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== Radio wave propagation ==
[[File:Maszt_radiowy_w_Konstantynowie.jpg|thumb|The radio mast in Konstantynów, Poland.]]
Radio Wave Propagation deals with the behavior of radio waves when the waves travel.  It is sometimes known as simply wave propagation or propagation of electromagnetic waves.  Radio wave propagation is used in radio communication in order to transmit signals to short or long ranges. Along with this, it also finds applications in radar, direction finding, remote machine controlling, etc.

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====RF Propagation Properties====
The behavior of RF signals during propagation is influenced by several key properties, each of which contributes to the complexity of wireless communication systems:

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* Attenuation: As RF signals travel through space, they experience attenuation, which is the reduction in signal strength due to absorption, scattering, and divergence. Higher frequencies generally suffer more attenuation than lower ones, making them suitable for short-range communication.
* Fading occurs when there are variations in signal strength at the receiver due to constructive and destructive interference of multiple signal paths. This phenomenon can be caused by changes in the environment, such as moving obstacles or changes in atmospheric conditions.
* Refraction: When RF signals encounter a change in the density of the medium they are traveling through, such as transitioning from air to water or from one layer of the atmosphere to another, refraction can occur. This bending of the signal path is what is claimed leads to signal coverage beyond the line of sight.
* Diffraction: As RF signals encounter obstacles like buildings, hills, or other obstructions, they can diffract or bend around these obstacles. This phenomenon enables communication in non-line-of-sight scenarios.
* Scattering refers to the redirection of RF energy in various directions when it interacts with irregular surfaces or small objects. Scattering can contribute to multipath propagation and may cause signal degradation in urban environments.

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====Methods of Radio wave propagation====
Radio wave propagation is associated with the phenomena that occur when a wave travels between transmitter and receiver. However, the wave can travel between transmitter and receiver in two ways:

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* By propagating in free space
* By guided within a medium such as coaxial cable or waveguide.

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When the wave is allowed to propagate through free space, then during propagation, the available spectrum must be shared amongst the existing users in a proper manner. This includes providing services to the devices which are in close geographical proximity to each other.  In this case, there are high chances of interference between the devices operating at nearly the same frequency.  The environment through which the radio waves propagate includes discontinuities like hurdles or variations in medium parameters. For practical radio wave propagation, the earth and the objects present in its surroundings are highly considerable.

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When propagation takes place through a guided media then the signal is subjected to less attenuation with other users. Also, in this case, the user gets access to the full range of frequency band. When a wave propagates through the earth environment then the manner of wave propagation does not depend only on the properties of waves but also on the environmental conditions. The effect of the medium in which the waves propagate is of great importance as it accounts for the intermediary objects in the path.

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===Types of Propagation=== 
====Free Space Propagation====
In an ideal, open space environment, electromagnetic waves propagate in a straight line from the transmitter to the receiver.  This type of propagation is characterized by minimal obstacles or interference and is commonly used for long-distance communication, such as satellite communication.  In free space, these waves adhere to the [[Tech/Inverse-square law|Inverse-square law]], which dictates that the power density exhibited by an electromagnetic wave correlates directly with the inverse of the square of the distance from the originating point source.  In simpler terms, when the distance between a transmitter and a receiver is doubled, the resulting effect is a reduction in the power density of the radiated wave at the new location to just one-quarter of its former value.

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[[File:Ground-wave-propagation.jpg|200px|thumb|right|Ground Wave Propagation Diagram]]
====Ground Wave Propagation====
Definition: It is also known as surface wave propagation in which the radio waves transmit by passing through the semi-conductive surface of the earth. Here the transmission of waves takes place at a region close to the surface of the earth traveling beyond the horizon. This mode of propagation requires vertically polarized waves as the horizontally polarized waves in this case will get absorbed by the earth.

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A major disadvantage associated with ground wave propagation is that it is suitable only for short-range operation. This is so because the induced wave in ground wave propagation causes attenuation of the propagated signal. Therefore, in order to transmit the signal with the least attenuation, it is preferred that the signal is transmitted only to short ranges, in the case of ground wave propagation.

