ELECTENG 721: Radio Propagation

What 2 design requirements must be satisfied when planning radio comms systems?

• Provide adequate signal strength

• Minimise interference to neighbouring systems

Why are radio waves transmitted in free-space

So that their wavefronts (surfaces of constant phase) are spherical and have radii of curvature which are continually increasing

What is an isotropic antenna

A theoretical antenna that radiates electromagnetic energy uniformly in all directions, meaning its power output is equal in all directions, like a perfect sphere.

What causes Free space attenuation

As the spherical wavefront propagates outwards, the energy per unit area of the wavefront decreases. Free-space attenuation arises due to this expansion of the wavefront and the resultant distribution of power over an increasingly larger area

What is the quantity Ptgt (W) also known as ?

Effective Isotropic Radiate Power

What is EIRP

Effective Isotropic Radiated Power, a measure of the power radiated by an antenna in a given direction compared to an isotropic antenna.

Define boresight

The direction along the axis of an antenna where it is most sensitive to incoming signals. It indicates the optimal angle for signal reception or transmission.

What is P_d and what are its units

The power density (with units W/m²) , representing the power per unit area received by an antenna.

What is P₀ and what is its formula

The power received by a loss free antenna.

P₀ = P_dA_e

What is A_e and what is its formula

Effective aperature of an antenna. It is a measure of how effectively an antenna can capture or transmit electromagnetic waves.

Ae = (gλ²)/4π,

where g is the gain and λ is the wavelength.

expressed in m²

What is the formula for rms field strength E at a point where the power density of a plane wave is P_d

E =sqrt(120π · P_d) (V/m)

What things can be designed using the basic free-space transmission formula

Satellite Links

Terrestrial Microwave links

UHF point to point links

What is the height gain effect and how can it be combatted?

The height gain effect occurs when the effective range of an antenna increases with height above the ground due to reduced obstructions. It can be combatted by using lower towers or antennas with higher directivity.

What is space diversity

A technique where signals are transmitted and received over multiple pathways or antennas in order to improve reliability and reduce the effects of fading.

What is the formula for the plane earth model

P_r (dBm) = 10 log₁₀ (P_tg_tg_r) − 40 log₁₀ (d) + 20 log₁₀ (h_th_r)

What is the troposphere

The lower part of the atmosphere

What can cause variations in signal level with time for radiowaves?

Changes in temperature, pressure and humidity, rain

What is ducting and what is it caused by

Ducting is a phenomenon where radio waves are trapped and guided within a specific layer of the atmosphere allowing them to travel father than they normally would, beyond the typical line of sight range.

Caused by extended negative gradients

What is ε_r in practical atmospheric applications and what is it dependent on

ε_r > 1
Air pressure/temp

water vapour content

normaly ε_r decreases with height

What is subrefraction

an (unwanted) condition where radtio waves ebnd upwards away from the earths surface, more than they would under standard atmospheric conditions.

What can introduce diffraction loss

Irregular terrain features (obstacles such as hills and dense forests)

What Environmental factors act to increase path loss

Natural and constructed obstacles

vegetation

What are some environmental effects in urban areas

Shadowing

Fading

What is fading and what causes it

the loss of signal as seen by the radio receiver at its input. Cause by atmospheric conditions and path geometry

What are the 2 types of fading

Fading due to obstruction

Fading due to multipath interference

When does fading due to obstruction occur and what are the consequences

can occur when k changes in subrefractive conditions, and consequences include:

Increase in the effective height of obstacles

Earth blocking

When does Fading due to multipath interference occur

Fading due to multipath obstruction occurs when obstacles like buildings or trees block or reflect radio signals, causing multiple delayed signal paths to reach the receiver. These signals can interfere with each other—sometimes canceling out—leading to reduced signal strength or quality at certain locations.

Why is Fading due to multipath interference a big concern in mobile radio systems.

because it causes signal strength to vary rapidly with movement, leading to dropped calls and data errors. This unreliability must be managed to ensure consistent communication quality in mobile environments.

In Mobile Radio Propagation, What is the received signal comprised of in a multipath channel

a number of attenuated, phase shifted and delayed copies of the original transmission

What cause the significant variations in amplitude for the received signal in mobile radio propagation

Caused by phasor addition of the various multipath components which result in constructive and destructive interference.

