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Dr David Maddison UNDERGROUND communications Communicating between people underground, or below and above ground, is challenging because rock and soil usually absorb the radio waves used to carry voice signals. In this article, we investigate the Image Source: https://unsplash.com/photos/5p-3r7kBhKc solutions to these problems. T he underwater communications discussed last month primarily concentrated on submarines and other submersibles. Underground, there are a wider variety of locations, including mines, tunnels and cave systems where people might need to communicate with each other or with the surface world. There are also cases like avalanches where people might be buried in snow, creating similar challenges. Even the seemingly unrelated issue of radio communications in aircraft cabins shares some of the same technology and solutions. We’ll start by describing some of the concepts used in all of these scenarios. Radiating feedlines Radiating feedlines, also known as ‘leaky coax’ or ‘leaky feeders’, are important for communications underground or in any enclosed area shielded from radio transmitters. They can be used in caves, tunnels, mines, car parks and even inside aircraft or ships. A radiating feeder is like an 16 Silicon Chip imperfect coaxial cable with slots or gaps fabricated into the shield (outer) wire, allowing electromagnetic radiation to escape – see Fig.30. This is the opposite of a regular coaxial cable, which is designed to contain or block as much electromagnetic radiation as possible. Because signal strength is lost in a signal conducted along a radiating feeder, the signal needs to be boosted with an amplifier at regular intervals, every 350-500m or so. An application of radiating feedlines that most readers would be familiar with is in road tunnels such as the Sydney Harbour Tunnel, Lane Cove Tunnel, Burnley Tunnel, AirportlinkM7, Northbridge Tunnel etc. In most of these tunnels (and others in our capital cities), radio and mobile reception operate normally, even when you’re a kilometre or more from either end of the tunnel. Road and rail tunnel communication Radio signals do not travel very far into tunnels. AM broadcast signals Australia's electronics magazine have wavelengths between 175m and 555m, so they will not travel far into a tunnel, given that its diameter will be much smaller than those wavelengths. FM broadcast signals with wavelengths between 2.8m and 3.4m can travel through a sufficiently wide tunnel, but for the signal to enter the tunnel cleanly, it would need to be line-of-sight from inside the tunnel; a reflected signal from outside would be much weaker. The signal would also be largely absorbed as it bounced off the tunnel surfaces multiple times unless the tunnel was perfectly straight and had a clear ‘view’ of the transmitter. DAB frequencies range from 1.3m to 1.6m and behave similarly to FM broadcast signals. Mobile phone telephone signals have even smaller wavelengths, from 43cm down to millimetres for 5G. They could travel some distance through a tunnel provided it had line-of-sight to the transmitter and the tunnel was perfectly straight. Those conditions are rarely met, so radio contact is usually maintained inside a tunnel via ‘rebroadcasting’. siliconchip.com.au Rebroadcasting commercial AM, FM and DAB channels improves driver satisfaction and reduces distraction by not having their favourite radio program interrupted. Given the expensive tolls we pay to use these tunnels, it’s the least they could do! Such rebroadcast systems generally also have a feature called ‘audio break-in’ so that emergency or service announcements can be made over all radio programs being rebroadcast, regardless of which channel the vehicle’s radio is tuned to. In an emergency, signs will usually come on overhead that read “turn on your radio” (or similar) so drivers can be advised of the best course of action. Passive versus active rebroadcasting Radio signals can be rebroadcast either passively or actively. Passive rebroadcast (see Fig.31) involves connecting an external antenna to one or more internal antennas to rebroadcast the signal in a different direction; in this case, through the tunnel. For shorter wavelengths, like FM or DAB, this could be Yagi antennas mounted at intervals in the tunnel. For longer wavelengths, it could be a leaky feeder. Passive rebroadcasting is only suitable for straight tunnels with line-ofsight to the rebroadcasting antenna(s); signal splitters are required for more than one