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[[File:Sky-wave-propagation.jpg|200px|thumb|right|Sky Wave Propagation Diagram]]
====Sky Wave Propagation====
Definition: A type of radio wave communication in which the electromagnetic wave propagates due to the reflection mechanism of the ionospheric layer of the atmosphere is known as sky wave propagation. Due to propagation through the ionosphere, it is also known as ionospheric wave propagation.

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The permissible frequency range in the case of sky wave propagation lies between 3 MHz to 30 MHz.

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Basically the electromagnetic waves in the range of 3 to 30 MHz get reflected by the ionosphere. '''However, the signals with frequency beyond 30 MHz despite undergoing reflection get penetrated.''' So, due to this reason, sky wave propagation is suitable only for this particular range of frequency.

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[[File:Tropospheric-scatter-propagation.jpg|200px|thumb|right|Tropospheric Scatter Propagation Diagram]]
====Tropospheric Scatter Propagation==== 
It is sometimes called forward scatter propagation or scatter propagation and is suitable for VHF, UHF and microwaves. In this, the waves propagate through forward scattering due to the irregularities of the troposphere. This propagation technique uses the properties of the troposphere.  This mode offers reliable communication between 160 km to 1600 km (99 to 994 miles).

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This popular belief of an ionosphere became mainstream, and has continued to setback science as did Einstein, as the ionosphere has never been actually empirically and independently validated and replicated the only evidence that the ionosphere exists there is that it must exist there for the earth to be a globe.

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====Tropospheric Ducting ==== 
Tropospheric ducting is a phenomenon where atmospheric conditions, such as temperature inversions, allow radio waves to magically follow the "curvature" of the Earth for extended distances within the troposphere. While tropospheric ducting can enhance the propagation of VHF (Very High Frequency) and UHF (Ultra High Frequency) signals, it is not typically responsible for radio waves at 105 MHz or higher frequencies to propagate around a "globe".

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Tropospheric propagated signals travel in the part of the atmosphere adjacent to the surface and extending to some 25,000 feet (8 km). Such signals are thus directly affected by weather conditions extending over some hundreds of miles. During very settled, warm anticyclonic weather (i.e., high pressure), usually weak signals from distant transmitters improve in strength. 

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A settled high-pressure system gives the characteristic conditions for enhanced tropospheric propagation, in particular favoring signals which travel along the prevailing isobar pattern (rather than across it). Such weather conditions can occur at any time, but generally the summer and autumn months are the best periods. In certain favorable locations, enhanced tropospheric propagation may enable reception of ultra high frequency (UHF) TV signals up to 1,000 miles (1,600 km) or more. 

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====Line-of-Sight (LOS) Propagation====
LOS propagation occurs when the transmitter and receiver have a direct line of sight with minimal obstacles between them. This type of propagation is crucial for microwave and millimeter-wave communication, often used in point-to-point communication and cellular networks. This method of transmission finds application in medium-distance radio communication, such as cell phones, cordless phones, walkie-talkies, wireless networks, FM radio, television broadcasting, radar, and satellite communication (e.g., satellite television). The distance of line-of-sight transmission at ground level is limited by the visual horizon, which relies on the heights of both transmitting and receiving antennas. Notably, this propagation technique is exclusively viable for microwave frequencies and higher frequencies.

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====Multipath Propagation====
In urban environments or areas with obstacles, RF signals can encounter reflections, diffractions, and scattering, leading to multipath propagation. This phenomenon can cause signal interference and fading, but it's also harnessed in technologies like MIMO (Multiple-Input Multiple-Output) to improve data rates and reliability.

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===Radio Frequency Bands===
Understanding the intricacies of RF propagation is essential for designing robust and reliable wireless communication systems. Different types of propagation and their associated properties have profound effects on signal coverage, interference, and overall system performance. As technology continues to advance, engineers and researchers must continue to explore and refine our understanding of RF propagation to meet the ever-increasing demands of wireless communication.

