Reference: Wayne Tomasi

Sender/Source Transmitter Receiver Destination

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TRANSMISSION MEDIA

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ELECTROMAGNETIC Electric + Magnetic field

Magnetic Field an invisible force field created by a magnet and is generated around a conductor when current flows through it

Electric Field an invisible force field produce by the presence of a potential difference between two conductors

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Whenever voltage is applied to the antenna, an electric field is set up.

At the same time, this voltage causes current to flow in the antenna, producing a magnetic field.

The electric and magnetic field are right angle to each other

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The fields vary together both in time and in space. Characteristic Impedance definite ratio

between the electric field intensity and the magnetic field intensity (ohms)

The relationship between the electric and magnetic intensities is analogous to the relation between voltage and current in circuits.

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For a lossless medium, this is equivalent to

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Definition:Permeability measure of the ability of a material to support the formation of magnetic field within itselfPermittivity measure of the resistance that is encountered when forming an electric field

For most media in which electromagnetic waves can propagate, the permeability is the same as that of free space. The permittivity is likely to be given as a dielectric constant which is simply the permittivity of the medium relative to that of free space

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After some equation manipulation, another formula for characteristic impedance is:

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Oscillate amplitudes of the electric and magnetic fields vary at a specific rate› Frequency

number of cycles of a repetitive wave that occurs in a given time period

Cycles per second, hertz› Wavelength

distance occupied by one cycle of wave normally expressed in meters

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Consists of traveling electric and magnetic fields but with the energy evenly divided between the two types of fields

Polarization orientation of the ELECTRIC FIELD VECTOR with respect to the surface of the Earth

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Linear polarization polarization remains constant

Horizontal polarization the electric field is propagating parallel to the Earth’s surfaceVertical polarization the electric field is propagating perpendicular to the Earth’s surface

Circular polarization the polarization vector rotates 360 degrees as the wave moves one wavelength through space and the field strength is equal at all angles of polarization

Elliptical polarization the field strength varies with changes in polarization

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RADIO WAVES ARE EM WAVES Signals carried by the cable are EM WAVES

(and may occupy the same spectrum) but ARE NOT RADIO WAVES

The term RADIO WAVE/RADIO PROPAGATION is unique to FREE-SPACE PROPAGATION

Although free space implies a vacuum, propagation through the Earth’s atmosphere is often referred to as free space propagation as well and is treated as such. The primary difference is that the Earth’s atmosphere introduces losses and impairments to the signal not encountered in a vacuum

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ISOTROPIC RADIATOR a point in space where EM waves would radiate equally in all direction

The wavefront (surface on which all waves have the same phase) of an isotropic radiator would be a spherical

An isotropic radiator is not a practical possibility but at distances from a real source that are large compared to the dimension of the source, this is a good approximation.

OMNIDIRECTION ANTENNA is a close approximation of an ideal isotropic radiator

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R1

R2A

B

CD

All points of the same distance from the source lie on the same wavefront and have EQUAL power densities

At any instance of time, the total power radiated (Prad) is uniformly distributed over the total surface of the sphere (assuming lossless transmission medium)

Based on the previous statements, the power density at any point on the sphere can be represented mathematically as:

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If we equate the two power density formula, we get a new formula for getting the electric field strength

The farther the wavefront moves from the source, the smaller the power density

Power density is inversely proportional to the square of the distance from the source

The total power distributed over the surface of the sphere remains the same

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FREE SPACE = VACUUM› NO loss of energy› Waves, however, SPREAD OUT resulting in

REDUCTION in POWER DENSITY - ATTENUATION

Attenuation occurs in free space as well as the Earth’s atmosphere

Wave attenuation or space attenuation

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Earth’s atmosphere is not a vacuum As an EM wave propagates through Earth’s atmosphere,

energy is transferred from the wave to the atoms and molecules of the atmosphere

It interchanges energy with free electrons and ions IF the ions do not collide with gas molecules or other ions,

all the energy is converted back into EM energy, and the wave continues propagating with no loss of intensity

IF ions collide with other particles, they dissipate the energy resulting in absorption

ABSORPTION is directly proportional to PARTICLE DENSITY

22GHz 1st peak of water vapor absorption 60GHz 1st peak of oxygen absorption >6GHz effect of rain is insignificant

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Does not depend on the distance from the radiating source but on the total distance that the wave propagates through the atmosphere

HOMOGENOUS MEDIUM› one with uniform properties› Absorption on the first mile is the same for the last mile

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INHOMOGENOUS MEDIUM› Absorption coefficient varies with location

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Real values of refractive index will be less than unity

Deviation of the refractive index from unity is greater the lower the frequency and the higher the electron density

When 81N > f2 the refractive index is imaginary and the atmosphere is unable to propagate the EM wave without attenuation

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When a ray passes from a less dense to a more dense medium it effectively bent toward the normal.