antenna, in which case the signal would be excessively weakened. However, such a signal could be amplified in the same way it is in, say, an apartment block with one antenna and many outlets. More commonly, active rebroadcasting is used. Receivers pick up and decode the signals using antennas outside the tunnel. They then feed the decoded signals (eg, audio) through audio break-in electronics to amplifiers and transmitters that re-radiate it at the original frequencies using antennas throughout the tunnel. See Fig.32 for a typical setup. Depending on the rebroadcast unit(s) and setup, it is possible to have AM and FM broadcast, DAB, VHF/ UHF/800MHz paging and two-way radio access in a tunnel. For mobile phones, it’s usually easier to install small mobile cell ‘towers’ throughout the tunnel linked back to the backhaul network rather than trying to preserve two-way siliconchip.com.au Fig.30: cutaway views of various radiating cables offered by Exlanta (http:// exlanta.com). Fig.31: a passive repeater as used in some tunnel installations. An outside signal is picked up by a Yagi antenna, connected to another Yagi antenna that redirects the signal into the tunnel. No electronics or power is required. For this type of installation to work, the tunnel would have to be straight with line-ofsight to the rebroadcasting antenna. Fig.32: an example of a tunnel with radiating feedline and ancillary equipment. Original source: https://alliancecorporation.ca/manufacturer/rfs-radiofrequency-systems/ Australia's electronics magazine April 2023  17 Fig.33: the radiation pattern of an EION Tunnel WiFi access point with a helical antenna. Original source: www.eionwireless.com/ assets/images/documents/datasheets/Tunnel-WiFi-Oct-14.pdf EION Tunnel WiFi Antenna Coverage Pattern Fig.34: the HeyPhone uses a ground dipole antenna and transmits 87kHz USB at ~10W. Source: https://bcra.org.uk/creg/ heyphone/pdf/heyphone-usermanual.pdf communications between phones in the tunnel and towers outside it. The phones are ‘handed off’ between the towers inside and outside the tunnel, just like they would be when moving between standard towers. Apart from tunnels, such systems can be used in other underground structures such as car parks, mines, and inside buildings where reception might be poor due to metal film on the windows or for other reasons. WiFi in tunnels WiFi can be installed in tunnels and other underground spaces. The most efficient way to do this in tunnels is to use WiFi access points with specially-­ designed helical antennas that have an extended radiation pattern in the direction of the tunnel, rather than a traditional circular pattern. Purpose-built access points are available for this usage from EION Inc – see Fig.33. Cave communications For cave radio, radio is transmitted through the earth (TtE) or via direct line-of-sight (LoS) with relays or multiple ever-weakening reflections. Regular radios can be used in caves for short hops with line-of-sight, but they are rarely suitable as caves rarely have many long and straight passages. Radio can also be transmitted and received via radiating feedlines but, of course, that involves running a wire, as does conventional one-wire (with earth return circuit) or two-wire telephony. The Molefone (TtE) The Molefone (Fig.35) was a radio developed for cave rescue and general use by Bob Mackin of Lancaster University in the 1970s, and used extensively in the 1980s and beyond. It used a multi-turn loop antenna of about 41cm diameter and could achieve a range of about 150m-200m through rock at 10W. It operated on 87kHz USB (upper side-band). No circuit diagrams are available. 87kHz became standard for other cave communications systems, such as the HeyPhone and System Nicola (both mentioned below), to retain compatibility. They are not being made now due to the unavailability of certain components and the resulting inability to repair failed units. The HeyPhone (TtE) Fig.35: operating a Molefone in the Matienzo Caves, Spain. Note the loop antenna made of computer ribbon cable. Source: http://matienzocaves.org.uk/ ugpics/2366-2007e-molep.htm 18 Silicon Chip Australia's electronics magazine The HeyPhone (https://bcra.org.uk/ creg/heyphone/ & Fig.34) was designed by John Hey and is something of a replacement for the Molefone. The British Cave Rescue Council (BCRC) initiated the project in conjunction with John Hey after a meeting in 1999. Unlike the Molefone, the HeyPhone uses a ground dipole as its primary antenna rather than a loop, although it is also capable of utilising