Update - The record for the longest ground to ground LoRaWAN transmission on The Things Network was set by TU Delft with their gateway. In this case, messages from the node in Manchester were picked up by the gateway as far as 467km away in Delft - TTN Mapper results. More details on that coming soon 🙂
On 8th July 2018, Ryan Walmsley from the East Coast community set a new record for ground to ground LoRaWAN transmission on The Things Network. Messages from his node were picked up by the gateways 235km away in Utrecht, Netherlands. Previously a record of 212km was achieved by Andreas Spiess from Switzerland and is described in his video.
It is interesting to see how Ryan's node was able to transmit from East Coast of the UK to gateways as far as Amsterdam and Utrecht. This long range is due to the formation of evaporation ducts in the atmosphere over the ocean. These 'ducts' are layers in the Earth's atmosphere where refractive index variation traps the signals, suppresses the vertical component and causes it to follow the curvature of the Earth. Atmospheric ducts are commonly found over water bodies due to vaporisation, and are specifically known as evaporation ducts.
Formation of Evaporation Ducts
The quantity of water vapour in the air closest to the surface of the water body is at highest possible amount that the air can hold, i.e., it has reached the saturation point and the relative humidity is 100%. The air that is further up and away from the surface of the sea, has lesser water vapour content. When the decrease in humidity with height is rapid, duct formation occurs.
For a few meters directly above the surface, the humidity rapidly drops with height. At this height, the water vapour pressure is the dominating influence on pressure, and the fall in this value causes the fall in the refractive index. After a certain height is reached, the water vapour pressure stabilizes and decrease in air pressure takes over as the dominant cause of pressure decrease. However, the amount of pressure decrease with height reduces and eventually causes the refractive index to increase. The height at which this occurs defines the thickness of the duct.
The formation of evaporation ducts is dependent on atmospheric factors like humidity, air pressure and temperature, and these factors vary with geographical location and climate. This dependency makes the occurrence and nature of ducts unpredictable and turbulent. However, they have been observed almost permanently, with heights varying up to a few 10s of meters, especially if the water body is calm.
The determination of the nature of the duct is performed using meteorological computations, as the varying nature makes it difficult to measure using instruments like refractometers and radiosondes. The variable nature also leads to a lack of reliability in wireless communication; this is further explored in the next section of the article.
Effect of Ducts on Radio Propagation
A significant impact on radio transmission has been observed in the presence of atmospheric ducts. This has led to substantial research on the topic, for example, regarding the change in the wave equation, or the impact on communication. The refractive index variation described in the previous section redirects radio waves towards the earth, overall causing the signal to transmit along the curvature of the earth. Due to this, receivers beyond the line of sight of the transmitters are able to receive the signal with enhanced signal strength.
Continuous refractions and reflections propagate the signal. The change in refractivity at the top of the duct causes refraction back towards the earth and the earth's surface reflects the signal upwards. This behaviour of a duct is comparable to a waveguide, however a small amount of energy leaks into free space as the duct boundary is not solid. This leakage does not significantly hamper radio signals and they can be transmitted over greater ranges via the duct.
The standard radio transmission patterns have been impacted by the presence of ducts, and this anomaly has resulted in extensive study, research and development of models to explain the nature and features of the phenomenon. For those of you reading and thinking, the earth is flat there is a scientific phenomenon due to which this happens 😅
So far, it appears that radio propagation via atmospheric ducts benefits the network, enabling longer ranges and better signal strength. However, it may also have a negative impact on the transmissions in the network, a few examples are mentioned here. A signal transmitted through a duct may cause interference in transmitters that might be located within the duct. Further, within the duct, the various reflections and refractions in different modes of transmission (one-hop, multi-hop etc.) are prone to destructive interference and may cancel each other out. There may also be refraction from different heights in the atmosphere, and these signals may interfere destructively at the receiver. Hence, mobile communication systems on coastal regions are impacted; the ducts cause anomalous transmissions to longer ranges, which may hinder regular transmissions.
This phenomenon can sometimes be beneficial for radio propagation while at times can be unfavourable. For Ryan it worked pretty well, congratulations once again for setting this record and we look forward to seeing many more cases like this in the future.
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The Things Network Team