Of overreach is talk when signals regional, terrestrial radio services received a much greater distance than usual. This is particularly noticeable with radio stations on VHF and TV stations, but can also occur with mobile radio networks . Frequencies above 30 MHz are affected , for which quasi-optical propagation conditions normally apply. Thus, even with many other wireless services, such as police radio , fire departments , emergency services , mobile radio , taxi radio or FM marine radio to feel the consequences of overreach. Communication is often made more difficult. Radio amateurs and DXers , on the other hand, use these conditions for rare connections.
One consequence of overreaches is that transmitters of the same frequency overlap. For example, if an analog TV station in Bavaria broadcasts on VHF channel 6 and an Austrian one uses the same channel , it is agreed between the two countries that the station ranges do not overlap. However, atmospheric and ionospheric influences can increase the range of a transmitter so that the receiver of one transmitter also receives signals from the other. In the case of analog signals, images or sound are then superimposed. If both channels broadcast the same program , the result is shifted images with the same content. That of the more distant transmitter appears weaker. With different programs you can see the images of the weaker station as a ghost image in the current program. In both cases, there may also be sound interference. Digital signals are also transported when the range is too high . These are initially more robust against interference. At a certain point, however, there is a total failure. Satellite links are not affected by the phenomena described here.
Different causes of overreach and their consequences
Radio waves in the frequency ranges VHF , UHF and above propagate quasi-optically. If there was a line of sight, distances of several 1000 km could be bridged. However, this prevents the earth's curvature.
Without overreach, the signals from a VHF radio station, for example, therefore only slightly exceed the viewing range of the transmitting antenna. If this is installed on a 200 meter high mast in the lowlands, there would be up to around 40 km of visual contact with a receiver. The signal is correspondingly strong in this area. However, a radio signal does not behave exactly like light. For example, it penetrates walls at a weakened level and also follows the curvature of the earth for a small distance.
The signal could be received on a good radio up to a distance of about 100 km. If the frequency is free, an FM radio signal can be received up to approx. 400 km with very weak, fluctuating signal strength even under normal conditions. However, directional antennas and suitable devices are required for this.
Tropospheric overreach (tropo)
Frequencies: VHF, UHF, SHF, range: 100 to 1000 km, signals: weak to strong
If, for example, Danish VHF radio stations are received in the Ruhr area, tropospheric overreaches are usually the reason for this. The overreaches are caused by inversion weather conditions in the troposphere , which extends up to an altitude of 15 km. Radio waves above about 50 MHz are then broken through layers of air with an inverse temperature profile (cold air below, warm above) and / or guided like in a waveguide. The refractive index gradient increases the normal radio horizon of radio stations anyway , which therefore extends beyond the curvature of the earth in the frequency ranges mentioned above - with inversion it can extend up to a distance of around 700 km. The field strengths to be observed can remain relatively constant over several hours or days. A slight "tropo" with only slightly increased field strengths occurs relatively frequently, especially in spring or autumn when there is relatively no wind. The upper layers of the air are warmed by the sun at sunset or sunrise, while the layers near the ground, especially in rural areas with rivers or lakes, cool down relatively quickly or warm up more slowly (indicator: ground fog). This soil inversion leads to overreaches of 300 to 400 km, while waveguide channels through higher inversion layers sometimes lead to overreaches of more than 1000 km.
Typically, tropospheric overreaches are favored by high pressure weather conditions and by ways of spreading over water. With strong inversion weather, in Germany about 10 days a year, stations up to 1000 km, in exceptional cases even more, can be received. In the Mediterranean, on the other hand, pronounced tropospheric overreaches are relatively common.
Spread beyond the quasi-optical horizon is also caused by the weather-independent troposcatter , but is then not referred to as overreach.
Sporadic-E (E s )
Frequencies: VHF up to a maximum of approx. 200 MHz, range (VHF): 800 to 2500 km, signals: strong, almost only in summer and during the day
If you can hear FM radio stations from southern Europe in Germany in the summer, it is very likely that it is " Sporadic-E ". The signals can become very strong, they fluctuate and can only be heard from a small geographical area in a narrowly defined direction of about ± 1 °, but at different distances of typically 1200 to 2200 kilometers. Reflection points are between the transmitter and receiver and are concentrated over a few kilometers.
In Germany, stations from Madrid and southern Portugal can be heard in a moment, while stations from Italy can be received in Great Britain at the same time. Sporadic-E occurs during the day almost every day in summer, sometimes only for a few minutes, occasionally for many hours. Until the 2010s, analogue, terrestrial television stations were also regularly used in Germany in this way, e.g. B. from Spain or Greece in VHF band I on channels 2 to 4. As a result of the switch to digital television, this band is rarely used in Europe.
Sporadic-E is completely different from Tropo. The radio waves are reflected in the approximately 150 km high E layer of the ionosphere. This means that large distances are bridged. Typical in the VHF radio band is the reception of stations from 1500 to 2000 km away. Because the angle of incidence of the reflection may only be very flat, distances below 800 km are hardly possible with Sporadic-E on VHF. Higher frequencies lead to greater distances.
