Electromagnetic Radiation & Free Space

Electromagnetic Radiation

• Is understood by focusing on the idea of electric power in a circuit
• When impedance is higher in a circuit, the voltage can be high and the voltage circuit is 90 degrees out of phase and becomes reactive
• When power escapes from free space, characteristics of free space govern it

Free Space

• Free Space: A space that does not interfere with the radiation and propagation of radio waves
• Doesn't exist on earth, but is still calculated to predict the effects
• Propagation conditions in free space: They are approximately those of free space, especially at high ultra-high frequencies (UHF)
• James Clerk Maxwell: A Scottish Physicist who theorized electromagnetic radiation in 1857
• Maxwell: Focused on mathematical explanations for behavior of electromagnetic waves and released it in 1873

Fundamentals of Electromagnetic Waves

• Electromagnetic waves: A form of energy that propagates or travels through space, carrying energy and momentum
• Similar to water ripples caused by throwing a pebble into water
• Electrostatic and electromagnetic radiation: The two energy reactions approximately formed
• The displacement of liquid: The energy created by displacement converted into vertical and horizontal components
• Energy level of a component: Inversely proportional to the distance

Waves in Free Space

• Electromagnetic wave spreads uniformly from a point source where there is no interference or obstacle.
• These rays radiate from the point source in all directions
• Rays are perpendicular to a tangential plane of the wavefront.
• P ray: Has a certain phase because it may leave the source at when voltage and current are maximum in the circuit
• Zero Electric and magnetic intensities: When a wave travels at a distance of 100,000.25 wavelengths
• Wave front: The plane joining all points of identical phase

Power Density

• Cursive P is power density at a distance r from an isotropic source
• pt is transmitted power
• Isotropic Source: Radiates uniformly in all directions in space; a theoretical concept in electromagnetics which describes a source radiating energy/power uniformly

Electric and Magnetic Field Intensities

• They are important in electromagnetic waves because they are the counterpart of voltage and current in circuits
• Field intensity: Inversely proportional to the distance from the source
• Proportional to the square root of the power density

Radiation and Reception

• Antennas: Radiate electromagnetic waves from electron flow in a suitable conductor, predicted by Maxwell's equation.
• Altering current causes magnetic field changes, and an electric field is present as well
• Polarisation: The physical orientation of the radiated wave in space.
• Waves have the same alignment; a characteristic of antennas that emit linearly polarized emitted radiation

Reception

• It is opposite of the process of transmitting.
• Transmitting and receiving antennas are basically interchangeable

Reciprocity Principle

• It states that the characteristics of antennas are identical regardless of reception or transmission use e.g. impedance and radiation pattern

Attenuation and Absorption

• States that the intensity of the effect is inversely proportional to the square of the distance from the source
• Attenuation: The loss of signal strength or energy as it propagates

Absorption

• The process when a wave transfers its energy to the medium
• Essentially causing the wave's amplitude to decrease as the medium takes in the energy
• Radio wave absorption: Does not occur in free space because there is nothing to absorb them
• The atmosphere tends to absorb radio waves

Effects of the environment

Reflection of Waves

• Angle of reflection: Is equal to the angle of incidence.
• Incident Ray, Reflected Ray, Normal: They lie in the same plane at the point of incidence, like light reflection
• Second Law of Reflection: The proof for the equality of the angles of reflection and incidence
• Incident and reflected waves: They travel with the same velocity

Reflection Coefficient

• It is the ratio of the intensity of the reflected electromagnetic wave to that of the incident wave
• R = 0: Where there is No reflection, and all the wave is transmitted
• 0 < R < 1: There is Partial reflection, which transmits part of the wave and reflects some
• R = 1: Indicates Total reflection meaning entire wave is reflected and no transmission occurs into the second medium.
• R = -1: Indicates Complete reflection with a phase reversal (i.e., the reflected wave is 180° out of phase with the incident wave).

Reflection of Waves, cont.

