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Hanyang University
ANTENNA THEORY
by Constantine A. Balanis
Chapter 1
Harim KIM
2017.01.20
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Contents
1. Antennas
1.1 Introduction
1.2 Types of Antennas
1.3 Radiation Mechanism
1.4 Current Distribution on a Thin Wire Antenna
1.5 Historical Advancement
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- Definition : “A usually metallic device for radiating or receiving radio waves.”
“A means for radiating or receiving radio waves”
Figure 1.1 Antenna as a transition device.
Figure 1.2 Transmission-line Thevenin equivalent of antenna
in transmitting mode.
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Conduction,
dielectric losses Radiation resistance
Figure 1.2 Transmission-line Thevenin equivalent of antenna
in transmitting mode.
Imaginary part of impedance
from radiation by the antenna
- Maximum power is delivered to the antenna under conjugate matching.
- Losses from line, antenna and the standing waves are undesirable.
when
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Figure 1.2 Transmission-line Thevenin equivalent of antenna
in transmitting mode.
- Maximum field intensities of the standing wave are sufficiently large
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How are the electromagnetic fields generated by the source.
Conducting wires are
- Current density
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- 1. Charge is not moving
No current, no radiation
- 2. Charge is moving uniform velocity
a. Wire is straight, and infinite, no
radiation
b. Wire is curved, bent, discontinuous,
terminated or truncated it makes
radiation.
- 3. Charge is oscillating
It radiates even if wire is straight.
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- Apply voltage, it creates an electric
field between the conductors.
- Electric lines of force ∝ Electric Field
Intensity
- Electric charges are required to excite
the fields but are not needed to sustain
them and may exist in their absence.
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- Dipole’s source
-
First, charge reached its maximum value
-
After next quarter, charge begins to diminish
-
Next quarter, charge have neutralized
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- Traveling wave current flow to
transmission line. At the end complete
reflection occurs.
- Reflected wave and incident wave
combined and forms standing wave
pattern.
- (a) Net result is non-radiation
- (b) Two line’s are not necessarily close
to each other, the fields radiated.
- (c) It’s called standing wave antenna
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- (a) 𝑘𝑙/2 is very small, can approximate
𝑘𝑙
sin 2 ≅ 𝑘𝑙/2
- (b) 𝑙 = 𝜆/2 at feed point, max 𝐼
𝜆
2
- (c) < 𝑙 < 𝜆
- (d) λ < 𝑙 <
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3𝜆
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Hanyang University
Thank you for your
attention
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