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:

Pr =PtGtGrλ

2

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