RF PropagationModule 3
Covers how RF signals travel through different environments — ground wave, sky wave, and line-of-sight propagation — with interactive ionosphere and SNR visualisers. Foundational for understanding real-world signal behaviour. Depends on Module 1.
RF Propagation Basics
RF propagation describes how radio frequency signals travel from a transmitter to a receiver. Unlike wired connections, RF signals radiate as electromagnetic waves through the air (and sometimes through materials), subject to the laws of physics governing wave behaviour.
The way a signal propagates depends primarily on its frequency. Lower frequencies hug the Earth's surface; HF signals bounce off the ionosphere; VHF and above travel in straight lines. Understanding these mechanisms is essential for system design, link budgeting, and predicting coverage.
Orientation of EM Waves
EM waves are transverse — the electric (E) and magnetic (B) fields oscillate perpendicular to each other and to the direction of travel. Polarisation refers to the orientation of the E-field: vertical, horizontal, circular, or elliptical. Matched polarisation between TX and RX antennas minimises losses.
- Vertical Polarisation
- E-field perpendicular to ground. Common in mobile communications and AM broadcast.
- Horizontal Polarisation
- E-field parallel to ground. Used in some TV broadcast and point-to-point links.
- Circular Polarisation
- E-field rotates as wave propagates. Used in satellite links to reduce Faraday rotation effects.
- Polarisation Loss
- Occurs when TX and RX polarisations are mismatched. Can cause 3 dB or more of additional loss.
Types of Radio Propagation — Interactive Diagram
Frequency Range
< 2 MHz (LF / MF)
Max Range
Up to 1,500 km
Use Cases
- • AM broadcast (MW)
- • Maritime communications
- • NAVTEX
- • NDB navigation beacons
Follows the curvature of the Earth along the surface. Reliable over-horizon propagation at lower frequencies. Attenuates with distance as energy is absorbed by the ground.
Electromagnetic Wave Propagation Characteristics
As an RF wave travels, it interacts with the environment in several ways that affect the received signal strength and quality.
- Free-Space Path Loss (FSPL)
- Power reduction due to spreading of the wavefront over distance. Doubles every time distance doubles (+6 dB per octave).
- Reflection
- Signal bounces off smooth surfaces (buildings, ground, water). Creates multipath components.
- Diffraction
- Signal bends around obstacles (knife-edge effect). Allows some coverage beyond hills and buildings.
- Scattering
- Signal disperses when it hits rough surfaces or small objects. Important in urban environments.
- Absorption
- Signal energy absorbed by materials (water, leaves, walls). Increases with frequency.
- Refraction
- Signal bends when passing through media of different density. Causes the tropospheric and ionospheric effects.
3.5.1 Environmental Factors Affecting RF Propagation
Real-world propagation differs significantly from free-space due to environmental conditions. The table below summarises key factors with their typical impact and affected frequency bands.
| Factor | Impact |
|---|---|
| Rain | high |
| Atmospheric Absorption | medium |
| Fog & Cloud | low |
| Terrain (Hills/Mountains) | high |
| Vegetation | medium |
| Buildings | high |
| Multipath Fading | high |
High Frequency Propagation
High Frequency (HF) signals — from 3 to 30 MHz — can travel thousands of kilometres by reflecting off the ionosphere, a region of the upper atmosphere ionised by solar radiation. This makes HF the basis for global shortwave, amateur (ham) radio, and military long-distance communications.
The Ionosphere
The ionosphere extends roughly 60 to 800 km above the Earth's surface. Solar UV and X-ray radiation ionise gas molecules, creating free electrons that can refract (bend) radio waves back to Earth. The degree of ionisation — and thus the frequencies that can be reflected — changes with time of day, season, and the 11-year solar cycle.
Ionosphere Layers — Interactive Visualiser
Use the slider to change the transmit frequency and see which ionosphere layers reflect or absorb the signal, and which it passes through.
↩ Reflected by: E Layer, F1 Layer, F2 Layer
F2 Layer
The most important layer for HF long-distance communications. Highest and most persistent — present day and night, though weaker at night. Reflects HF up to the MUF (Maximum Usable Frequency). A single F2 hop can reach 4,000 km.
Present: Day & Night
Ionosphere extends roughly 60–800 km above Earth's surface
- Critical Frequency (fc)
- Maximum frequency reflected straight up (vertical incidence). Measured by ionosondes.
- Maximum Usable Frequency (MUF)
- Highest frequency reflected at a given path angle. Approximately 3× fc for long paths.
- Lowest Usable Frequency (LUF)
- Minimum frequency that overcomes D-layer absorption on a given path.