• Importance of electric vector: Electric vector be perpendicular to conducting surface
• Reflection optical laws: For curved surfaces, they continue to be followed during reflection
• Reflection from rough & smooth surfaces: Similar if provided angle of incidence is in excess of Rayleigh criterion

Rayleigh Criterion

• Utilized for when a surface is considered smooth or rough, based on :
• The wavelength of the wave
• The angle of incidence

Refraction

• It happens when electromagnetic waves travel from one propagation medium to another of different densities.
• There is a relationship between:
• Incidence angle θ and refraction angle θ’
• If one medium is gradually varied, refraction happens in a complex way
• When the boundary of two media are curved, then refraction still takes place

Interference of Electromagnetic Waves

• Interference: Occurs when two waves originate from one source and have traveled by different paths to a point
• Interference Pattern: Successive points with alternating cancellations and reinforcements create

Conditions for Interference

• Multiple Paths: More than two paths should be available for the wave to travel between receiver and source
• Path Difference: Varying path lengths must be creating a phase difference when the waves meet
• Coherent Sources: The interfering waves must originate from the same source, which ensures a stable phase relationship.

Diffraction of Radio Waves

• Waves spread out after passing through slits or around obstacles

Diffraction results when:

• A wave meets the edge of an obstacle
• Signals propagated between space wave may be received behind tall buildings, mountains and similar obstacles

Propagation of Waves

• Space Waves: Travel in straight lines over high frequencies, with some refraction thanks to the atmosphere
• Ground Waves: Combination of how waves diffract along the Earth's surface and E layers
• Sky Waves: Waves in the HF range that are reflected by the ionized layers of the atmosphere e.g. ionosphere.
• Beyond-the-Horizon Propagation: Tropospheric scatter and stationary satellite communications are two additional methods

Ground (Surface) Waves

• Field strength at a distance
• VHF Propagation

Sky-Wave Propagation - The Ionosphere

• Ionosphere its effects
• Reflection Mechanism
• Virtual height
• Critical Frequency (fc)
• Maximum Usable Frequency

Ground Waves

• Earth-guided EM waves close to the earth

Must be vertically polarized to prevent electrical component short-circuiting

• Signal disappears over distance
• Due to tilting, radio waves lose energy by the earth's curvature
• Attenuation due to absorption depends on the conductivity of the earth's surface and the frequency of the EM wave

Field Strength & Distance

• Stronger signal means better voltage
• Weaker signals as distance increases
• Antenna height affects reception
• Environmental Factors are additional source of signal loss

VLF Propagation

• VLF (below 100 kHz) : Travels far and with minimal absorption and stays as ground waves below 1,000 km and the E-Layer
• Use cases:
  • Maritime Comms: Ship navigation
  • Time and Frequency Transmissions :Global time synchronisation
  • Military naval comms: Secured communications
• VLF-antennas have specific requirements: Inefficient , need HIGH power and LONG masts to transmit
  • The power requirement is > 1 MW)
  • Size of masts is 387 m

Relative Conductivity of Earth Surfaces

Sky Wave Propagation

• Experiments after 1925 done by scientist Sir. Edward Appleton confirmed that sun's energy ionizes atmospheric molecules to then reflect HF radio back to Earth.
• EM have to directed above horizon level
• Waves are radiated based angle relative to the Earth, and either reflected/refracted to Earth

Sky Wave Propagation- The Ionosphere & Its Effects

• It is the uppermost part of atmosphere absorbing largest amounts of radiant solar energy to be IONIZED
• Ionization - Conversion of atom/molecule into an ion using light from the sun
• Variance in density, radiation and temperature are key to FOUR Main Layers (D,E,F1,F2)
  • Each affects the propagation of radio waves differently

LAYERS OF THE IONOSPHERE

D LAYER

• Located lowest in atmosphere
• Height -70km
• Depth 10km
• ionization occurs with presence of sunlight -disappears during night time

E LAYER

• Height- 100km
• Depths ~ 25km
• MF surface is added
  • Some HF's are reflected during the day
• Sporadic E Layer:
  • Very high ionization
  • Occasionally appears within E layer and can maintain its existence at dark

F1 LAYER

• Occurs during the day time
• Located at -180 km
• Can combine with the F2 layer when is dark
• Thicnkess is ~20km
• Can reflect some HF's, although many to pass towards F2
• Key results
  • Increases absorption that affects HF'S
    • Both are absorbed up and down

F2 LAYER

• Key to reflection for high-frequencies
• Can have ~ 200 KM of size
  • Heights range between 250 km, and 400-km by the sunlight, and over 300-km dark
• Melds the F2 when sun has set

LAYERS OF THE IONOSPHERE

Sky-Wave Propagation -the ionosphere: RELECTION MECHANISM

• Radio appears that are in ionosphere are bend when density increases resulting to refraction. By altering ionization

  • This affects a wave refraction based on that increase and the rate that ionization is present
  • Low index's are bent -Correct angle for incidence is ideal to travel far distances

VIRTUAL HEIGHT

- Calculated by time it takes from wave is sent to receive at receiver of ionized layer.

