Radio Wave Propagation

Radio wave Propagation

Radio Wave Transmission

• Radio waves propagate or travel as Ionospheric and Non-ionospheric propagation
• Ionospheric layer is found in the upper levels of stratosphere in atmosphere
• Radio waves travel or propagate with or without use of Ionosphere
• Non-Ionospheric propagation takes place without use of ionosphere
• Ionospheric propagation takes place using the ionospheric layer

Non Ionospheric Propagation

• Non-Ionospheric propagation does not pass through Ionosphere
• Ground waves propagate without passing through Ionosphere
• Ground waves are sub-divided into Surface and Space waves

Surface Waves

• Low, Medium and High Frequency waves from 20 KHz – 50 MHz travel as surface waves
• Normally 20 KHz to 2 MHz frequencies are used for aviation purposes
• LF, MF and HF waves have less atmospheric and surface attenuation
• High diffraction enables surface waves to bend around the earth’s surface
• Surface waves are used in Long range NDB and Long distance communication

Space Waves

• VHF and UHF waves greater than 50 MHz have very less diffraction and propagate as Space waves
• Maximum range of ground waves is given by the line of sight formula
• Range in NM is equal to 1.23 times the sum of square roots of heights of transmitter and receiver
• Space waves are sub-divided into Direct and Ground reflected waves
• Direct waves travel as per the Line of sight principle from transmitter to receiver
• Ground reflected waves are received after reflection from obstacles which could cause an error in navigational equipments

Ionosphere

• Ionospheric layer is found in the upper levels of stratosphere in atmosphere
• Consists of negatively charged free electrons due to ionized gasses
• Ionisation is the process of ejecting electrons from an atom
• Ionospheric propagation involves passes through Ionospheric layer
• Direct and Sky waves are types of Ionospheric propagation

Layers of Ionosphere

• Ionosphere exists in D, E and F bands or layers
• Kennely or D layer is found at an average height of 75 km over earth and has low ionization levels
• Therefore, D Layer and is capable of refracting only lower frequencies
• Heavyside or E layer is found at an average height of 125 km over earth and has medium ionization levels
• E layer is capable of refracting up to 10 MHz frequencies
• Appleton or F layer is found at an average height of 225 km over earth and has high ionization levels
• High levels of ionisation in F layer is capable of refracting up to 3 MHz frequencies

Variation in Ionosphere

• Height and intensity of the ionosphere varies on seasons as well as time of day
• Intense ionisation is found at the centre of D, E and F layers
• D layer forms at sunrise and disappears at sunset
• E layer reduces in altitude at sunrise and increases in altitude after sunset
• F layer splits into two at sunrise and rejoins at sunset
• Altitude of F1 reduce below F layer at sunrise
• Altitude of F2 layer is dependent on time of year
• In summers, F2 increases in altitude up to 400 Km
• In winters, F2 remains at approximately same at 225 Km

Ionospheric Attenuation and Refraction

• Collisions between wave particles and free electrons causes Ionospheric Attenuation and Refraction
• Ionospheric Refraction cause some of the waves to return to ground that can be useful in Radio Navigation
• Ionospheric attenuation is higher in lower frequencies and increases with increase in Solar Radiation

Sky Waves

• Radio waves returning back to earth on refraction by the ionospheric layer are called as Sky waves
• Sky waves are possible in LF, MF ad HF bands within the frequency range of 2 to 50 MHz

Critical Angle and Critical Frequency

• The distance a sky wave returns to earth is determined by its frequency and transmission angle
• Critical angle is the minimum transmission angle for return of sky wave of a certain frequency
• Transmission at less than the critical angle would make the waves to exit the atmosphere as Escape Rays
• Critical angle is dependent on Ionisation levels and Frequency of wave
• Critical angle increases with Frequency and reduces with Ionisation Levels
• Critical frequency is the lowest usable frequency for a given critical angle
• Higher ionisation levels increase critical frequency for a certain critical angle

Maximum Usable Frequency

• Maximum usable frequency is the highest usable frequency for High Frequency Radio (HF Radio)
• High ionisation levels at noon results in higher Maximum Usable Frequencies
• MUF has minimum attenuation and static interference since it travels the shortest path through the ionosphere
• HF Radio Transmission at frequencies higher than MUF will form escape rays and will not return to earth

Optimum Working Frequency

• OWF is derived from MUF in order to ensure that the transmission does not exceed MUF due to ionospheric fluctuations
• Optimum working frequency is 0.85 times the Maximum Usable Frequency
• HF Radio transmission frequencies are halved at night due to low ionisation levels

Skip Distance and Dead Space

• Skip distance is the distance from transmitter to first returning sky wave
• Higher ionisation levels reduce skip distance while higher Frequency or refraction layer increases skip distance
• Dead Space is the distance from Surface wave range to the Skip distance
• Since the range of surface wave is constant, dead space reduces with decrease in skip distance

Multi Hop Propagation

• Multi-hop propagation unexpectedly increase the range of sky waves in high power transmissions
• Reflection of sky waves back to the ionosphere causes multi-hop propagation
• Ground Based Transmitters transmit tangential to earth in order to maximise sky wave range

Sky Waves at Night

• Sky wave propagation in LF, MF and HF bands vary between day and night
• During day, low powered sky waves vanish due to absorption by D layer
• At night, sky waves refracted by E layer are unusable due to weak signals
• Range of sky waves increase at night but interfere with the same surface wave resulting in incorrect direction indications

Fading and Multi Path Fading

• Increasing and decreasing signal quality in received signal due to same wave received by two paths is called Fading
• Fading is caused due to weak signal strength as a result of higher distances, low power, weather and terrain
• Multi-path fading is due to out of phase reception of sky and surface wave of same signal
• Multi-path fading can be overcome by frequency or space diversity
• Frequency diversity is the use of different frequencies for transmission and reception
• Space diversity by using two or more antenna for reception in large aircrafts

Best of luck