Chapter 14: Antennas EET-223: RF Communication Circuits Walter Lara - - PowerPoint PPT Presentation

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
• pen-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

• r 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: Pr = PtGtGrλ2 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