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{| class="wikitable"
|+ Frequency, Wavelength, and Band Name of RF Signal and Associated Propagation Methods
|-
! Band !! Frequency !! Wavelength !! Propagation Method / Usage
|-
| ELF || 3–30 Hz || 100,000-10,000 km<br>62,137-6,213 miles || Guided between the Earth and the ionosphere.  This frequency band is used for underwater communication, especially for pipeline transportation.
|-
| SLF || 30–300 Hz || 10,000-1,000 km<br>6,213-621 miles || Guided between the Earth and the ionosphere.  Used for submarine communications and in the electric grid (not as a transmitted wave).
|-
| ULF || 0.3–3 kHz || 1,000-100 km<br>621-62 miles || Guided between the Earth and the ionosphere.  Used for mining communications and military applications.
|-
| VLF || 3–30 kHz || 100-10 km<br>62-6.2 miles || Guided between the Earth and the ionosphere and ground wave propagation.  As this band of frequency exhibits penetration properties through dirt and rock, it is used for geophysics applications, navigation, wireless heart monitoring, etc.
|-
| LF || 30–300 kHz || 10-1 km<br>6.2-0.6 miles || Guided between the Earth and the ionosphere and ground waves propagation.  In Europe and some parts of Asia, the LF band is used in AM broadcasting. Other LF band applications include RFID, amateur radio, and navigation.
|-
| MF || 300–3000 kHz || 1000-100 m<br>3,280-328 feet || Ground waves propagation and slight ionospheric refraction.  This frequency band covers AM broadcasting, coast-to-sea communication, emergency distress signals, etc.
|-
| HF || 3–30 MHz || 100-10 m<br>328-32 feet || Ionospheric refraction.  This band is also called the short wave band. It is most useful in aviation communication, amateur radio communication, and weather broadcasting applications.
|-
| VHF || 30–300 MHz || 10-1 m<br>32-3.2 feet || * Line-of-sight propagation.  This band is used for analog television broadcasting, FM radio broadcasting, medical equipment utilizing magnetic resonance imaging, mobile-land, and marine communication systems.
|-
| UHF || 300–3000 MHz || 100-10 cm<br>39-3.9 inches || * Line-of-sight propagation.  This frequency band is significant in modern wireless communication systems with applications in satellite television, WiFi, GPS, Bluetooth, television broadcasting, mobile communications such as GSM, CDMA, and LTE services.
|-
| SHF || 3–30 GHz || 10-1 cm<br>3.9-0.39 inches || * Line-of-sight propagation.  Modern communications technologies, modern radars, DTH services, 5GHz Wi-Fi channel, radio astronomy, mobile networks, TV broadcasting satellites, microwave devices, broadcasting satellites, and amateur radio are some of the applications of SHF.
|-
| EHF || 30–300 GHz || 10-1 mm<br>0.39-0.039 inches || ** Line-of-sight propagation, limited by atmospheric absorption to a few kilometers (miles).  EHF is used in radio astronomy, amateur radio, remote sensing at microwave frequency, and high-frequency microwave relays.
|-
| THF || 0.3–3 THz || 1-0.1 mm<br>0.039-0.0039 inches || ** Line-of-sight propagation, limited by atmospheric absorption to a few meters.  THF is utilized as an alternative to X-ray and is used in TeraHertz frequency imaging. Other applications include terahertz space-time spectroscopy, solid-state physics, and terahertz computability.
|}
<nowiki>*</nowiki> ''Line-of-sight propagation means the wave is not expected to be able to follow a curve nor propagate around and behind objects such as mountains.''<br>
<nowiki>**</nowiki> ''Limited by atmospheric absorption means the radio wave will not bounce off the atmos back down to the surface.''
[[File:Electromagnetic spectrum.jpg|350px|Electromagnetic spectrum]]

