Propagation of radio waves is something that has intrigued me from the very first time I made a radio contact. In the past 25 years that I studied propagation, I gathered a lot of knowledge. I believe that I should share my knowledge so I wrote this article hoping that it can explain how radio wave propagation works.
1. Radio waves:
Radio waves are electromagnetic waves that propagate with a speed near 300,000 km/s. Electromagnetic waves have a frequency and wavelength. There are different type of waves, with high and low frequency. Even visible light is an electromagnetic wave, that has a very short wavelength. Your own eye is in fact an antenna, and you probably did not know!
You can calculate frequency or wavelength with these two formulas:
Frequency (MHz) = 300 / Wavelength (m). For example a wavelength of 10m has the following frequency: 300 / 10 = 30 MHz.
Wavelength (m) = 300 / Frequency (MHz). For example a frequency 28.495 MHz has the following wavelength: 300 / 28.495 = 10.528 m.
In the world of communication, different wavelengths or frequencies are divided into:
LF (Low Frequency) = 0.03 - 0.3 MHz (1000 m to 10000 m band)
MF (Middle Frequency) = 0,3 - 3 MHz (100 m to 1000 m band)
HF (High Frequency) = 3 - 30 MHZ (10 m to 100 m band)
VHF (Very High Frequency) = 30 - 300 MHz (1 m to 10 m band)
UHF (Ultra High Frequency) = 300 - 3000 MHz (1 m to 10 cm band)
SHF (Super High Frequency) = 3000 - 30000 MHz (10 cm to 1 cm band)
EHF (Extremely High Frequency = 30000 - 300000 MHz (1 cm to 1 mm band)
By the way:
1000 Hz = 1 kHz
1000 kHz = 1 MHz
1000 MHz = 1 GHz
There are also differences in properties for these wavelengths. LF waves easily penetrate through dense materials like concrete, rock, soil, etc. They follow the curve of the Earth, which makes LF ideal for long distance ground wave communication over more than 1000 km. Submarines even use VLF (Very Low Frequency) radio waves, because they can even travel right through the Earth's core!
The higher the frequency becomes, the less they bend along the Earth's curve, and the less they can penetrate dense materials. 10 m band waves follow the Earth's curve only little.
2. Earth's atmosphere
Before we explain the physics of propagation, it is good to that the Earth's atmosphere has a great influence on propagation:
The lowest part of the atmosphere is the troposphere. This part of the atmosphere holds for our weather. The troposphere ends at roughly 14 km under a small layer, the tropopause. Only the tops of large thunderstorm or supercell clouds (Cumulonimbus Incus) occasionally reach over 14 km, and some even push up the tropopause. The troposphere does not have great influence on HF propagation, but can sometimes extend normal 'ground wave' propagation, especially on the short HF wavelengths like 10 m and 12 m.
This part of the atmosphere does not influence HF propagation. The upper part of the stratosphere holds the ozone layer, which filters harmful ultraviolet (UV) radiation. The only clouds you find here are so called Noctilucent Iceclouds which are sometimes visible after midsummer sunsets.
The mesosphere harbours the so called D-layer. The most lower part of the ionosphere. The D-layer absorbs HF radiowaves, especially up to 10 MHz.
The ionosphere is very important for propagation of HF radiowaves. It harbours the so called E-layer, F1-layer and F2-layer. These layers emerge under influence of solar radiation.
3. Three types of propagation
We divide propagation in roughly three types:
3a. Ground waves
Ground waves are waves that propagate along the Earth's surface. Your favourite station on FM for example uses ground waves. How far ground waves can travel depends on the height of the antenna. That is why commercial broadcast stations on FM use large towers or are located on the highest mountains. 10 m band radiowaves usually travel around 30-50 km on ground wave, with the antenna at an average height of 10 m. The less obstacles the radio wave encounters, the stronger the signal will be. Ground waves over large surfaces of water travel much further than ground waves in mountainous or rural areas. Once the ground wave cannot follow the Earth's curve anymore, it travels into the sky and into space!
3b. Tropospheric skywaves
Sometimes ground waves travel further than theoretically possible. Distances up to 100 km and even more are possible on 10 m. This type of propagation is called Troposheric or 'Tropo'. In the troposhere, there are different layers of air, with different temperatures and different humidities. When it's windy, these layers of air mix together. But when it is not windy, different layers of air are present at different altitudes. When you are in the center of a high pressure area, weather is very quiet. In the morning, the air layer close to the ground is relatively cool and moist (sometimes it produces fog or mist!), while the area above it is relatively warm and dry. The change of temperature can be easily 10 degees Celcius over 100m, and is called an inversion. Now the cold layer of air is more dense than the warm layer of air. The sharp transition between cold and warm air, a temperature inversion, refracts radiowaves in VHF and UHF bands.
On some occasions there can be multiple inversions. Once a radio signal has been caught between two inversions, it can travel in between like travelling through a kind tunnel. This propagation mode is called tropospheric ducting. Ducting has only been reported on VHF and UHF.
Tropo is also observed along cold fronts, where there is a very sharp transition between cold dry and warm moist air. But even in violent atmospheres like hurricanes and typhoons.
