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4.3 Ionospheric Layers (D, E, F1, F2) and Skywave Propagation
ACMA Amateur Radio Standard Recognition Certificate - Theory Exam
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1 Chapter 1: Electrical Principles and Components
1.1 Mathematics, Units and Metric Prefixes 1.2 DC Circuits, Voltage, Current, and Ohm's Law 1.3 Power, Energy, and Resistance Calculations 1.4 Resistor Colour Code and Component Symbol Identification 1.5 Capacitors: Construction, Ratings and Safety 1.6 Inductors: Construction, Factors Affecting Inductance and Combinations 1.7 Capacitor and Inductor Combination Calculations 1.8 AC Circuits, Frequency, Wavelength, and RMS Values 1.9 Reactance and Impedance in AC Circuits 1.10 Resonance, Q Factor and Tuned Circuits 1.11 Transformers: Turns Ratio, Impedance Matching and Eddy Currents 1.12 Rectifiers, PIV and Power Supply Smoothing 1.13 Active Components: Semiconductors, Diodes, and Transistors 1.14 Transistor and FET Symbols, Configurations and External Behaviour
2 Chapter 2: Transmitters and Receivers
2.1 Superheterodyne Receiver Architecture and Block Diagrams 2.2 Frequency Converters, IF Amplifiers and Transceiver Sharing 2.3 Receiver Performance: Sensitivity, Selectivity, and AGC 2.4 Signal-to-Noise Ratio and Receiver Noise Performance 2.5 Transmitter Architecture, Oscillators and Frequency Synthesisers 2.6 Transmission Quality: Frequency Drift, Parasitic Oscillation and Over-Modulation 2.7 Modulation Techniques: AM, FM, and SSB 2.8 Amplifier Classes and Power Amplifiers (Class A, B, C, and AB) 2.9 Modulation, Duty Cycle and Rated Transmitter Power 2.10 Transceiver Control: ALC, Squelch, and Digital Modes 2.11 Digital Transmission Modes and Bandwidth
3 Chapter 3: Transmission Lines and Antennas
3.1 Transmission Lines: Coaxial, Twin-Lead, and Characteristic Impedance 3.2 Standing Wave Ratio (SWR) and Reflection Loss 3.3 Impedance Matching, Baluns, and Antenna Tuning Units (ATUs) 3.4 Resonant Antennas: Dipoles, Verticals, and End-Fed Antennas 3.5 Angle of Radiation, Antenna Height and Current Distribution 3.6 Directional Antennas: Yagis, Gain and Radiation Patterns 3.7 Effective Radiated Power: System Gains and Losses
4 Chapter 4: Radio Wave Propagation
4.1 Electromagnetic Waves: Velocity, Wavelength and Polarisation 4.2 Ground Wave, Direct Wave and Line-of-Sight Propagation 4.3 Ionospheric Layers (D, E, F1, F2) and Skywave Propagation 4.4 Critical Frequency, Maximum Usable Frequency (MUF), and Fading 4.5 Solar Activity, Sunspot Cycles, and VHF Anomalous Propagation 4.6 Seasonal and Diurnal Effects on Band Selection
5 Chapter 5: Electrical and RF Measurements
5.1 Multimeter Operation: Voltage, Current, and Resistance Measurements 5.2 RF Power and SWR Measurement (PEP vs. Average Power) 5.3 Frequency Counters, Dummy Loads, and Test Equipment 5.4 Frequency Standards, Crystal Calibrators and Standard-Frequency Transmissions
6 Chapter 6: Interference and Electromagnetic Compatibility (EMC)
6.1 Harmonic Emissions, Spurious Signals, and Transmitter Filtering 6.2 How Interference Enters Television and Broadcast Receivers 6.3 Masthead Amplifier Overload and Audio-Stage Demodulation 6.4 Electromagnetic Interference (EMI), Breakthrough, Ferrites, and EMC 6.5 Filter Types, Response Curves and EMC Filter Selection 6.6 EMC Good Practice: Minimum Power, Balanced Antennas and Feedline Routing 6.7 Vehicle EMC and Resolving Community Interference Complaints
7 Chapter 7: Safety and Environmental Hazards
7.1 Mains Voltage Safety, Earthing, and High-Voltage Capacitors 7.2 Equipment Approval, Mains Wiring and Fuse Practice 7.3 Station Layout, Cabling and Emergency Shut-Off 7.4 Lightning Protection and Station Security 7.5 Electric Shock: Prevention, Rescue and Emergency Response 7.6 Battery, Chemical and Hearing Safety in the Amateur Station 7.7 RF Exposure: EMR Hazards and Safe Distance 7.8 Antenna Erection, Siting and Power Line Safety
4.3 Ionospheric Layers (D, E, F1, F2) and Skywave Propagation
Key Takeaways Explain This Topic AI
The ionosphere is formed in the upper atmosphere (50 km to 500 km) when solar ultraviolet (UV) and X-ray radiation ionise atmospheric gas molecules.
