question: Why are FM signals interrupted when underneath buildings or bridges?
comment: Radios use special waves to send their messages. Dense stuff like bridges usually have metal in them, and metal is really good at blocking those special waves. If those waves can't get through, your car's antenna can't pick them up. It's like putting a pillow over your ears- you might be able to still hear a little bit, but it's really muffled because the sound is being absorbed by the pillow. Slightly > 5 explanation: bridges/buildings often have rebar, which is concrete that is strengthened with metal bars. The metal bars often create a mesh which acts like a Faraday cage, which generally blocks incoming radio transmission. This is why you often get poor cell service in big buildings. Someone please correct me if I'm wrong here.
passages: Completion of contacts using these weak signal modes involves the exchange of signal level reports and location by grid square which is known as the Maidenhead Locator System. Two way ducting contacts can have very strong signals and are often made with moderate power, small antennas and other types of modes. Long distance ducting contacts do occur using FM modes as well but for the most part go unnoticed by many FM operators.
  
  Section::::Communications beyond 50 miles.:Sporadic E.
   BULLET::::- 2,000 watt solid-state AM transmitter with standby
  
  BULLET::::- two antenna masts providing directional coverage
  
  BULLET::::- emergency power-plant
  
  Section::::Effect of NDIS.
   BULLET::::- The Lady Godiva Memorial Bnad , made up of engineering students at the University of Toronto, attended the Bloor-Danforth subway opening on February 26, 1966, and "leapt over turnstiles", with 400 students piling onto a train. One student then pulled the emergency power switch, interfering with regular service for more than 5 hours on the first day of the subway line's operation. 
   Low-power transmitters such as those mentioned above are also sometimes used for neighborhood or campus radio stations, though campus radio stations are often run over carrier current. This is generally considered a form of microbroadcasting. As a general rule, enforcement towards low-power FM stations is stricter than with AM stations, due to problems such as the capture effect, and as a result, FM microbroadcasters generally do not reach as far as their AM competitors.
  
  Section::::Small-scale use of the FM broadcast band.:Clandestine use of FM transmitters.
   Two forms of transmitting antennas are employed. Most commonly used are standard non-directional vertical antennas. However, an alternate implementation, called "leaky cable", is a form of carrier current transmission, which employs long horizonal conductors, commonly run alongside roadways. Stations using a standard antenna are generally limited to a coverage radius of , with an antenna height of no more than 15 meters (49.2 feet), and a maximum power of 10 watts, although critical evacuation systems, such as those in the Florida Keys and near chemical and nuclear facilities, have been granted waivers to exceed that limit for emergency operations, typically for up to 100 watts. 
   Unlike some other long distance modes, high power and large antennas are often not required to make contact with distant stations via a sporadic E event. A two-way conversation can take place over a distance of several hundred miles or more, often using low levels of RF power. Sporadic E is a rare and completely random propagation phenomenon lasting anywhere from a matter of minutes to several hours.
  
  Section::::Communications beyond 50 miles.:Satellite communications.
   Section::::Reasons for weak signal.:Building size.
  
  Large buildings, such as warehouses, hospitals, and factories, often lack cellular reception. Low signal strength also tends to occur in underground areas (such as basements, and in shops and restaurants located towards the centre of shopping malls). In these cases, an external antenna is usually used.
  
  Section::::Reasons for weak signal.:Multipath interference.
   BULLET::::- KRFH at Humboldt State University
  
  BULLET::::- KRIO 1660 at Rio Linda High School
  
  BULLET::::- K-ROCKS RadioOne AM Stereo 1670 and AM Stereo 710 in Casper Wyoming
  
  BULLET::::- KSSU 1580 AM at California State University, Sacramento
  
  BULLET::::- KSUB at Seattle University in conjunction with 8 mW low-power broadcasting KXSU-LP 102.1 FM and Internet radio
  
  BULLET::::- KUR 1670/88.3 at Kutztown University of Pennsylvania
  
  BULLET::::- KUTE/1620 University of Utah
  
  BULLET::::- WALT/1610 at Davidson College
  
  BULLET::::- WCXQ 1690 AM in Isabela, Puerto Rico
  
  BULLET::::- WERW 1670 AM at Syracuse University
  
  BULLET::::- WEXP at La Salle University
  
  BULLET::::- WGCC 650 AM at Genesee Community College
   BULLET::::- WTAS 610 AM at Hope College in Holland, Michigan
  