Shortwave signals are usually reflected in the ionosphere. The maximum frequency for this is usually between 15 and 30 MHz. With Sporadic-E it increases up to 150 MHz, in exceptional cases even above. In summer, light Sporadic-E is almost daily for several hours, and strong for up to 20 days. The following table gives an example of distances for different forms of Sporadic-E:
TV channels 2,3,4; 6 m amateur radio
2 m amateur radio
|normal conditions||2000 km||-||-||-||-|
|light Sporadic-E||1300 km||1500 km||-||-||-|
|medium sporadic E||1000 km||1300 km||1800 km||-||-|
|strong Sporadic-E||600 km||800 km||1000 km||1600 km||1800 km|
In rare cases, double jumps of a signal can be observed with Sporadic-E.
Frequencies: up to 200 MHz, range: approx. 1000 km, signals: weak
Weak, heavily distorted signals from stations in northern regions, such as Scotland, indicate “Aurora”. Radio signals are reflected in temporarily ionized areas of the atmosphere ( polar light , also called aurora ). The phenomenon can only be observed in Germany without great effort if the aurora effect is very strong. North of the Arctic Circle, it occurs many days a year.
Meteor Scatter (MS)
Frequencies: up to 200 MHz, range: approx. 1000 km, signals: weak
When meteors enter the atmosphere, they leave ionized tracks that allow radio waves to be reflected. This effect is sporadic, limited depending on the angle and can only be observed for a few seconds to minutes. Stations up to 2400 km away can be received. Radio amateurs use the phenomenon for particularly long-range radio connections ( QSOs ).
Also on airplanes, English Aircraft , can be radio signals reflect and utilize km for over ranges of up to 800th These connections are usually only available for a short time in the time range from 30 s to a few minutes. Due to reflections on aircraft, weak signals on VHF and UHF can be heard up to about 550 km away in regions with permanent air traffic. This corresponds to an altitude of a good 10 km. These reflections can be detected using Doppler effects on strong, analog TV carrier signals.
Transequatorial Propagation (TEP)
Frequencies: up to 200 MHz, range: 4000 to 8000 km, signals: strong
TEP (trans equatorial propagation) made possible by reflection takes place on the ionosphere signal paths between stations that are located at the same distance from 2000 to 3000 km north and south of the (geomagnetic) equator . This usually takes place at frequencies between 30 and 70 MHz, in the sunspot maximum up to 108 MHz. Reception of signals above 220 MHz is extremely rare, but can go as high as 432 MHz (70 cm amateur radio band ).
The first major occurrence of TEP compounds in the VHF region was observed in 1957-58 during the maximum of the 19th sunspot cycle . Around 1970, the maximum of the subsequent 20th cycle, many connections between Australian and Japanese radio amateurs occurred. With the rise of the 21st cycle, signaling pathways appeared between southern Europe ( Greece / Italy ) and southern Africa ( South Africa / Zimbabwe ) as well as between central and southern Africa.
TEP cannot be used in Germany. However, there have been reports of isolated belt openings.
There are two different types of TEP:
- afternoon TEP, short aTEP (afternoon TEP)
- evening TEP, short eTEP (Abend-TEP).
Afternoon TEP (aTEP)
Afternoon TEP usually occurs between 2 p.m. and 7 p.m. local time and is usually limited to distances between 4,000 and 8,000 km. It occurs preferentially during the equinoxes (March / April or September / October). Usable frequencies are typically 40-55 MHz, occasionally as high as 60-70 MHz.
aTEP is traced back to areas of increased plasma density in the ionosphere symmetrically north and south of the geomagnetic equator. So there is a double reflection at the ionosphere.
Evening TEP (eTEP)
Evening TEP usually occurs between 8 p.m. and 11 p.m. local time and is usually limited to distances between 3,000 and 6,000 km. Frequencies up to 432 MHz were observed.
eTEP events are attributed to bubbles with a reduced plasma density , which extend symmetrically north to south of the equator and are aligned with the magnetic field lines running in the north-south direction. They rise in height, usually at speeds of 125 to 350 m / s, in peaks they were measured at up to 2 km / s. The diameter of the individual bubbles varies from 40 to 350 km. Higher frequency radio waves can penetrate the bubbles and propagate through them like a waveguide .
Short, medium and long wave
When receiving on these wavebands, we do not speak of overreach, as distant stations can already be heard under normal conditions. On shortwave , the spread changes constantly, depending on frequency, time of day, time of year and number of sunspots. With Sporadic-E on shortwave the distances are shortened, one speaks of short skip . On medium and long waves , strong signals usually reach about 500 km during the day and about 2000 to 3000 km at night. The cause is the ionosphere, which, depending on the time of day and solar activity, extends downwards to different degrees so that radio waves are attenuated, guided or reflected depending on their wavelength.
- The ARRL Handbook for Radio Communications. Newington, 2013.
- VHF portal Make More Miles on VHF with distance maps of overreaches
- FM station cards and station tables
- VHF TV working group: Club for the Hobby Overreach
- Photos from analog TV DX receptions
- Frank Helmbold: mirages. The propagation of ultrashort waves in the troposphere , in Hear Worldwide No. 2/1982, January 1982, accessed on June 20, 2020
- VK2KFJ’s TEP information: Transequatorial Propagation
- https://www.electronics-notes.com/articles/antennas-propagation/ionospheric/transequatorial-propagation.php Ian Poole: Transequatorial Propagation, TEP , accessed on 20th Jubi 2020