- Optimum Working Frequency (OWF)
- About 85% of MUF — provides reliable propagation with some margin.
- Skip Zone
- Area between the ground-wave range and the first sky-wave return — no signal received.
- Multi-hop
- Signal bounces between ionosphere and Earth multiple times, extending range globally.
Tropospheric Scatter
Tropospheric scatter (troposcatter or troposcatter) exploits the scattering of VHF/UHF signals by irregularities in the lower atmosphere (troposphere, up to ~12 km). A small fraction of the transmitted power is forward-scattered toward a distant receiver, enabling beyond-line-of-sight links of 300–800 km.
Troposcatter links are typically used for military, government, and remote communications where reliable beyond-horizon coverage is needed without satellite infrastructure. They require high transmit power, large antennas, and diversity reception to overcome the severe path loss (often 50–80 dB worse than free-space).
- Scatter Volume
- The common volume of troposphere illuminated by both TX and RX antenna beams — determines received power.
- Path Loss
- Typically 240–270 dB for a 500 km troposcatter link — much higher than FSPL alone.
- Frequency Range
- Best performance at 900 MHz to 5 GHz. Lower frequencies suffer more ionospheric interference; higher frequencies attenuate more.
- Diversity
- Space, frequency, or angle diversity is used to combat the severe multipath fading inherent in troposcatter.
Common Interference Methods
Interference degrades or prevents the reception of a wanted signal. Understanding common interference mechanisms helps in designing resilient RF systems.
- Co-channel Interference (CCI)
- Two transmitters on the same frequency — dominant in cellular frequency reuse. Managed by cell planning and power control.
- Adjacent Channel Interference (ACI)
- Bleed-over from transmitters on nearby frequencies. Managed by channel spacing and receiver filter selectivity.
- Intermodulation (IMD)
- Non-linear mixing of two or more signals in a receiver or amplifier produces spurious products at unwanted frequencies.
- Multipath Interference
- Reflected copies of the same signal arrive with different delays, causing constructive or destructive addition (fading).
- Jamming
- Intentional interference to disrupt communications. Electronic warfare countermeasure.
- Passive Intermodulation (PIM)
- Non-linear mixing in passive components (connectors, cables, antennas) — problematic in base stations.
Noise and RF Signals
All electronic systems generate noise — random fluctuations of electrical signal that limit the minimum detectable signal. The fundamental source is thermal noise: the random motion of electrons due to temperature.
Thermal noise power: P = k · T · B where k = 1.38×10⁻²³ J/K (Boltzmann), T = temperature (K), and B = bandwidth (Hz). At room temperature (290 K), the noise floor is approximately −174 dBm/Hz, or −174 + 10·log₁₀(B) dBm.
SNR Calculator
Enter signal and noise powers in dBm to calculate the SNR and assess link quality.
e.g. −50 dBm for a typical received signal
e.g. −90 dBm noise floor for a typical receiver
| SNR Range | Quality | Typical Use |
|---|---|---|
| ≥ 25 dB | Excellent | High-speed data, MIMO, 64-QAM |
| 15–25 dB | Good | Video streaming, QPSK |
| 5–15 dB | Marginal | Voice, BPSK, basic data |
| < 5 dB | Poor | Link at risk of outage |
- Noise Figure (NF)
- How much a component degrades SNR, in dB. An ideal (noiseless) device has NF = 0 dB.
- Noise Floor
- Minimum detectable signal level. Determined by thermal noise + receiver noise figure + bandwidth.
- Eb/N₀
- Energy per bit / noise density — the digital equivalent of SNR, independent of data rate.
- Shannon Capacity
- C = B·log₂(1 + SNR). Maximum error-free data rate for a given bandwidth and SNR.
RF Propagation & Operating Bands
Changing the operating frequency band fundamentally changes propagation behaviour. RF engineers choose bands based on the required range, data rate, licensing, antenna size constraints, and environment.
| Band | Freq Range | Propagation | Key Uses |
|---|---|---|---|
| LF | 30–300 kHz | Ground wave, 1000+ km | Navigation, maritime |
| MF | 300 kHz–3 MHz | Ground wave (day), sky wave (night) | AM broadcast, amateur |
| HF | 3–30 MHz | Sky wave (ionosphere), up to global | Shortwave, military, ham |
| VHF | 30–300 MHz | Line of sight, ~150 km | FM radio, DAB, aviation, marine |
| UHF | 300 MHz–3 GHz | Line of sight, limited | 4G/5G, Wi-Fi, GPS, TV |
| SHF | 3–30 GHz | LoS only, rain attenuation | Satellite, radar, 5G mmWave |
Module 3 complete
Next: Modulation Fundamentals