) would be modified as

Power density=

PtGt

4 πR^2 LtLta

(15.1.20)

We shall assume that the transmitting and receiving antennas (reciprocal elements) point directly

toward each other, so that their gains are maximum. LettingLchdenote any losses incurred by the

wave in the channel (medium), andArebe the effective area of the receiving antenna, the power

that the receiving antenna is able to produce is given by

S=

PtGtAre

4 πR^2 LtLtaLch

(15.1.21)

Accounting for the receiving antenna loss and receiving-path losses, a total system lossLcan be

defined as