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===The Evolution of Radio wave propagation===
{| class="wikitable"
|-
! Year !! Achievement
|-
| 1873 || Scottish physicist James Clark Maxwell publishes his Treatise on Electricity and Magnetism, the first time anyone laid out a solid mathematical case for the existence of electromagnetic waves.
|-
| 1885-1889 || German physicist Heinrich Hertz successfully proves the existence of electromagnetic waves through a series of experiments at the University of Kalsruche.  Despite this incredible finding, Hertz's interest in radio waves was purely scientific; he did not consider his discovery to be of any practical or commercial significance.
|-
| 1890 || The Coherer, the worlds fist functional radio signal receiver is invented by Edouard Branly in France.  While not very reliable, the development of the Coherer was a major milestone in the development of radio wave technology which allowed more advanced experiments to take place in the coming years.
|-
| 1894 || A young amateur inventor named Guglielmo Marconi conducts his first experiments with radio wave transmission in his parents' attic in Italy. Heinrich Hertz dies this same year.
|-
| 1897-1899 || Marconi relocates to England and puts on many successful demonstrations of short and medium-distance "wireless telegraphy" via radio waves. In 1889, he successfully transmits radio signals across the English Channel.
|-
| '''1901''' || Italian physicist and radio pioneer Guglielmo Marconi succeeds in sending the first radio transmission across the Atlantic Ocean, disproving detractors who told him that the curvature of the earth would limit transmission to 200 miles or less. The message–simply the Morse-code signal for the letter “s” traveled more than 2,000 miles from Poldhu in Cornwall, England, to Newfoundland, Canada.
|-
| 1904 || Frustrated by the unreliable performance of the transmitters and receivers in their experiments, Fleming perceives the solution might lie in the design of Edison's incandescent bulbs. He adapts the design to develop a two-electrode valve inside a vacuum bulb. It only passed current in one direction, and when used with a galvanometer as part of a tuned electrical circuit, it could successfully detect high-frequency radio wave currents. The diode valve or "Fleming Valve" drastically improves radio performance.
|-
| 1906 || American inventor Lee de Forest develops the "Audion" triode, a Fleming Valve with an electrode grid between the plate and filament to control the current. In 1912, the triode's ability to not only receive current but amplify it was discovered. This leads to the development of amplitude modulation (AM) transmission, which in turn allows the development of loudspeakers. For the first time, multiple people can listen to radio signals without headphones.
|-
| 1914-1919 || Civilian radio activity suspended in Europe. Radio communication taken over for government and military use, and continues to develop and advance.
|-
| 1920s || The "Golden Age of Radio". Civilian and commercial use of radio begins. Superheterodyne ("superhet") radios become available to consumers. Marconi's company begins daily programming and the British Broadcast Company (BBC) is formed. Radio becomes the dominant electronic medium for news, sports, and entertainment.
|-
| 1930s || Bliley Piezo-Electric Company (precursor to Bliley Technologies, Inc.) opens its doors and begins quartz crystal manufacturing for the amateur radio industry.
|-
| 1939 || Robert Watson-Watt develops the "Chain Home" network of radio towers for the British military, capable of detecting incoming enemy planes up to 100 miles away by transmitting the radio waves and calculating the time it took for the waves to bounce off an object and return to the receiver. US Navy adopts the technology and coins the acronym "RADAR" (Radio Detection and Ranging).
|-
| 1939-1945 || World War II leads to major advances in radio wave technology. Radio communication plays an integral role in military and government operations on all sides. Jamming and interception leads to the development of encryption and early forms of electronic warfare. The first experimental radio-controlled unmanned aerial vehicles are designed.
|-
| 1945 || Arthur C. Clarke publishes article proposing the idea of geostationary satellites which broadcast radio wave signals all over the world to facilitate "global" communications.
|-
| 1950s || The first commercially viable transistors available for industrial use. Televisions become available.
|-
| 1957 || Soviet Union launches the Sputnik satellite into orbit, prompting the launch of the Space Race between the US and USSR.
|-
| 1962 || Telstar satellite goes online, and successfully relays telephone calls and television broadcasts "around" the world.
|-
| 1960-70s || Diodes and triodes are phased out of most consumer electronics in favor of transistors. Mobile telephone systems in vehicles are developed, and the first wireless call on a prototype handheld mobile phone is placed by Motorola executive Martin Cooper in 1973. Electronic Countermeasures (ECM) and Electronic Counter-Countermeasures (ECCM) continue to advance at a rapid rate. "Global" (Ground) Positioning System (GPS) goes online in 1978.
|-
| 1980s || Ameritech launches the DynaTAC 8000X, the first commercially available cellular phone. Satellite TV becomes popular. GPS approved for civilian use.
|-
| 1990s || Cellphone use continues to rise, and the first 2G networks go online. GSM and DCMA standards are developed. First SMS text message with the words "Merry Christmas" is sent over the Vodaphone network in 1992.  Bluetooth technology developed by Ericsson.
|-
| Mid/Late 90s || The Gnat, a radio-controlled surveillance drone with a long-range mounted video camera, is launched.  First version of WiFi (IEEE 802.11 protocol) is released. Satellite phone service becomes available.
|-
| 2000-2010 || Northrup Grumman releases the Predator UAV, a weaponized update to the Gnat drone, in 2001.  First 3G networks go online, allowing high-speed wireless internet and streaming video via smartphone.  Wi-Fi and 3G-equipped tablets such as the iPad become popular.  4G networks become commercially available by 2009.  XM and Sirius Satellite radio services are launched and compete for listeners before merging in 2008.
|-
| 2010-Present || 4G speeds set a new standard for performance expectations in PEDs. The Internet of Things (IoT) and big data collection and analytics revolutionizes and disrupts dozens of industries. Spectrum crowding leads to commercial expansion into the Ku and Ka bands. Small UAVS become available for personal and commercial use.
|-
| 2017+ || 5G networks facilitate high-speed connectivity across the loT. In-Flight Entertainment and Connectivity (IFEC) options are greater than ever before, and driverless vehicles and cargo drones change the shipping industry. Electronic warfare tech advances at a dizzying rate and satellite constellations beam down Wi-Fi all over the developing world, bringing hundreds of millions more people online.
|}