  BULLET::::- WTGR 530 AM (1969) at Memphis State University in Memphis, Tennessee, now the University of Memphis — now WUMR on 91.7 FM
  
  BULLET::::- WTMC 530 AM (1973) at the Boston Housing Authority's Bromley-Heath Housing Project, Jamaica Plain, Massachusetts
  
  BULLET::::- WUFI-540AM at Florida International University, now WRGP 88.1 & 95.3 FM
  
  BULLET::::- WUVA 640 AM at the University of Virginia in Charlottesville from 1947 until 1979, when it was licensed to 92.7 MHz FM
  
  BULLET::::- WUVT 640 AM at Virginia Tech
   BULLET::::- PIMRC 2002 in Lisbon, Portugal
  
  BULLET::::- PIMRC 2003 in Beijing, China
  
  BULLET::::- PIMRC 2004 in Barcelona, Spain
  
  BULLET::::- PIMRC 2005 in Berlin, Germany
  
  BULLET::::- PIMRC 2006 in Helsinki, Finland
  
  BULLET::::- PIMRC 2007 in Athens, Greece
  
  BULLET::::- PIMRC 2008 in Cannes, France
  
  BULLET::::- PIMRC 2009 in Tokyo, Japan
  
  BULLET::::- PIMRC 2010 in Istanbul, Turkey
  
  BULLET::::- PIMRC 2011 in Toronto, ON, Canada
  
  BULLET::::- PIMRC 2012 in Sydney, Australia
  
  BULLET::::- PIMRC 2013 in London, UK
  
  BULLET::::- PIMRC 2014 in Washington, DC, USA
  
  BULLET::::- PIMRC 2015 in Hong Kong, China
  
  BULLET::::- PIMRC 2016 in Valencia, Spain
  
  BULLET::::- PIMRC 2017 in Montreal, QC, Canada.
   Diffraction phenomena by small obstacles are also important at high frequencies. Signals for urban cellular telephony tend to be dominated by ground-plane effects as they travel over the rooftops of the urban environment. They then diffract over roof edges into the street, where multipath propagation, absorption and diffraction phenomena dominate.
  
  Section::::Modes.:Other effects.:Absorption.
   Section::::Adverse weather conditions.
  
  For FM, vertical layering of moisture content and temperature in the atmosphere (inversion layers) can occasionally cause signals to travel hundreds or thousands of kilometres further than usual. An inversion layer (or duct) is most commonly observed over high pressure regions and may affect radio signals for several hours to several days. The phenomenon is commonly referred to as anomalous propagation and is more likely in hot, dry weather in late summer.
  
  Section::::Poor frequency planning.
   Section::::Types of DXing.:VHF DXing.
  
  Though sporadic in nature, signals on the FM broadcast and VHF television bands – especially those stations at the lower end of these bands – can "skip" for hundreds, even thousands of miles. North American FM stations have been received in Western Europe, and European TV signals have been received on the West Coast of the U.S.
   In practice, the propagation characteristics of these radio waves vary substantially depending on the exact frequency and the strength of the transmitted signal (a function of both the transmitter and the antenna characteristics). Broadcast FM radio, at comparatively low frequencies of around 100 MHz, are less affected by the presence of buildings and forests.
  
  Section::::Impairments to line-of-sight propagation.
   On November 9, 1972 in Washington, D.C., the United States and Mexico signed an "Agreement Concerning Frequency Modulation Broadcasting in the 87.5 to 108 MHz Band". Since then, in the FM band power levels and frequency assignments have been set by mutual agreement between the two countries. AM radio border blasters still exist, though they are largely ignored due to the decline of AM radio in the U.S. and in Mexico. There are several such stations licensed by Mexico's Secretariat of Communications and Transportation using transmitters with an effective radiated power similar to those of major licensed commercial stations located within the USA.
   BULLET::::- Birmingham NEC Link bridge, with a lateral frequency of 0.7 Hz
  
  BULLET::::- Dockwray Footbridge, Kendal Cumbria built 1907 with a lateral frequency of about 2 Hz
  
  BULLET::::- Groves Suspension Bridge, Chester, in 1977 during the Jubilee river regatta
  
  BULLET::::- Tacoma Narrows Bridge (1940) collapsed in wind. Frequency about 0.25 Hz
   The radiation pattern of WWVB antennas is designed to present a field strength of at least 100 μV/m over most of the continental United States and Southern Canada during some portion of the day. Although this value is well above the thermal noise floor, man-made noise and local interference from a wide range of electronic equipment can easily mask the signal. Positioning receiving antennas away from electronic equipment helps to reduce the effects of local interference.
  