When a ray passes from a more dense to a less dense medium it effectively bent away from the normal

NORMAL imaginary line drawn perpendicular to the interface at the point of incidence

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ANGLE OF INCIDENCE angle formed between the incident wave and the normal

ANGLE OF REFRACTION angle formed between the refracted wave and the normal

SNELL’S LAW dictates how an EM wave reacts when it meets the interface of two transmissive materials that have different index of refraction

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The portion of the total incident power that is not reflected is called the power transmission coefficient (T)

For a perfect conductor, T = 0 LAW OF CONSERVATION OF ENERGY for a

perfect reflective surface, the total reflected power must equal the total incident power

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Reflection also occurs when the reflective surface is irregular or rough but such surface may destroy the shape of the wavefront

DIFFUSE RELECTION When an incident wavefront strikes an irregular surface and scatters in many direction

SPECULAR REFLECTION reflection from a perfectly smooth surface

Surfaces that fall between smooth and irregular are called semirough surfaces and causes a combination of diffuse and specular reflection

A semirough surface will not totally destroy the shape of the reflected wavefront but it will reduce total power

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Modulation or redistribution of energy within a wavefront when it passes near the edge of an opaque object

Phenomenon that allows radio waves to propagate (peek) around corners

HUYGEN’S PRINCIPLE› Every point on a given spherical wavefront

can be considered as a secondary point source of EM waves from which other secondary waves are radiated

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Occurs when 2 or more EM waves combine in such a way that system performance is degraded

Subject to LINEAR SUPERPOSITION whenever 2 waves occupy the same point in space

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EM waves traveling within Earth’s atmosphere are called TERRESTRIAL WAVES

TERRESTRIAL RADIO COMMUNICATION communication between 2 or more points on Earth

EM waves can be propagated in a number of ways depending on TYPE OF SYSTEM AND ENVIRONMENT most of which are FREQUENCY DEPENDENT

EM waves travel in STRAIGHT LINES/PATH except when Earth and its atmosphere alter their path

3 WAYS OF EM WAVE PROPAGATION› Surface Wave or Ground Wave› Space Wave or Line-of-Site› Sky Wave or Ionospheric

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3 WAYS OF EM WAVE PROPAGATION› Surface Wave or Ground Wave› Space Wave› Sky Wave or Ionospheric

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>2MHz Earth guided EM wave that travels over

the surface of the Earth As it moves over the Earth’s surface, it is

accompanied by charges induced in the Earth producing current

Earth offers a resistance to the flow of current thus energy is dissipated in a manner very similar to those in transmission line

Earth’s surface is also has DIELECTRIC LOSSES

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Energy is absorbed from the surface wave thus the portion of the wave in contact with the Earth’s surface is continuously wiped out

The energy is replenished by diffraction of energy downward from the portions of the ground wave immediately above the Earth’s surface

Earth has gradient density resulting it to tilt progressively Wave propagates close to the surface

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Attenuation of the surface wave due to absorption depends on the conductivity of Earth’s surface and frequency of the EM wave (refer to table)

Attenuation on terrain increases rapidly with respect to frequency NOT RECOMMENDED FOR HF TRANSMISSION

Vertically polarized because if horizontally it would be shortcircuited with the conductivity of the ground

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Includes radiated energy that travels in the lower few miles of the Earth’s atmosphere

DIRECT WAVE + GROUND REFLECTED

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DIRECT WAVE › Travel in a straight line between the transmit and

receive antennas› Limited by the curvature of the earth› Line-of-sight transmission

GROUND REFLECTED WAVE› Waves reflected by the Earth’s surface as they

propagate between the transmit and receive antenna

RADIO HORIZON Curvature of the Earth Radio Horizon can be lengthened simply by

elevating the transmit or receive antennas

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Effect of antenna height on the radio horizon

For TX and RX antenna

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EM waves directed above the horizon Radiated in a direction that produces a

relatively large angle with reference to the earth

Either reflected or refracted IONOSPHERE upper portion of the

earth’s atmosphere which absorbs large quantities of the sun’s radiant energy which ionizes the air molecules creating free electrons

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Electric field of the wave exerts a force on the free electrons causing them to vibrate

Vibrating electrons decrease current, which is equivalent to reducing the dielectric constant

Reducing the dielectric constant increases propagation velocity and causes EM waves to bend away from the region of high electron density to lower electron density

The farther from the earth the higher the ionization upper atmosphere has a higher percentage of ionized molecules than the lower atmosphere

Stratified nonuniform density depending on temperature

Most dense during maximum sunlight and on summer

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D-layer› Lowest layer› Little ionization› Little effect on direction of propagation› Depends on the altitude of the sun so it

disappears at night› Reflects VLF and LF› Absorbs MF and HF

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E-layer› Kennelly-Heaviside› Maximum density at noon when the sun is

at its highest point› Almost disappears at night› Aids MF and reflects HF during daytime› SPORADIC E solar flares and sunspot

with high ionization density improves long-distance radio transmission

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F-layer› F1 and F2› Separate during day but combines at night› F1 absorbs and attenuates some HF waves

although most of the wave passes through F2 where they are refracted back to earth