Transcript Slide 1

Chapter 3
Concept of
Radiation
Chapter Outlines
Chapter 3
Concept of Radiation
 Radiation Mechanism
 Basic Radiation Source – Single Wire
 Basic Radiation Source – Two Wires
 Current Distribution on a Thin Wire
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)
Radiation Mechanism (Cont’d..)


Theoretically, a transmission line that ends with open
circuit will get fully reflected waves, but practically not
most of them get reflects but 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 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?
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.
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.
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.
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, charge must be oscillating in a
time harmonic motion as for a λ/2 dipole.

Single Wire (Cont’d..)
Important notes from Balanis:
Single Wire (Cont’d..)
Wire configurations for radiation:
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 energized, 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
It is suggested that radiated fields are produced at each
end and along the remaining part of the wire.
Single Wire (Cont’d..)
Where pulses in a wire:
Single Wire (Cont’d..)
Where, we can conclude that:
3.3 Basic Radiation Source - Two
Wires
Consider a voltage source connected to a two conductor
transmission line which connected to an antenna.

It creates an E field between the conductors.
The E field has associated with it electric lines of force that
tangent to the E field at each point and its strength is due to
its intensity.

Have tendency 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.

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:
Two Wires (Cont’d..)
The EM waves enter the antenna and associated with them
electric charges and corresponding currents. If remove part of
the antenna, free space waves can be formed by connecting the
open ends of the E lines.
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 the guided waves detached??
Remember the water waves created by the dropping of
pebble in a calm body of water, where once the
disturbance initiated, water waves are created which
begin to travel outwardly.
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.
This leads us to conclude that electric charges are
required to excite the fields but are not needed to sustain
them and may exist in their absence. This is direct
analogy with water waves.
Two Wires – Small DIPOLE Antenna
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.
Current Distribution on a Thin Wire
(Cont’d..)
Radiation for each wire occurs  time varying nature of current
and the termination of the wire.
For 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.
If the spacing between two wires is very small (s<<λ) , the fields
radiated by the current of each wire are cancelled each other.
The net result is an almost ideal nonradiating transmission line.
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 due to 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.
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
throughput its length. Spatially it is oriented in the same
direction as that of the other arm. The field radiated by the two
arms of the dipole (vertical parts of a flared TLine).
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
Current Distribution on a Thin Wire
(Cont’d..)
λ /2 < L < λ
λ < L < 3λ/2
Concept of Radiation
End