  Section::::See also.
  
  BULLET::::- Radio clock
  
  BULLET::::- Watch (electronic movements)
  
  Section::::External links.
  
  BULLET::::- – official website
   Section::::Broadcast technologies.:Infrastructure-independent.
  
  Broadcast technologies that do not depend on man-made infrastructure to convey communication may be least susceptible to disruption during disasters and emergencies. Some examples of infrastructure-independent technologies are:
  
  BULLET::::- Short-wave Radio
  
  BULLET::::- Two-way Radio
  
  BULLET::::- Weather Radio
  
  BULLET::::- Internet Based Communication
  
  Each device has its own pros and cons given the emergency situation.
  
  Section::::Broadcast technologies.:Infrastructure-dependent.
  
  Broadcast technologies that depend on man-made infrastructure to convey communication are susceptible to disruption if any part of that infrastructure is overloaded, damaged or otherwise destroyed. Some examples of infrastructure-dependent technologies are:
  
  BULLET::::- Audio Public Address Systems
  
  BULLET::::- WMT Public Address Systems
   During normal operations, WUSB's audio signal is sent from its studios to the transmitter site via a T1 line. However, the old antenna location, atop the Graduate Chemistry building, remains as an emergency alternative to the Farmingville transmitter site, as well as an emergency studio-transmitter link to the Farmingville tower.
   Tap your foot 'cause you love it so, 
  
  Hearin' that good old rock and roll.
  
  This here station's got a lotta power, 
  
  But that's because of the water tower, 
  
  If you dig it we do know why, 
  
  And with that verse we'll say "bye bye."
   Early in its lifetime, electrical interference problems caused by GWEN system operation began to surface. Since the stations were using LF, the chosen frequency was within 1 kHz of the operating frequency of nearby electrical carrier current systems. With GWEN handling constant voice, teletype and other data traffic, it caused interference to the power companies diagnostic two kilohertz side carrier tone. When the side carrier tone disappeared due to interference from GWEN, the power grid would interpret that as a system fault.
  
  Section::::Site layout.
   BULLET::::- Scott Fybush, international tower photographer who has documented thousands of towers in his travels
  
  BULLET::::- Tom Bosscher of Western Michigan's website on towers of Michigan
  
  BULLET::::- Mike Fitzpatrick's NECRAT.US tower based website contains tower pictures from New England, New York, and beyond.
  
  BULLET::::- Turkish towers (in Turkish)
  
  BULLET::::- Amateur Radio Tower construction project
  
  BULLET::::- Displays over 12,000 80+ meter tall towers used in wind resource assessment
  
  BULLET::::- French towers (in French)
  
  BULLET::::- The Legal Landscape When a Tower Collapses
  
  BULLET::::- Richard Moore's Anorak Zone Photo Gallery of UK TV and Radio transmission sites
   When all factors are at their optimum, worldwide communication is possible on HF. At many other times it is possible to make contact across and between continents or oceans. At worst, when a band is "dead", no communication beyond the limited groundwave paths is possible no matter what powers, antennas or other technologies are brought to bear. When a transcontinental or worldwide path is open on a particular frequency, , SSB and Morse code communication is possible using surprisingly low transmission powers, often of the order of milliwatts, provided suitable antennas are in use at both ends and that there is little or no man-made or natural interference. On such an open band, interference originating over a wide area affects many potential users. These issues are significant to military, safety and amateur radio users of the HF bands.
   Section::::History.
  
  After the Alexandra Palace, London 405-line BBC channel B1 TV service was introduced in 1936, it soon became apparent that television could be received well outside the original intended service area.
  
  For example, in February 1938, engineers at the RCA Research Station, Riverhead, Long Island, accidentally received a 3,000-mile (4,800 km) transatlantic F2 reception of the London 45.0 MHz, 405-line channel B1 TV service.
   To solve this the best thing to do is to locate the "rusty bolt" and either remove it or render it harmless. Possible examples of rusty bolts include the pole holding an aerial, cast iron drainpipes, loose manhole covers and guy wires on radio masts.
  
  Section::::TV and radio equipment.
  
  Section::::TV and radio equipment.:Common problems.
  
  Section::::TV and radio equipment.:Common problems.:Motorboating.
  