loops. The ground dipole comprises two earthed electrodes 25-100m apart. Ground dipole antennas have greater siliconchip.com.au penetration than the loop antennas used by the Molefone. Like the Molefone, the HeyPhone used 87kHz USB at about 10W, and the two radios were compatible. This project is now no longer active or supported, but if you are an experimenter, you can obtain circuit diagrams and other documentation to build your own: https://bcra.org.uk/creg/heyphone/ documentation.html You can also get a user manual for the device at: https://bcra.org.uk/creg/heyphone/ pdf/heyphone-usermanual.pdf HeyPhones were said to be used in the Tham Luang cave rescue (Thailand; June-July 2018), along with Maxtech mesh radio units (see below). System Nicola (TtE) Following the death of Nicola Dollimore in a caving accident in 1996, funds were collected to make the “ultimate cave radio”. It was a collaborative effort between the French, Swiss and British and based on the HeyPhone. The Mk2 was released in 1998 and is the system used throughout France. The Mk3 digital version was developed in the early 2000s, while the Mk4 is currently under development; see Fig.36. The Mk2 radio operates at about 87kHz & 3W with USB modulation. The ground dipole antenna uses two electrodes in the earth about 40m80m apart. The through-rock transmission distance is about 500m-1200m, depending on conditions. Unfortunately, there is little information on this radio. System Nicola does not have a website, but they do have a Facebook page, www.facebook. com/AssociationNicola/ Cave-Link (TtE) Cave-Link (www.cavelink.com/ cl3x_neu/index.php/en/) is a throughthe-earth cave communications system that uses VLF frequencies to conduct text data transfer, not voice, to a depth of 1300m or possibly more. The above-ground part of the system, which the manufacturer calls an ‘earth current modem’ (see Fig.37), can also be connected to the mobile phone system to transfer SMS messages. Some European cave rescue organisations use Cave-Link and it is also used for data logging from sensors located inside caves (eg, water flow, siliconchip.com.au water depth, temperature, CO2 level, pH, pressure etc). It operates between 20kHz and 140kHz using 4PSK (quadrature phase shift keying) modulation and the ARQ (automatic repeat request) error correction protocol. The antennas on the surface and in the cave consist of two metal plates, each connected to one conductor of the feedline from the transmitter or receiver, buried in the ground connected by a cable. This forms an antenna known as a ground dipole (see Fig.16 from last month). The distance between the plates corresponds to a vertical depth of transmission approximately ten times the horizontal distance between the plates. Fig.36: two Nicola Mk4 radios (stacked on each other), which are currently under development. Source: System Nicola Facebook page HF Radio (TtE) HF radio has some capability of penetrating the earth, primarily through dry rock in arid regions. Some experiments have been done at 1.8750MHz using an Elecraft KX3 transceiver (see https://youtu.be/WTnrDwIPKrI). Other experiments reported are: 1. Paul Jorgensen, KE7HR, with an FT817ND transceiver on 3.9MHz SSB and 5W, demonstrated voice communication to a depth of 238m in Carlsbad Cavern, New Mexico, USA. 2. In 2015, the UK Cave Radio and Electronics Group communicated to a depth of 100m with a slant distance of 692m using 20W at 7.135MHz SSB with an IC-706 transceiver. 3. The BCRA Cave Radio and Electronics Group Journal 97, March 2017, reported the reception of 7MHz WSPR signals (weak signal propagation reporter, described in our article on Digital Radio Modes) 100m underground in the UK from nine countries. Fig.37: a Cave-link terminal for sending text data via VLF through the earth. Source: https://expo.survex. com/expofiles/documents/hardware/ Cavelink2.13_en_2014-3.pdf Two-wire telephones (wired) Cavers used surplus army two-wire field telephones in the past. However, they have mostly been replaced by single-­wire telephones or Michiephones. Single-wire telephone (wired) A single-wire cave telephone, also known as a Michiephone, uses only one wire instead of the two used by classic analog telephones. The return circuit is through the earth (see Fig.38). With one wire, the spool weighs less and it is easier to deploy the wire. They work for days on batteries; see www.speleonics.com.au/business/ michiephones/ Australia's electronics magazine Fig.38: Jill Rowling from Speleonics using a single-wire Michiephone. Source: www.speleonics.com. au/business/ (reproduced with permission) April 2023  19 Fig.40 shows the classic circuit for a typical device, designed by Australian Neville Michie in the 1970s. They are very simple, the main component being an operational amplifier. Speleonics is an Australian manufacturer of these devices, although they do not appear to be making any at the moment. The main difference between the device they manufacture and the original design is that theirs also has a filter to remove 50Hz mains hum. VHF & UHF Mesh Radio (LoS) Fig.39: the Entel/Maxtech MaxMesh SDR radio, as used in the Thai cave rescue. Source: www.entelkorea. com/assets/resources/brochures/ HT786-MaxMesh.pdf During the 2018 rescue of a Thai youth soccer team trapped in a cave, rescuers established communications with equipment flown from Israel, made by Maxtech Networks (https:// max-mesh.com/). The equipment fit in one suitcase and comprised walkie-­ talkie-like software-defined radios (SDRs). Either 17 or 19 radios were brought (depending on which report you read) but only 11 were ultimately used to establish a communications link 4km into the cave by forming a mesh network. Maxtech produced the mesh software, while UK-based firm Entel produced the radio platform (Fig.39). The radios operate in the VHF and UHF ranges (225MHz-470MHz). Without a mesh network, communications in a cave between two radios at these frequencies would be line-of-sight or via a limited number of reflections around corners. However, in a mesh network, each radio can act as a relay station for the next one. Individual radios still communicate with each other via line-of-sight or reflections. Despite this, a radio at the start of the network of radios (eg, at the tunnel entrance) can seamlessly communicate with a radio at the far end. Each consecutive radio in the mesh network passes the message on to the next, even though there is no direct link between the communicating radios (first and last). Audio and video communications were established for the cave rescue using 11 radios (siliconchip.au/link/ abir), each with a battery life of 10 hours. In certain places, the only path was through water, so they laid underwater data cables to connect pairs of software-defined radios. The mesh network established by the radios was self-forming, self-­ routing, self-healing and required no other infrastructure. It was a ‘mobile ad hoc network’ (MANET) and used a time division multiple access (TDMA) Media Access Control (MAC) scheme with an innovative routing algorithm. Note that these radios are not explicitly designed for cave rescues; they would be helpful in any hostile environment, such as in collapsed buildings after an earthquake. Fig.40: the Michiephone circuit as produced by Speleonics. The microphone used is extremely hard to get; it is from an old-style telephone handset, and there is no modern replacement. Original source: www.speleonics.com.au/ business/ 20 Silicon Chip Australia's electronics magazine siliconchip.com.au The system can also establish gateways to 3G and 4G phones, analog radios and other networks. For more information, see the video at siliconchip.au/link/abis and the one titled “Maxtech networks video over radio” on YouTube at https://youtu.be/ C2q9L8iAOyA UHF Mesh Radio (LoS) The video “Underground & ThroughThe-Earth Communications” at https:// youtu.be/WTnrDwIPKrI describes an experimental mesh network made of Ubiquity M2 (2.4GHz) and M900 (900Mhz) MIMO (multiple-input and multiple-output) wireless bridges using custom firmware from http:// hsmm-mesh.org/ The result was an IEEE 802.148 mesh network for cave communications. Voice entered the cave via an HF radio link and then was digitised and transmitted through the cave. Fig.41: the results of an APRS UHF radio test in Mammoth Cave, USA, showing the location of radios (numbered in blue) in the cave system, the communications path in red and distances in feet (700ft = 213m). Source: www.aprs.org/cave-link.html APRS (LoS) Using APRS (Automatic Packet Reporting System) radios in caves is also possible. APRS is an amateur radio protocol, so it is not currently available for general cave use, but ham operators who are also cavers are exploring its use. As per mesh radio networks discussed above, the VHF and UHF frequencies are line-of-sight only or via limited reflections. Unlike the Maxtech radios, only data can be transmitted with APRS. Like Maxtech, individual radios can act as repeater stations (‘digipeaters’) for several radios in a chain. An experiment was performed with APRS