Concept of Radiation

Chapter

Chapter OutlinesChapter Concept of Radiation

Radiation MechanismBasic Radiation Source – Single WireBasic Radiation Source – Two WiresCurrent Distribution on a Thin Wire

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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)

3.1 Radiation Mechanism

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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?

Radiation Mechanism (Cont’d..)

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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. 5

Single Wire (Cont’d..)

The current density, Jz over the cross section of the wire:

zvz vqJ

If the wire is ideal conductor, the current density Js resides on the surface as:

zsS vqJ

Where qs is the surface charge density. If the wire is very thin (ideally zero radius), the current in the wire:

zlz vqI Where ql is the charge per unit length.

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If the current is time varying (in a very thin wire), the derivative of the current is:

zlz

lz aq

dtdv

qdtdI

If the wire is of length l, then it can be written as:

zlz

lz alq

dtdv

lqdtdIl

This is the basic relation between current and charge, and it also serves as the fundamental relation of EM radiation.

Single Wire (Cont’d..)

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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.

Single Wire (Cont’d..)

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Important notes from Balanis:1.If a charge is not moving, current is not created = no radiation2.If a charge is moving with a uniform velocity:

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)

3.If the charge is oscillating in a time motion, it radiates even if the wire is straight.

Single Wire (Cont’d..)

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Wire configurations for radiation:

Single Wire (Cont’d..)

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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 endRadiated fields are produced at each end and along the remaining part of the wire.

Single Wire (Cont’d..)

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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

Single Wire (Cont’d..)

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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.

Single Wire (Cont’d..)

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3.3 Basic Radiation Source - Two WiresConsider 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 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. 17

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.

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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. 20

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 cancels each other. The net result is an almost ideal non-radiating transmission line.

Current Distribution on a Thin Wire (Cont’d..)

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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 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.

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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 throughput 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). 23

Current Distribution on a Thin Wire (Cont’d..)The fields radiated will primarily reinforce each other toward most directions of observationIf 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<<λL = λ/2

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λ /2 < L < λ

λ < L < 3λ/2

Current Distribution on a Thin Wire (Cont’d..)

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Concept of RadiationEnd