Download Chap2 P1 Concept of Radiation

Chapter
Concept of
Radiation
Chapter Outlines
Chapter Concept of Radiation
 Radiation Mechanism
 Basic Radiation Source – Single Wire
 Basic Radiation Source – Two Wires
 Current Distribution on a Thin Wire
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3.1 Radiation Mechanism



Vibration of EM waves from radiation source.
Vibration produced from electric time varying current
source, which is in form of scattering electrical charges.
Mismatch between the characteristic impedance of
transmission line and open circuit at the other end
produces or generates reflected waves (as static wave)
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Radiation Mechanism (Cont’d..)


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Theoretically, a transmission line that ends with open
circuit will get fully reflected waves.
But practically not most of them get reflected, some of
them transmits or radiates into open free space.
Why? The field line in the transmission line suppose to
phase shifted when it reached open circuit but some
still radiates.
But, how is radiation accomplished? How are EM
waves generated by the source, contained and guided
within the transmission line and antenna and finally
detached from the antenna to form a free space
wave?
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3.2 Basic Radiation Source – Single
Wire
Conducting wires are material
whose prominent characteristic is
the motion of electric charges and
the creation of current flow.
Assume electric volume charge
density, qv is distributed uniformly
in a circular wire with cross
sectional area A and volume V.
A total charge Q within volume is
moving in the z direction with
uniform velocity vz.
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Single Wire (Cont’d..)
The current density, Jz over the cross section of the wire:
J z  qv v z
If the wire is ideal conductor, the current density Js resides
on the surface as:
J S  qs v z
Where qs is the surface charge density. If the wire is very
thin (ideally zero radius), the current in the wire:
I z  ql v z
Where ql is the charge per unit length.
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Single Wire (Cont’d..)
If the current is time varying (in a very thin wire), the
derivative of the current is:
dI z
dvz
 ql
 ql a z
dt
dt
If the wire is of length l, then it can be written as:
dI z
dvz
l
 lql
 lql a z
dt
dt
This is the basic relation between current and charge, and
it also serves as the fundamental relation of EM radiation.
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Single Wire (Cont’d..)
It states that to create radiation, there must be a time
varying current or an acceleration or deceleration of
charge.
To create charge acceleration or deceleration, the wire
must be curved, bent, discontinuous or terminated.

To create periodic charge acceleration or deceleration
or time varying current, charges must be oscillating in a
time harmonic motion as for a λ/2 dipole.

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Single Wire (Cont’d..)
Important notes from Balanis:
1.
If a charge is not moving, current is not created = no
radiation
2.
If a charge is moving with a uniform velocity:
3.
a)
There is no radiation if the wire is straight or
infinite in extent
b)
There is radiation if the wire is curved, bent,
discontinuous, terminated or truncated (Fig. 10)
If the charge is oscillating in a time motion, it
radiates even if the wire is straight.
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Single Wire (Cont’d..)
Wire configurations for radiation:
10
Single Wire (Cont’d..)
Consider a pulse source attached to an open ended
conducting wire, connected to ground through a discrete
load at its open end:
• When the wire energizes, free electron/charges are
in motion due to electrical lines of force created by
the source.
• The charges accelerate in the source end of the
wire, and decelerated during reflection from its end
Radiated fields are produced at each end and along the
remaining part of the wire.
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Single Wire (Cont’d..)
Behavior of pulses in a wire:
• Stronger radiation with a more broad freq spectrum
occurs if the pulses are shorter/more compact duration
• Continuous time-harmonic oscillating charge produces
ideally radiation of single frequency determined by f
oscillation
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Single Wire (Cont’d..)
We can conclude that:
• Pulses radiate a broad bandwidth (spectrum of
radiation). The shorter the pulse, the broader the
spectrum
• A sinusoidal (smooth) waveform of current or
charge leads to a narrow spectrum of radiation:
ideally zero bandwidth at the frequency of the
sinusoid, if it continues indefinitely.
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3.3 Basic Radiation Source - Two
Wires
Consider a voltage source connected to a two conductor
transmission line which is connected to an antenna.

It creates an E field between the conductors.
The E field has associated electric lines of force that are
tangential to the E field at each point, and its strength is due
to its intensity.

Tends to act on free electrons (easily detachable from atoms)
and force them to be displaced.

The movement creates currents and in turn creates H field
intensity.

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Two Wires (Cont’d..)
The creation of time varying electric and magnetic
fields between the conductors forms EM waves which
travel along the transmission line:
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Two Wires (Cont’d..)
The EM waves enter the antenna and associated with them
electric charges and corresponding currents. If the antenna part
is removed, free space waves can be formed by connecting the
open ends of the E lines.
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Two Wires (Cont’d..)
The free space waves are also periodic.
But a constant phase point P0 moves outwardly with the speed
of light and travels a distance of λ/2 (to P1) in the time of one
half of period.
Close to the antenna, the constant phase point P0 moves faster
than the speed of light but approaches the speed of light at
points far away from the antenna.
But how are the guided waves detached??
Remember the water waves created by the dropping of
pebble in a calm body of water
Once the disturbance initiated, water waves are created
which begin to travel outwardly.
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Two Wires (Cont’d..)
When the EM waves are within the transmission line and
antenna, their existence is associated with the presence of the
charges inside the conductors.
When the waves are radiated, they form closed loops and there
are no charges to sustain their existence.
Conclusions:
• Electric charges are required to excite the fields
• But they are not needed to sustain fields and may exist in their
absence.
• This is direct analogy with water waves.
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Two Wires – Small DIPOLE Antenna
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3.4 Current Distribution on a Thin
Wire
For a lossless two wire TLines, movement of charges creates a
traveling wave current, I0/2 along each wires.
At the end, it undergoes a complete reflection (equal
magnitude and 1800 phase reversal).
When it combines with incident traveling wave, forms a pure
standing wave pattern.
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Current Distribution on a Thin Wire
(Cont’d..)
Radiation for each wire occurs  time varying nature of current
and the termination of the wire.
Two wire balanced (symmetrical) Tline: the current in a half
cycle of one wire is the same magnitude but 1800 out of phase
for corresponding half cycle other wire.
Two wires Tline with very small spacing (s<<λ): radiated fields
by the current of each wire cancels each other. The net result is
an almost ideal non-radiating transmission line.
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Current Distribution on a Thin Wire
(Cont’d..)
As the section begins to flare, it can be assumed that the current
distribution is essentially unaltered in form in each of the wires.
But the two wires of the flared section are not close to each
other, the fields radiated by one do not cancel those of the other.
Ideally, there is a net radiation by the TLine system.
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Current Distribution on a Thin Wire
(Cont’d..)
This is the geometry of widely used dipole antenna.
If l<λ, the phase of current standing wave pattern in each arm
is the same throughout its length.
Spatially it is oriented in the same direction as that of the other
arm. The fields are radiated by the two arms of the dipole
(vertical parts of a flared TLine).
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Current Distribution on a Thin Wire
(Cont’d..)
The fields radiated will primarily reinforce each other toward most
directions of observation
If the diameter of each wire is very small (d<<λ) , the ideal standing
wave pattern along the arms of dipole is sinusoidal with a null at the
end.
For center-fed dipoles, the current patterns are:
L<<λ
L = λ/2
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Current Distribution on a Thin Wire
(Cont’d..)
λ /2 < L < λ
λ < L < 3λ/2
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Concept of Radiation
End