question: Why does a radio app on a phone require a headphone as antenna?
comment: FM radio signals have a fairly long wavelength - at least, compared to any other use of radio in a phone. That longer wavelength needs a longer antenna, which you can't fit inside the phone. So they instead use the shielding in the headphone cable, which is plenty long enough. The app could try to work without the headphone plugged in - many do - but it is hard for them to pick up a radio signal without it.
passages: The FM Radio can also use accessories connected to the bottom port of the phone as an antenna, such as the wired stereo earpiece. It can also play back through the phone's speaker.
   Portable devices have also become usable when worn. For example, most walkie talkies come with a voice operated transmit capability so that they will work hands-free, when used with a wearable microphone. Many telephones such as the Motorola Star Tac also feature an earpiece that allows the phone to be worn and used hands-free. The Star Tac was the first wearable cellular telephone, in the sense that it was the first that could be used while being worn.
   The base of the phone also features a 4-part 2.5 mm socket for the supplied headset. When the supplied 4-part 2.5 mm headset is plugged in, it can function as an antenna for the stereo FM radio that also has support for Visual Radio.
   The predominant cellular network implementation is to have multiple antennas at the base station and a single antenna on the mobile device. This minimizes the cost of the mobile radio. As the costs for radio frequency (RF) components in mobile devices go down, second antennas in mobile device may become more common. Multiple mobile device antennas are currently used in Wi-Fi technology (e.g. IEEE 802.11n), where WiFi-enabled cellular phones, laptops and other devices often have two or more antennas.
  
  Section::::MIMO Technology in WiMAX.
   Alternatively, in November 2016, Google added the option to run Android Auto as a regular app on an Android device, i.e., not tethered to a car's head unit, which allows it to be used on Android-powered head units, or simply on a personal phone or tablet in the vehicle. In addition, on January 1, 2018, it was announced that JVCKenwood would be exhibiting wireless Android Auto-enabled head units at CES 2018, which would be capable of operating without the need for a wired connection.
  
  Section::::Availability.
   Nokia is making the Visual Radio client available to users of other phones, and have announced that they are producing a Java applet to enable this.
   Umbrella antennas were invented during the wireless telegraphy era, about 1900 to 1920, and used with spark-gap transmitters on longwave bands to transmit information by Morse code. Small umbrella antennas were widely used with portable transmitters by military signal corps during World War I, since there was no possibility of setting up full-sized quarter-wave antennas.
  
  Umbrella antennas were used at most OMEGA Navigation System transmitters, operating around 10 kHz, and at LORAN-C stations, operating at 100 kHz with central masts approximately 200 metres tall, before those systems were shut down.
  
  Section::::Structure.
   These antennas are typically designed to be used in the unlicensed ISM band radio frequency bands (900 MHz, 1.8 GHz, 2.4 GHz and 5 GHz), however, in most commercial installations, licensed frequencies may be used to ensure quality of service (QoS) or to provide higher connection speeds.
  
  Section::::Fixed wireless broadband.
   Prior to the final transition to ATSC digital television in the United States on June 11, 2009, two smart antenna models were brought to market:
  
  BULLET::::- RCA ANT2000 – no longer available from retailers
  
  BULLET::::- DTA-5000 – manufactured by Funai Electric, marketed under the "DX Antenna" brand name, sometimes associated with the Sylvania brand name; no longer available from retailers
  
  And two models are causing consumer confusion:
  
  BULLET::::- Although the Apex SM550 is capable of connecting to a CEA-909 port for the purpose of drawing electrical power, it is not a true smart antenna.
   The E220 antenna is internal; the USB cable does not act as an antenna. The external antenna connector is not accessible without opening the device (voiding any warranty). It is still possible to connect an external antenna without opening the device and using the (internal) external antenna connector. Together with a high gain antenna like an LPDA or an OMNI booster antenna and an adapter, signal strength can be considerably improved.
   In 2016, two manufacturers unveiled phone lines with modular accessory systems. LG Electronics unveiled its LG G5 smartphone, which allows add-on modules to be installed by removing its "chin" and battery, and attaching the battery to an accessory that is then re-inserted into the phone. LG unveiled camera grip and audio enhancement accessories as part of the launch of the device. Motorola later unveiled the Moto Z, which allows the installation of case-like accessories known as "MotoMods", mounted using magnets to the rear of the device and a pogo pin connector for communication.
   Starting in April 2004 Icom began releasing new "D-STAR optional" hardware. The first to be released commercially, was a 2-meter mobile unit designated IC-2200H. Icom followed up with 2 meter and 440 MHz handheld transceivers the next year. However, the yet to be released UT-118 add-on card was required for these radios to operate in D-STAR mode. Eventually, the card became available and once installed into the radios, it provided D-STAR connectivity. The June 2005 edition of the ARRL's QST magazine reviewed the Icom IC-V82.
   Each radio works over a single band of frequencies. If a tow car company had a frequency on the same band as its auto club, a single radio with scanning might be employed for both systems. Since a mobile radio typically works on a single frequency band, multiple radios may be required in cases where communications take place over systems on more than one frequency band.
  