What factors affect path loss

path length

frequency

antenna height

terrain

environment in vicinity of terminals

What is propagation loss on obstructed paths inluenced by

object size and shape

placement

frequency

number of obstacles

What are the 2 categories that terrestrial radio links are broken down into

Point to Point

Area Coverage: either Broadcasting or Mobile radio

What is the formula for the total system loss in a point to point system

As = A₀ + A_Z - (G₁ + G₂) + A_F1 + A_F2

A₀ = basic free space transmission loss (dB)

A_z = additional loss due to obstructions (dB)

A_F = feeder losses (dB)

G =antenna gains (dB)

What is the formula for received power P_r and give its units

P_r = P_t -A_S

basically just total power - total system loss

_{Units are dBm}

What are the 2 alternative path profiles in any link budget calculations

Curved earth of radius ka and straight rays

Flat earth with rays of curvature kr

What are the possible ways that we can compensate for the diffraction loss in link budget design?

increasin antenna gain(s) or if this is not feasible then try relocating the link terminal(s)

What is the High-Low Technique

Uses high sites to cover areas with blocked or non-reflected paths, and low sites to handle reflected paths and improve coverage in obstructed or dense environments.

What is an example of area coverage systems

Mobile (cellular) radio systems

What is the β symbol used for in area coverage systems design

This is a clutter factor aka the excess loss. It is multiplied onto the rest of the Plane earth formula (Pr = Ptgtgr…)

What does the clutter factor depend on

Both frequency and levels of environmental clutter

What is the formula for thermal noise

V_n² = 4kTBR
where
V_n² = mean square value of the thermal noise voltage

k = Boltzmann’s constant = 1.38 × 10−23 J/K

T = absolute temperature (K) B = system noise bandwidth (Hz)

R = resistance (Ω)

What is thermal noise fundamentally caused by

caused by resistive components that are not at a temperature of absolute zero

What is the Noise Figure/ Noise Factor formula

NF = 10log₁₀F

F can only have a max value of 1, so the noise figure of an ideal noiseless network is 0dB

units in (dB)

Receiver thermal noise formula

p_A = FkTB

_{units in W}

What is mds

minimum discernable signal or receiver noise floor is the signal with a power level equal to the noise level

How can we increase sensitivity

Cool receiver front end (Reduces T)

Reduce bandwidth

Redesign receiver to reduce NF

Why do we never design a system fo S/N =1

because the signal is barely distinguishable from noise, leading to unreliable communication with high error rates. For effective and robust performance, systems need a higher S/N to ensure clear signal detection and data integrity.

What is the required input signal level for a given S/N refered to as ?

system sensitivity

What is the mixer operation?

a three terminal deive (2 inputs, 1 output) that functions as an analogue multiplier. The 2 inputes are multiplied to produce the output.

What is gain compression`

As the input power increases, a point is reached when the output no longer increases in the same rate.

What is the dynamic range

The range between the minimum detectable signal (MDS) and the signal level where distortion matches the noise floor. It shows how well a receiver can handle both weak and strong signals without distortion.

What is the basic idea for a radar

illuminate a region with RF energy and observe the reflections to determine if something is there or estimate the distance (range).

What are some uses of radar

Tracking aircraft, people, weather, space objects, and mapping Earth's surface—even at night or through clouds.

How does radar detect targets?

Sends short pulses via directional antenna; reflected signals depend on target shape, material, and wavelength.

How is range calculated in radar?

The range of a target can be found from the time delay between when a pulse is transmitted and when it is received

Break down the steps on how a radar detects targets

1. Transmitter produces a short pulse, Directional antenna used to produce a focused incident wave.

2. Incident waves scatter from the target, scattering depends on the shape, material properties and wavelength.

3. Some of the scattered waves reach the radar antenna + receiver.

Time delay formula for a radar

T = 2R/C

Maximum unambiguous range formula

R_u = cT_p /2

R_u = c /2f_p

Range resolution formula

∆𝑅= 𝑅₂ − 𝑅₁ = 𝑐𝜏 / 2

What conditions must be met to detect a target using radar?

power of reflected signal > power of noise

How can SNR (radar) be increased

increase P_t and t , although a short t is preferred to improve ΔR

Multiple radar returns (assuming obeject does not move too much) can be integrated to increase SNR.