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===Going the distance===
====1901====
Marconi's transatlantic transmission won him worldwide fame.  Detractors of the project declared that radio waves would not follow the curvature of the earth, as Marconi believed, and would only travel about 200 miles, however Marconi's first attempt was successful in transmitting over '''2,000 miles'''.  This lead to advances in both science and technology.  

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Since this success demonstrated that radio transmission was not bounded by the horizon, it prompted Arthur Kennelly and Oliver Heaviside to suggest, shortly thereafter, the existence of a layer of ionized air in the upper atmos (the Kennelly-Heaviside layer, now called the Ionosphere).  

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Marconi's technology also gave the new technology of "wireless telegraphy" a "global" dimension that made radio one of the major forms of communication in the twentieth century.

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====1979====
World's longest microwave link was a hop commissioned decades ago in 1979. The laws of physics have not changed. This record setting link was '''360 km (223 mile)''' long and crossed the Red Sea over a good part of its path connecting Jebel Ebra, Sudan with Jabal Dakka, Saudi Arabia. This microwave link was deployed by Telettra using the '''2 GHz''' band and clearly some excellent engineering. 

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====2015====
Two Amateur Radio microwave enthusiasts in Australia are claiming a new distance record on '''10 GHz'''.  On January 5, during a tropo opening across the Great Australian Bight, VK6DZ and VK7MO exchanged reports over a '''2,732 km (1,697 mile)''' path, using JT4f mode as well as SSB. The distance surpasses by 36 km the previous World Record of 2,696 km (1,675 mile) from Southern Portugal to Cape Verde Island.

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VK6DZ was portable at Torbay Hill, 24 km west of Albany, Western Australia. He was running 10 W to a 60 cm dish. VK7MO was portable Cape Portland in northeastern Tasmania, running 50 W to a 77 cm dish.

