Chapter 14: Antennas

EET-223: RF Communication Circuits

Walter Lara
 Basic Antenna Theory
• In a transmitter system, an antenna provides a
transition from a guided wave on a transmission
line to an electromagnetic wave
– RF currents flowing through the antenna produce
electromagnetic waves that radiate into the atmosphere
• In a receiver system, an antenna provides a mean
for the collection of electromagnetic waves
– Electromagnetic waves “cutting” through the antenna
induce RF currents for use by the receiver
 Basic Antenna Theory – Cont’d
• The reciprocity principle states that any antenna
can transmit or receive with the same efficiency
– Occurs because antenna characteristics are the same
regardless of whether the antenna is transmitting or
receiving
– Because of that, typically we only study transmission
• Polarization is the direction of the electric field of a
given electromagnetic radiated signal
– Vertical antenna transmits vertically polarized wave and
received signal is theoretically zero if vertical electric field
cuts through horizontal receiving antenna
– Same idea applies for horizontal or any other angle
 Half-Wave Dipole Antenna: Introduction
• Two open-ended wires physically oriented 180°
apart hooked to opposite voltage terminals
• Physical length equals half-wavelength (λ/2) of
applied frequency
• Predominantly used with frequencies above 2 MHz
– Lower frequencies require impractical antenna sizes
– What antenna size would be require for 60-Hz? 2-MHz?
Half-Wave Dipole Antenna: How does it work?
• As discussed in Chapter 12, the characteristics of an
open-ended, quarter-wave transmission line
segment (Fig 14-1) are such that:
– Voltage is close to zero at input and maximum at end
– Current is maximum at input and close to zero at end
• But two-wire line cannot maximize radiation
because magnetic field surrounding each conductor
is in a direction that opposes the lines of forces
about the other conductor
– Solution: bending each line outward 90° to form λ/2
antenna (see Fig 14-2)
Figure 14-1 Quarter-wave transmission line segment (open-ended).
Figure 14-2 Basic half-wave dipole antenna.
Half-Wave Dipole Antenna: How does it work?
– Cont’d
• Another problem of the open-ended line is that it
cannot absorb and radiate power because source
sees close to zero impedance
– Solution: gap between input terminals of λ/2 antenna
forces input impedance to be close to 73 Ω, instead of
close to 0 Ω (see Fig 14-3)
Figure 14-3 Impedance along a half-wave antenna.
 Half-Wave Dipole Antenna: Radiation and
Induction Field
• Radiation Field (Far Field): radiation that surrounds
an antenna but does not collapse its field back into
the antenna but rather radiate it out into the
atmosphere
• Induction Field (Near Field): radiation that
surrounds an antenna and collapses its field back
into the antenna
• The effects of the Near Field become negligible at a
distance more than about one-half wavelength
from the antenna
 Half-Wave Dipole Antenna: Radiation and
Induction Field – Cont’d
• The Far-Field region begins when the distance:
Rff = 1.6λ if D/ λ < 0.32
Rff = 5D if 0.32 < D/ λ < 2.5
Rff = 2D2/ λ if D/ λ ≥ 2.5

Where: Rff = far field distance from antenna (m)

D = dimension of the antenna (m)
λ = wavelength of transmitted signal (m/cycle)
 Half-Wave Dipole Antenna: Radiation Pattern
• A radiation pattern is a diagram indicating the
radiated field strength around an antenna as a
function of direction
• Because of the reciprocity principle, the radiation
pattern is the same for receiving or transmitting
• The beamwidth is the angular separation between
half-power points of an antenna’s radiation pattern
• The radiation pattern of a half-dipole antenna is
doughnut-shaped (see Fig 14-5)
Half-Wave Dipole Antenna: Radiation Pattern –
Cont’d
• The maximum field strength for the half-dipole
antenna occurs at right angles to the antenna and it
is close to zero “off the ends” (see Fig 14-4a)
• The radiation pattern of an isotropic source is a
sphere or, if shown in two dimensions, it is circular
or omnidirectional (see Fig 14-4b)
Figure 14-5 Three-dimensional radiation pattern for a /2 dipole.
Figure 14-4 Radiation patterns.
 Half-Wave Dipole Antenna: Antenna Gain
• The antenna gain is a measure of how much more
power an antenna radiates in a certain direction
with respect to that which would be radiated by an
isotropic source (measured in dBi)
– Half-wave dipole antenna at right angles: 1.15 dB
• When the gain of an antenna is multiplied by its
power input, the result is termed as effective
radiated power (ERP)
– Can be in Watts, dBW or dBm
– Characterizes both antenna pattern and transmitted
power in one quantity
 Half-Wave Dipole Antenna: Antenna Gain –
Cont’d
• The amount of power received by an antenna (Pr)
can be computed as:
PtGtGrλ2
Pr =
16π2d2
Where: Pt = power transmitted (W)
Gt = transmitting antenna to isotropic gain ratio
Gr = receiving antenna to isotropic gain ratio
λ = wavelength (m)
d = distance between antennas (m)
• For half-wave dipole antenna at optimum reception
alignment: Gt = Gr = 1.64