radios on the 2nd-3rd of March 2013 in Mammoth Cave, Kentucky, USA, the world’s longest known cave system (Fig.41). It was found that for VHF radios, the average hop length was 119m with a maximum of 162m. For UHF, the average hop length was 134m with a maximum of 207m. Fig.42: a MagneLink unit alongside a miner. Source: www.teslasociety.ch/info/ magnetlink/2.pdf They also found that signals would go around a 90° bend in the cave passage without a significant difference in range compared to a straight section. Increasing the power to 50W did not make much difference compared to 5W or less; even ½W was satisfactory. The cave passages were reasonably large, about 9m to 15m wide and 3m to 6m tall. Radiating feedline in caves (wired) Like mines and tunnels, a radiating feedline can be used in caves to enable Using 87kHz through-the-earth comms in Australia Even though 87kHz through-the-earth communications has been established as an international standard for cave rescue communications, it is apparently not approved by ACMA (the Australian Communications and Media Authority) and would be illegal to use in Australia for that purpose. That is why Speleonics only produces the wired Michiephone device and not wireless devices. As it is an international standard and the risk of interference is low-to-nonexistent, ACMA should revisit their objection to such usage and make an exception, at least for cave rescue or exploration purposes. siliconchip.com.au Australia's electronics magazine normal radio operation within line-ofsight of the wire. Such an arrangement would typically be used in tourist caves; however, feedlines have been used experimentally in other caves. Due to the high cost of purpose-made radiating cable, with the experiment described in the PDF at siliconchip. au/link/abit, the objective was to find a cheap substitute for the expensive purpose-made cable. They discovered that low-cost domestic satellite cable was sufficiently leaky (unintentionally) to be useful for this application. Communications in mines Wireless radio communications in mines may be through the earth, via radiating feed lines or wired telephone systems. MagneLink (wireless, through the earth) Magnetic Communication System (MCS) by Lockheed Martin (see Fig.42) is an emergency communications system used in mines to communicate April 2023  21 Mine Emergency Responder Loop antenna on surface MCS Rescue Team Loop antenna in mine entry MCS MCS – strategically positioned along escape routes or with emergency refuge shelters Fig.43: the MagneLink Magnetic Communications System (MCS) in a rescue scenario. Source: www.teslasociety.ch/info/magnetlink/2.pdf with trapped miners and rescue teams that provides two-way voice and text. Trapped miners with access to a MagneLink can activate it to send out a beacon signal, helping emergency teams find the trapped miners. It can be used either vertically between the ground and the mine, or horizontally along a mine tunnel with a blockage – see Fig.43. The system uses loop antennas, so communication is via the magnetic field component of a radio signal rather than the electric field component (see Fig.44). This allows much smaller antennas to be used rather than the alternative type, the ground dipole, which might need to be tens or hundreds of metres long. The part of the system installed in the mine is intended to be kept in designated locations such as ‘refuge areas’. The loop antenna is wrapped horizontally around a mine structure, such as a support pillar (an unexcavated area for roof support). In tests, the MagneLink system has achieved communication depths of radio signal 457m for voice and 610m for text. Radiating feedlines in mines (wired + wireless) Radiating feedlines (leaky feeders) work in mines much as they do in other locations such as tunnels. They are designed for bidirectional communications using handheld devices. On the surface or at some other command centre, a base station is responsible for sending and receiving transmissions (see Figs.45 & 46). There are also amplifiers about every 350-500m, and power for these can be carried by the feedline itself, typically at 12V. Frequencies used are usually in the VHF and UHF bands. The basic building block of a radiating feedline in a mine is a power cell, with one cell per section of a mine. Many power cells may be connected together. Having many cells provides redundancy in case of damage to one section – see Fig.48. Nodes/mesh (wireless) Another way handheld radios can Fig.44: how MagneLink and other through-the-earth