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Also in 2015, Two California radio amateurs — one of them in Hawaii — have set new world distance records on the '''2.3 and 3.4 GHz''' microwave amateur bands. Wayne Overbeck, N6NB, operating from a radio-equipped rental car on the big island of Hawaii, worked Gregory Campbell, W6IT, operating Overbeck’s own fixed station near Orange, California, on both bands — a distance of more than '''4,024 km (2,495 miles)'''. 

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The North American distance record on '''47 GHz''' is '''344.8 kilometers (213.8 miles)''', set in 2015.

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====2016====
Exalt Wireless, the leading innovator of next-generation wireless connectivity systems for private networks and Internet infrastructures, today announced that its ExploreAir LR '''7 GHz''' microwave systems have been used to establish a reliable, high-speed link spanning what it believes is a record-breaking '''235 kilometers (146 miles)''' over water.

===Radio wave Flat Earth Observations===
{| class="wikitable"
!Observation
!Distance
!Observer Height
!Target Height
!Globe Predicted Horizon Distance
!Globe Predicted Hidden by Horizon
!Relevant Axis Mismatch
|-
|Knickebein Stollberg Hill Beam #1
|413 miles (665 km)
|141 feet (43 m)
|19,200 feet (3.64 mi)
|14.54 miles (23.40 km)
|20.1 miles (32.3 km)
|Y-axis
|-
|Knickebein Beam #2 Kleve
|325 miles (523 km)
|1,145 feet (349 m)
|19,200 feet (3.64 mi)
|41.4 miles (66.6 km)
|10.1 miles (16.3 km)
|Y-axis
|-
|Marconi Wireless Telegraph  UK<>Australia
|10,553 miles (16,983 km)
|850 feet (260 m)
|300 feet (0.057 mi)
|35 miles (56 km)
|DOES NOT COMPUTE
|Y-axis
|-
|Admiral Byrd Little America to Riverhead,  NY
|9,000 miles (14,000 km)
|1,000 feet (300 m)
|1,000 feet (0.19 mi)
|38.75 miles (62.36 km)
|DOES NOT COMPUTE
|Y-axis
|-
|OTH Radar
|3,400 miles (5,500 km)
|1,000 feet (300 m)
|40,000 feet (7.6 mi)
|38.75 miles (62.36 km)
|2,032.9 miles (3,271.6 km)
|Y-axis
|}

==== Microwave Flat Earth Observations ====
{| class="wikitable"
!Observation
!Distance
!Observer Height
!Target Height
!Globe Predicted Horizon Distance
!Globe Predicted Hidden by Horizon
!Relevant Axis Mismatch
|-
|El Aguila to Platillón
|237.3 miles (381.9 km)
|4,200 metres (13,800 ft)
|1,500 metres (4,900 ft)
|232 kilometres (144 mi)
|1,719 metres (5,640 ft)
|Y-axis
|}

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===Gallery===
<gallery>
Coherer.jpg|1890: The coherer was a primitive radio signal detector
Triode_tube_1906.jpg|1906: The Audion triode, the first triode.
Marconi's_first_radio_transmitter.jpg|Marconi's first radio transmitter
Guglielmo_Marconi.jpg|1909: Guglielmo Marconi
Guglielmo_Marconi_1901_wireless_signal.jpg|Marconi demonstrating apparatus he used in his first long-distance radio transmissions in the 1890s. The transmitter is at right, the receiver with paper tape recorder at left.
East-West Effect on VLF transmission.png|Article clip: East-West effect on VLF transmission across earths magnetic field 
</gallery>

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===See Also===
* [https://www.arrl.org/distance-records arrl.org: distance records]
* [https://www.arrl.org/news/new-world-distance-records-set-on-2-3-and-3-4-ghz-ham-bands arrl.org: new world distance records]
* [[:File:ARRL Distance-Records-19-May-2023-Rev.pdf|ARRL RF Distance Records May 19, 2023]]
* [https://isp.today/what-is-the-longest-range-of-a-ham-radio/#gsc.tab=0 isp.today: longest range of ham radio]
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