4.3 Ionospheric Layers (D, E, F1, F2) and Skywave Propagation

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.

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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 ($N_2$, $O_2$, $O$), knocking bound electrons free to produce positively charged ions and free electrons.

$\text{Gas Molecule} + \text{Solar Photon (EUV/X-ray)} \longrightarrow \text{Positive Ion} + \text{Free Electron}$

The density of free electrons (electron density, $N_e$) 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

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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

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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 ($A \propto 1/f^2$). 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.

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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 ($n$) of the medium below 1.0:

$n = \sqrt{1 - \frac{81 N_e}{f^2}}$

Where $N_e$ is free electron density ($\text{electrons/m}^3$) and $f$ is operating frequency ($\text{Hz}$). As the wave travels into regions of higher electron density ($N_e$), 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.

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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
  =================================================================

1. Ground Wave Range: The distance over which the ground wave signal remains readable before being absorbed by the Earth.
2. Skip Distance: The distance from the transmitter to the point where the first skywave returns to Earth after ionospheric refraction.
3. 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.

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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.