0195136047.pdf

15.1 WAVES, TRANSMISSION LINES, WAVEGUIDES, AND ANTENNA FUNDAMENTALS 677

system that involves antennas (with no conductors in the propagation medium) and the following

nomenclature:

Pt Power generated by the transmitter

Lt,Lr Transmitting-path loss (representing the power reduction caused by the transmission line

or waveguide that connects to the transmitting antenna) and receiving-path loss

Lta,Lra Transmitting antenna loss and receiving antenna loss

R Distance of separation between the antennas

S Signal power available at the lossless antenna output

Sr Signal power available at the receiver input

Radio transmission consists of antennas at the source and at the destination. It requires the

signal to be modulated on a high-frequency carrier, which usually is a sinusoid. Driven by an

appropriate carrier, the transmitting antenna launches an electromagnetic wave that propagates

through space without the help of a transmission line. A portion of the radiated power is collected

at the receiving antenna.

Thewavelengthλof the radio wave in air is related to the carrier frequencyfcby

fcλ=c= 3 × 108 m/s (15.1.16)

The radio-transmission loss differs from that of a transmission line in two ways: (i) it increases as

the square of the separating distance instead of exponentially, and (ii) it can be partly compensated

by the antenna gains.

Antenna gain depends on both shape and size.Dipole antennas,commonly used at lower

radio frequencies, are made up of a rod or wire of lengthλ/10 toλ/2, and have an antenna-gain

range of 1.5 to 1.64 (1.8 to 2.1 dB).Horn antennasandparabolic dishes(so named after their

shapes) have much more gain at higher frequencies. A useful, although approximate, expression

for gain is given by

Transmitter

Information-bearing

signal

Transmitting

path losses

Lt

Transmitting

antenna loss

Receiving

path losses

Receiving

antenna loss

Lossless

antenna Medium

Lta

Pt

Receiver

Information-bearing

signal Sr Lr Lra S

R

Figure 15.1.5Communication system elements involving antennas.