Such inversions affects VHF and UHF bands most, but the higher HF bands like 10 m are also affected, however much less. On 10 m band contacts have been made over 350 km on a day with paths across stationary high pressure areas, and no reports of any ionospheric propagation at all on the same day.
Radio waves can travel far distances because they can be reflected to the Earth's ionosphere. They call such a reflection a 'hop'. The radio wave that is being reflected by the ionosphere can travel back to Earth. It bounces of the Earth' s surface back up again into the ionosphere. There it will be reflected down and again. Multiple bounces and reflections are called 'multi hop' propagation.
4. The Ionosphere
The ionosphere is a thin layer of air. It is called ionosphere because it is formed by ions. Ions are charged particles that appear under the influence of solar radiation (ultraviolet and X-rays). These ions have the capability to bend or reflect a radiowave. That capability depends on the density of ions, the more ions the stronger the reflection. The maximum frequency that the ionosphere can reflect is called the MUF or Maximum Usable Frequency.
We read earlier that the ionosphere consists out of 4 layers:
D-layer: only absorbs radiowaves, especially under 4-5 MHz. Appears very fast after sunrise, and disappears almost immediately during sunset.
E-Layer: reflects radiowaves up to 5 MHz, radiowaves above 5 MHz are absorbed, but less than in the D-layer. Appears shortly after sunrise, and disappears shortly after sunset.
F1-layer: reflects radiowaves up to 10 MHz. Appears shortly after sunrise and after sunset it merges with the F2-layer to become the F-layer.
F2-layer: reflects radiowaves up to 50 MHz (occasionally MUF's of 70 MHz have been reported). Appears after sunrise and disappears shortly and after sunset it merges with the F2-layer to become the F-layer. Is stronger in the winter than in the summer, due to seasonal effects.
As you can see the F2-layer is the most important one for us, it reflects our HF radiowaves along great distances, and at nighttime the F-layer does the same.
5. Solar Activity
You now know that the ionosphere appears under the influence of solar radiation, mainly Ultraviolet (UV) and X-ray. This solar radiation varies under the influence of:
5a. Sunspots and Solar Flux
sunspots are dark spots on the sun's surface, and can be compared with the crater of an active volcano. They produce the intense radiation which causes ionization of the ionosphere. The index for this radiation is called the Solar Flux and is measured at 2800 MHz. The higher this Solar Flux, the higher the level of ionization. The lowest possible Solar Flux is 64 (no sunspot regions), and the highest numbers go well into 200. Conditions on 10 m band generally start to become interesting on all latitudes, when the solar flux index passes the 100 number.
Sunspots are classified by their magnetic complexity. The more complex the magnetic configuration, the more active they are producing lots of radiation and all kind of other events like solar flares and CME's. Magnetic configuration is classified as:
In the picture below you can see many sunspots, and just above the middle, the largest sunspot ever recorded in modern history. These sunspots are clustered together in so called sunspot regions. The active regions are assigned a number when they appear on the sun's surface.
5b. Solar Wind
The solar wind is a constant stream of charged particles which flows form the sun into our solar system. Solar wind can reach speeds up to 1000 km per second.
5c. EGF (Earth's Geomagnetic Field)
Our planet has a core that consists mainly of iron. The funny thing about iron is when you rotate its, it produces a magnetic field like in the picture below. This magnetic field protects the Earth from the charged particles from the solar wind. When solar wind speed is very low, the EGF is quiet, but when the solar wind's speed is very high, the EGF becomes unsettled to active, and in some occasions we even talk about a solar wind storm. The EGF is very important for the production of a stable ionosphere. A quiet EGF means a stable ionosphere, with relative high MUF's. An active or stormy EGF means unstable propagation with a relative low MUF.
The EGF is strongest around the equator and weakest on the north pole and south pole, as you can see in the picture below.
5d. Solar Flares
When groups of sunspots are active, they are likely to produces solar flares. These solar flares are like vulcanic eruptions with large flames shooting millions of kilometres into space, like in the pictures below. These solar flares produce a lot of radiation, like X-ray which causes the D-layer to grow stronger. Usually after a large solar flare, propagation blacks out, because of very high absorption of the D-layer. Solar Flares also cause and ejection of large masses of charged particles, which is called a Coronal Mass Ejection (CME). The strength of a solar flare is measured from C-class followed by a number, up to M-class and X-class. M-class and X-class flares are likely to produce a radio blackout. A-level means very low levels of X-ray radiation.
Now the chance for a solar flare, depends on the magnetic configuration of the sunspot group:
Up to low M-class
Up to low X-class
Up to extreme large X-class
The magnetic configuration is very important. For example, a sunspot group or region with a 5 spot count in Delta-class can produce much more and bigger solar flares, than a 30 spot count group in Beta-class!
Solar Flares can even be heard on your own radio, especially the larger X-class flares, but also C-class flares that spit out lots of radiation. You can hear the level of background static rise for a short period, as the radiation reaches Earth.
5e. Coronal Holes
that is a a hole through the sun's outer shell (the corona). Coronal holes are always there, and they always produces a stream of charged particles. When a coronal hole faces Earth, it's stream is likely to hit the EGF within 1-5 days, and push solar wind speeds up to 600-700 km/s, bringing the K-index to active or storm levels. The black area in the picture below is a very large coronal hole.