The D layer (50–90 km) exists only during daylight hours, absorbing lower HF frequencies (below 7–10 MHz) and preventing daytime skywave propagation on 160m and 80m.
The E layer (90–150 km) refracts medium HF signals during the day and recombines quickly after sunset.
The F layer splits into F1 (150–250 km) and F2 (250–450 km) during the day, merging into a single F layer (~300 km) at night to provide long-distance worldwide HF skywave propagation.
The skip zone (dead zone) is the region between the outer limit of ground wave coverage and the point where the first skywave returns to Earth.
Last updated: July 2026
4.3 Ionospheric Layers (D, E, F1, F2) and Skywave Propagation
Long-distance High Frequency (HF) communication depends on skywave propagation (often referred to as "skip"). Radio waves transmitted upward toward space are bent (refracted) by the ionosphere back down to the Earth's surface thousands of kilometres away.
Formation of the Ionosphere
The ionosphere is a region of the Earth's upper atmosphere extending from roughly 50 km to over 500 km above the Earth's surface. High-energy solar radiation—specifically Extreme Ultraviolet (EUV) light and X-rays —strikes neutral atmospheric gas molecules ( N 2 N_2 N 2 , O 2 O_2 O 2 , O O O ), knocking bound electrons free to produce positively charged ions and free electrons. Gas Molecule + Solar Photon (EUV/X-ray) ⟶ Positive Ion + Free Electron \text{Gas Molecule} + \text{Solar Photon (EUV/X-ray)} \longrightarrow \text{Positive Ion} + \text{Free Electron} Gas Molecule + Solar Photon (EUV/X-ray) ⟶ Positive Ion + Free Electron
The density of free electrons (electron density, N e N_e N e ) determines how strongly the ionosphere will bend radio waves. Solar ionisation varies according to:
- Time of day (diurnal variation)
- Season of the year
- Geographic latitude
- 11-year solar sunspot cycle
- Ionospheric Layers and Their Characteristics
The ionosphere is stratified into distinct ionised regions or "layers," designated by the letters D, E, F1, and F2 in order of increasing altitude. DAYTIME NIGHTTIME
- Altitude
- (km)
- 500 +--------------------------+ +--------------------------+
- | F2 Layer | | |
- 300 |--------------------------| | Merged F Layer |
- | F1 Layer | | (~300 km) |
- 150 +--------------------------+ +--------------------------+
- | E Layer | | E Layer (Residual) |
- 90 +--------------------------+ +--------------------------+
- | D Layer | | D Layer Disappears |
- 50 +--------------------------+ +--------------------------+
- 0 ==============================================================
- Earth's Surface
- Comprehensive Layer Comparison
Layer Altitude Range Diurnal Existence Recombination Rate Primary Effect on Radio Signals
D Layer 50 km – 90 km Daytime only (vanishes at night) Extremely Rapid Absorbs low HF frequencies (1.8–7 MHz); causes heavy attenuation
E Layer 90 km – 150 km Max at solar noon (weak at night) Rapid Refracts lower HF signals (1.8–7 MHz) for daytime single-hop (~2000 km)
F1 Layer 150 km – 250 km Daytime only (merges at night) Moderate Refracts medium HF signals; merges into F layer at night
F2 Layer 250 km – 450 km Day & Night (highest electron density) Very Slow Primary DX layer ; single-hop range up to 4000 km; merges at night
- Detailed Behavior of Individual Layers
- 1. The D Layer (50 km – 90 km)
High Atmospheric Density: Because the D layer is located in relatively dense air, free electrons collide frequently with neutral gas molecules and rapidly recombine.