"CRITICAL FREQUENCY"

• The frequency range where energy will come back to ground surface By one area that can be beamed up Is dependent on

• Angle is normal

• Varies based on

• Density of ionization

  • The season

• Is ~5-12MHZ over F2 layer as a general comparisons

Maximum Usable Frequency (MUF)

• Occurs when highest frequency is directed back to Earth thanks to another, specific level above angle normal
• It reaches ~ (8-and 35-MHz)

Example Problem

• 150-m tower-antenna, emits with 1.2 Megahertz thanks to a ground location, includes 8 amp antenna. What Voltage could have? How, If was to 2 Meters from ground with 40- km.

Maximum Usable Frequency

• 12.5Megahertz is critical when calculating if signal incidence is 65°

maximum usable frequency

• Two areas on Earth are at ~ 1500-km distanced one to other communicate with use of High Frequencies
• The critical point stands at 7 mega Hertz - And conditions ideal to calculate the point value using height - 300 km Ionoshperic Level

Skip Distance

- Distance for a sky wave with a specific frequency to come come to Earth

• If the corner for incidence, it can escape to area.
• In case corner increases, high-layer will happen afar

Multipath Propagation

- Happens when the wave is not the same as receiver

• It results to signal destortion

Long-distance sky wave

• Includes limitation from Earth curvature + area + Long areas will go past ~ >34000 km above F2 level

North- South and East -West locations

 -East and South areas will need timing tuning

-The terminator impacts

• Can be issue to undershoot or reject signal or height

FAIDING

• Impacts to signals as reception to the area

• Interference inbetween with paths

• Change pass is important so signals go through

higher area

_Shorts frequencies are susceptible

Ionospheric Variance an Solar Acvitity

• Occur due to variances and years
• Areas near sunlight have problems due "Sudden disturbance"
  • That result signal rejection by solar flare for an HR
• As high areas are highy affected, then it means that smaller ones aew more stable.

Space Wave and Radio Levels

• Used inside Very High freq + Tv and cellphone companies
• Goes straigh -> It must with sight
• Longest areas require long range

Microwave Propagation and Ducting

• How a micro freq operates: 300- 300 GHZ

  • Radio systems

  • Air absorve/ rain

  • A long side Superfractor: Traps the levels bends are back into the Earth over >10000 km

Tropospheric scatter (troposcatter)

• communication that will use land as space point because they can easily go through long corners. For more UHF+ very side lines. -It depends

How it works

• HIGH pow -> Wavelength-direction that level

• 2. Scatters is what goes on with moisture density of a cornered layer

• Receptors areas at horizans

  4 Signal = Must directional antenna in point-like form

Advantages of Tropospheric scatter/ limitations in the modern world

• Good
• The signal doesn't shift a lot
• Bad
• Large cables high in power
• It will need senesitve signal - due to high reduction
• The impact on multi-point -Interence

Extraterrestrial communication

 - It is wave to transfer - Radio/ Earth - Satellitas or ships

Sub divided = Divided- Into Three

• Low Lands (LES)

  • Sat is high as radius ~ =145 , needing constnts follow
  • This is near as +36

• GEO- (STATIONARY) - Occurs with earths cycle

  • Has constsnt communiton

• INTERPLANATARY PRObes Used to -> Connect with land and space (Long time + tech)

Transioners Space-wave

• Wave

• MUST BE ~100 mhz

• below than ~ GHZ

• Transioners to the areas Has impacts with polarization's through interactions+ fields

• Variance = Loss of Signals

"SOLUTION'"

• Use circular with is divided woth sides (to not shift) "Is by frequencies "

Probe tracking

• Tracking near cable + global
• Tracking probers needs hugh wave for great distant
• The technology used : Like Radio

Follow with the cables - to use at smaller stations

• Needs rapid waves for those points
• Medium energy works

Interplaners

• Even "less strong"
  • Some measure to meter in side with 60

• NASA uses to send 10^ over cables