  Section::::Walkie talkie converters in place of mobile radios.
   In October 2013, Motorola Mobility announced Project Ara, a concept for a modular smartphone platform that would allow users to customize and upgrade their phones with add-on modules that attached magnetically to a frame. Ara was retained by Google following its sale of Motorola Mobility to Lenovo, but was shelved in 2016. That year, LG and Motorola both unveiled smartphones featuring a limited form of modularity for accessories; the LG G5 allowed accessories to be installed via the removal of its battery compartment, while the Moto Z utilizes accessories attached magnetically to the rear of the device.
   The point-to-point signal transmissions occur through the air over a terrestrial microwave platform rather than through copper or optical fiber; therefore, fixed wireless does not require satellite feeds or local telephone service. The advantages of fixed wireless include the ability to connect with users in remote areas without the need for laying new cables and the capacity for broad bandwidth that is not impeded by fiber or cable capacities. Fixed wireless devices usually derive their electrical power from the public utility mains, unlike mobile wireless or portable wireless devices which tend to be battery powered.
  
  Section::::Antennas.
   Section::::Applications.
  
  Inverted-F antennae are widely used in compact hand-held wireless devices where space is at a premium. This includes mobile phones and tablet computers using wireless transmissions such as GSM, Bluetooth, and Wi-Fi. The planar inverted-F antenna is the most frequently used internal antenna in mobile phone designs.
  
  These antennae are also of use for vehicle telematics. Vehicle manufacturers like to use antennae that follow the contours of the vehicle for style and aerodynamic reasons. Multiband PIFAs can be used to combine the antennae feeds for mobile phone, satellite navigation, and car radio.
   Applications on the market that offer this walkie-talkie style interaction for audio include Voxer, Zello, Orion Labs, Motorola Wave, and HeyTell, among others.
  
  Other smartphone-based walkie-talkie products are made by companies like goTenna, Fantom Dynamics and BearTooth, and offer a radio interface. Unlike mobile data dependent applications, these products work by pairing to an app on the user's smartphone and working over a radio interface. These products are consumer oriented implementations of technologies that have been widely available to HAM radio enthusiasts for years.
  
  Section::::Specialized uses.
   Desktop computers that use tower cases can easily swap parts such as hard drives, memory, and graphics cards. Among early mobile devices, the Handspring Visor PDA had a Springboard Expansion Slot which could give it the capabilities such as a phone, GPS, a modem, or a camera - but only one at a time. The Israeli startup Modu created a phone and screen core that could be added to various cases that gave the device features such as a keyboard or camera; the company failed and sold its patents to Google in 2011.
   Conventional MIMO systems, known as point-to-point MIMO or collocated MIMO, require both the transmitter and receiver of a communication link to be equipped with multiple antennas. While MIMO has become an essential element of wireless communication standards, including IEEE 802.11n (Wi-Fi), IEEE 802.11ac (Wi-Fi), HSPA+ (3G), WiMAX (4G), and Long Term Evolution (4G), many wireless devices cannot support multiple antennas due to size, cost, and/or hardware limitations. More importantly, the separation between antennas on a mobile device and even on fixed radio platforms is often insufficient to allow meaningful performance gains. Furthermore, as the number of antennas is increased, the actual MIMO performance falls farther behind the theoretical gains.
   Using phone numbers as identifiers may also create security risks that arise from the possibility of an attacker taking over a phone number. This can be mitigated by enabling an optional Registration Lock PIN in Signal's privacy settings.
  
  Section::::Limitations.:Android specific.
   The Audion provided power gain; with other detectors, all of the power to operate the headphones had to come from the antenna circuit itself. Consequently, weak transmitters could be heard at greater distances.
  
  Section::::History.:Patents and disputes.
   With the growing infrastructure of wireless networks, and improving speed and reliability, fixed wireless has also become a viable solution for broadband access. Businesses and homes can use fixed-wireless antenna technology to access broadband Internet and Layer 2 networks using fixed wireless broadband. Networks which have redundancy and saturation and antennas that can aggregate signal from multiple carriers are able to offer fail-over and redundancy for connectivity not generally afforded by wired connections. In rural areas where wired infrastructure is not yet available, fixed-wireless broadband can be a viable option for Internet access.
  
  Section::::See also.
  
  BULLET::::- Internet access
   Section::::Applications.
   Because all bloks are external of the main board, signals have significantly farther to travel between components. This extra distance could lead to drops in signal quality and increase power consumption due to added interfacing resistance. In addition, the quality of connection needed would require expensive pins and sockets, and developing a system to allow the user to arbitrarily place bloks without considerable cost would be very difficult. The end product would likely be much thicker than today's smartphones, and bloks would be at risk of dislodging from the main board(a case to help hold the blocks together during high movement activities, maybe a case with extra antennas for the RF?). RF design of a modular phone is also a major challenge. Antennas integrated into the main body of the phone would require RF routing to the RF modem(s) in the modules and could limit the frequencies on which the phone could operate, necessitating different phones for different networks. Antennas integrated into modules could be less efficient due to limited space and EM interactions with adjacent modules, limiting range.
   This technique, known as radio direction finding (RDF), is useful but only moderately accurate. Measurements better than a few degrees are difficult with a small antenna, and because of the electrical characteristics, it is not always easy to make a larger version that might provide more accuracy. Moreover, the addition of a loop antenna may be difficult on smaller vehicles, or difficult to operate for those without a dedicated navigator.
  
  Section::::Description.:Orfordness' solution.
  
presuppositions: Headphones acting as antenna are required for radio apps to work. 
corrections: Radio apps will work without antenna, however the radio signal is harder for the apps to pick up. 
raw_presuppositions: Headphones acting as antenna are required for radio apps to work.  radio app requires a headphone as antenna
raw_corrections: Radio apps will work without antenna, however the radio signal is harder for the apps to pick up.  app could try to work without it, many do