communications systems that use loop antennas work. Source: www.cdc.gov/niosh/mining/ UserFiles/Works/pdfs/2013-105.pdf be used in a mine is as part of a nodebased system. While the range of radios underground is generally limited, small repeater stations or nodes can significantly extend radio range – see Fig.47. These nodes and the radios used with them are microprocessor controlled. As discussed earlier, the system forms a mesh network when many nodes are used. The mesh network routes signals between nodes as it deems appropriate (Fig.49). If one node is out of action, an alternate path is established. Medium-frequency system (wired + wireless) Medium-frequency radio waves in enclosed underground spaces will couple into any existing conductors such as power lines, data cables or a radiating feedline. Unlike VHF and UHF radio, medium frequencies can use any existing conductor. So if a suitable conductor is present, MF radios can be used over an extended distance inside a mine, and no repeater is needed. Fig.45: the basic architecture of radiating feedline inside a mine. 22 Silicon Chip Australia's electronics magazine siliconchip.com.au Fig.46: how a section of radiating feedline might be laid out in a mine. The dots show the signal between two miners. The downside is that handheld MF radios are considerably larger than VHF and UHF radios. A solution is to use VHF/UHF to MF converters. This enables a small handheld radio to be used within range of a converter which then retransmits the signal at MF, coupling it into nearby conductors. At the other end of the link, the MF is upconverted to VHF/UHF to allow another miner to receive the transmission. Avalanche beacons Avalanches occur when an unstable layer of snow breaks free and slides down a mountain, burying any unfortunate skiers or snowshoe walkers in its path. They are common in areas of Europe and North America. People in avalanche risk zones often carry a form of emergency locator beacon called an avalanche transceiver. Avalanche emergency locator beacons were first invented in 1968, and commercial units were first sold in 1971. They operated at 2.275kHz (ULF). In 1986, 457kHz (MF) was adopted as the standard frequency. The 457kHz (656m) frequency was adopted because it is not subject to significant attenuation by snow, rocks, trees, debris or people, and is less prone to problems resulting from multipath reflections compared to the much lower 2.275kHz frequency. Fig.47: repeater nodes can be used to communicate between two radios that are otherwise out of range of each other. Extending this concept results in a mesh network. Fig.48: example of how a radiating feedline, with above- and below-ground redundancy, can continue to operate after a disaster. Original source: www.technowired.net/wp-content/uploads/2017/02/4.-Sistema-MCA1000-Digital-en.pdf siliconchip.com.au Australia's electronics magazine April 2023  23 Fig.49: multiple repeater nodes can be used to communicate between two radios in a range of locations that would otherwise be out of range of each other. Together, these nodes comprise a mesh network. By necessity, the antenna length can only be a small portion of the wavelength, making transmission very inefficient. Still, the effective electrical length can be increased by using a ferrite core loop antenna with many turns. In use, when each party member heads out into the avalanche-prone area, they turn on their transceiver, and it emits a beep over the radio once per second. If any party members become buried in an avalanche, the remaining members switch their units from transmit to receive to pick up signals from the buried members. The range of the beacons is 40-80m. Due to the shape of the radiated signal, there is a specific technique for finding someone buried in the snow; practice is required to refine the technique, as time is of the essence. Fig.50 shows the radiation pattern, and there are various YouTube videos that explain the required search technique. More modern beacons use digital transmission modes and some use W-Link in addition to the standard 457kHz signal. W-Link operates on either 869.8MHz or 916-926MHz, depending upon the region. W-Link transmits additional information, such as device ID and allows signals Fig.50: the shape of the radiated signal affects the search pattern during avalanche rescues. Practice is required to quickly locate people buried under the snow using their beacons. Original source: https://youtu.be/tXpEUBDzbu0 24 Silicon Chip Australia's electronics magazine of people already rescued