Daytime Absorption: When an HF radio wave passes through the D layer during daylight hours, the oscillating electric field causes free electrons to vibrate. In the high-density D layer, these vibrating electrons collide with neutral gas molecules before re-radiating their energy, converting the RF energy into heat.
Frequency Dependence of Absorption: D-layer absorption is inversely proportional to the square of the frequency ( A ∝ 1 / f 2 A \propto 1/f^2 A ∝ 1/ f 2 ). Lower HF frequencies (1.8 MHz and 3.5 MHz) suffer total absorption during the day. Higher HF frequencies (21 MHz and 28 MHz) pass through the D layer with minimal attenuation.
Nighttime Disappearance: After sunset, solar UV radiation ceases. Rapid electron recombination causes the D layer to vanish completely within minutes, opening up the 160m, 80m, and 40m bands for worldwide skywave communication.
2. The E Layer (90 km – 150 km)
Peak ionisation occurs at local solar noon.
Provides daytime refraction for 1.8 MHz to 7 MHz signals over short-to-medium paths (up to 2000 km).
De-ionises rapidly after sunset, leaving only weak residual ionisation.
3. The F1 and F2 Layers (150 km – 450 km)
Daytime Splitting: During daylight hours, solar heating and ionisation split the F region into two distinct layers: F1 (lower) and F2 (upper).
The F2 Layer: Located between 250 km and 450 km, the F2 layer has the highest electron density and altitude of any ionospheric layer. It is responsible for almost all long-distance worldwide (DX) HF communication. A single hop from the F2 layer can span up to 4000 km .
Nighttime Merging: Because atmospheric gas density at 300+ km is extremely low, free electrons collide with positive ions very infrequently. Consequently, ionisation persists throughout the night. After sunset, the F1 and F2 layers recombine into a single F layer located at an altitude of approximately 300 km.
Skywave Refraction Mechanism
Radio waves returned to Earth by the ionosphere are refracted (bent) , not reflected like light off a mirror. Ionospheric Plasma
Electron Density Increases With Altitude
.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.~.
- / Gradual bending away from dense plasma \
- / \
- Transmitter Receiver
- | |
- =====|=====================================================|=====
- Earth's Surface
When an electromagnetic wave enters an ionised plasma layer, the free electrons reduce the phase refractive index ( n n n ) of the medium below 1.0: n = 1 − 81 N e f 2 n = \sqrt{1 - \frac{81 N_e}{f^2}} n = 1 − f 2 81 N e
Where N e N_e N e is free electron density ( electrons/m 3 \text{electrons/m}^3 electrons/m 3 ) and f f f is operating frequency ( Hz \text{Hz} Hz ). As the wave travels into regions of higher electron density ( N e N_e N e ), the refractive index drops, causing the wavefront to bend progressively away from the region of higher density until it is directed back down toward the Earth.
Skip Distance and Skip Zone
When an HF signal is transmitted from an antenna, two distinct wave components exist near the Earth's surface: SKIP ZONE (DEAD ZONE)
- <---------------------------------------->
- [Ground Wave Limit] [First Skywave Arrival]
- Tx | |
- |========>| v
- | Ground | No Signal Received /\ First Skywave
- | Wave | (Silent Dead Zone) / \ Return
- =================================================================
Ground Wave Range: The distance over which the ground wave signal remains readable before being absorbed by the Earth.
Skip Distance: The distance from the transmitter to the point where the first skywave returns to Earth after ionospheric refraction.
Skip Zone (Dead Zone): The region between the outer limit of ground wave coverage and the inner limit of first skywave return. No signal can be received within the skip zone because the ground wave has died out and the skywave has not yet returned to Earth.
Multi-Hop Propagation
A single-hop skywave path from the F2 layer has a maximum theoretical limit of approximately 4000 km due to the Earth's curvature. To reach distances beyond 4000 km (such as Australia to Europe or North America, spanning 15,000+ km), radio signals travel via multi-hop propagation . Ionosphere (F2 Layer ~300 km)
- /\ /\
- / \ / \
- / \ / \
- / \ / \
- Transmitter \ Ground/Ocean / Receiver
- | \ Reflection / |
- =====|===========V======/\========V==========|=====
- / \
In multi-hop propagation, the signal travels up to the ionosphere, refracts down to the Earth's surface, reflects off the ground or ocean , and travels back up to the ionosphere for a second or third hop. Sea water provides an excellent reflection surface due to its high electrical conductivity.