to be ignored. Modern beacons (Fig.51) also employ two or three 457kHz antennas in receive mode to make the receiver more sensitive in certain directions depending on the relative alignment of the transmitter and receiver. If you wear one, keep it under your outer clothing to prevent the batteries from freezing and to stop the device from being torn off if you are caught in an avalanche. Many people are not aware that avalanches can occur in Australia. Although rare and not as large as overseas, they occur in certain alpine Fig.51: the Mammut Barryvox S Avalanche beacon for finding buried victims. It has a feature to assist in the search pattern, W-Link and three antennas. Source: https:// varuste.net/p77030/mammutbarryvox-s siliconchip.com.au Related Silicon Chip articles Fig.52: a pipeline pig can be located through steel, soil and concrete by picking up the 22Hz signal transmitted from the pig. regions, although not typically in areas frequented by skiers and are not as dangerous as the ones that occur in the Americas, Asia or Europe. Australia’s Mountain Safety Collective (https:// mountainsafetycollective.org/) conducts training and has rescue teams for avalanche incidents. Pigging communications (pipelines) Pigging involves inserting a ‘pig’ into a pipeline for cleaning or inspection (see Figs.52 & 53). The pig is a device that tightly fills the internal diameter of the pipe and is pushed along by fluid or gas pressure behind it. Some are equipped with electronics to communicate their position or other data to the world above. We have discussed various aspects of VLF and ELF frequencies and comms before and aspects of underground communications in the following articles: ● Radio Time Signals throughout the World (February 2021; siliconchip.au/ Article/14736) ● Underground mapping, leak detection & pipe inspection (February 2020; siliconchip.au/Article/12334) ● Atmospheric Electricity: Nature’s Spectacular Fireworks (May 2016; siliconchip.au/Article/9922) ● How Omega Ruled The World Before GPS (September 2014; siliconchip. au/Article/8002) ● HAARP: Researching The Ionosphere (October 2012; siliconchip.au/ Article/492) ● Digital Radio Modes (April & May 2021; siliconchip.au/Series/360) The industry standard frequency for pig communication is 22Hz. Such signals penetrate the metal of a pipeline and soil or reinforced concrete above it. Leaky feedlines on aircraft Aircraft are not exactly underground [I’m sure the passengers are relieved to hear that! – Editor], but some of the same problems apply to radio reception onboard planes as inside tunnels. Leaky feed line systems have been developed by companies like W. L. Gore & Associates for use in the cabins of widebody and single-aisle aircraft – see the PDF at siliconchip.au/link/abiq These airborne systems provide ‘picocells’ for mobile phone coverage, Fig.53: a pipeline cleaning pig on display in a cutaway length of pipe. Some have electronics and communicate at 22Hz. Source: https://w.wiki/6Exp (CC BY-SA 2.0) siliconchip.com.au Australia's electronics magazine access points for WiFi and support Bluetooth, DECT, DECT2, Globalstar, GSM, IRIDIUM Sat, MMS, PDC and TETRA protocols. They reduce dead zones and reduce the weight of the required equipment. The antennas are suitable for frequencies from 400MHz to 6GHz. See the YouTube video titled: “GORE Leaky Feeder Antennas” at https:// youtu.be/ZK7wBCfJJa0 Conclusion In summary, there are two main techniques for underground communications without having to run wires throughout the enclosed space: the use of low frequencies (typically VLF or LF, 3kHz to 300kHz) for better penetration of rock and soil, or the use of repeaters (possibly in a mesh) to overcome line-of-sight difficulties in curved tunnels or a series of cave/ mine chambers. The main advantage of the VLF/ LF approach is that only two radios are required; however, the low frequencies involved generally require the use of relatively large antennas (somewhat mitigated by using loops). In cases like tunnels or mines where there is frequency activity and significant infrastructure already exists, mesh networks or leaky feeders allow for greater flexibility. For rescue situations, likely a mix of the two approaches will be required. VLF/LF radios can be used initially until a mesh network can be built, allowing rescuers to communicate with small hand-held radios. Given the low cost of powerful RF chips these days, it probably won’t be long before low-cost mesh radios are widely available; possibly even open-source designs. SC April 2023  25