From: nospam@127.0.0.1   
      
   JF Mezei  writes:   
      
   > On 2020-10-17 09:49, David Spain wrote:   
   >   
   >>Why does range vary by frequency?   
   >   
   > Can you elaborate, now you made me curious. Why would shape of Earth   
   > affect radio wave range?  I know that high frequencies penetrate   
   > water/snow less, penetrate buildings less. But wasn't aware that their   
   > range would be affected by Earth's shape.   
   >   
   Because radio emissions travel in straight lines unless bent. The higher   
   the frequency the less likely they will be reflected off the ionosphere   
   and "bounced" back down to the Earth.   
      
   > I know that for AM frequencies, at night, they are able to bound off the   
   > ionosphere and thus have effective range far greater than during the   
   > day. As a teenager, I used to be able to catch WLS in Chicago from   
   > montréal.  Also caught a station from North Carolina, new York City etc.   
      
   Quite true. That's why most AM stations run lower power at night. At one   
   time the FCC (the manager of the airwaves in the USA) granted exception   
   to the so-called "clear channel" radio stations. Those used to be   
   distingushed by their special 3 letter call signs: WLS, WGN, WBZ, KGO,   
   etc. The "clear channel" station license allowed them to run maximum   
   power (50kW in the USA) 24/7 into an omni-directional antenna. Which at   
   night provided quite a distant signal. "Clear channel" meant they were   
   the only station authorized to transmit on that specific AM frequency in   
   the country. That was life back in the 60's and before. I don't know   
   where all this stands today. I lost track of it all near the end of the   
   80s. I believe I've read that "clear channel" exclusivity is a thing of   
   the past.   
      
   The distance was not because of "ground wave" i.e. the signal travelling   
   directly from the transmitting antenna to your radio, but because of   
   ionospheric reflection of the signal. After a few hundred miles the   
   radio station is no longer "line of sight" as the top of it's tower has   
   dipped well below the horizon. As such the "ground wave" signal is   
   blocked by the Earth itself. Radio frequencies below about 30MHz all   
   experience some degree of ionopsheric bounce. Frequencies below about   
   100kHz (ULF and ELF) can acutally propogate through the Earth itself   
   and is used (very low bandwidth) to signal submerged submarines when   
   it's time to surface an antenna and receive an important message via   
   higher bandwidth frequencies. The frequencies the ionosphere reflect as   
   you note change during the time of day. Some shortwave frequencies   
   (those above 12MHz running up to about 30MHz) actually reflect during   
   the day and give better distance reception then. You may have heard of   
   the old Citizen's Band (27MHz/11m) radio lingo called "skip", which   
   meant the ability to hear or converse with someone hundreds of miles   
   away during the day. At night these frequencies go quiet as the   
   ionosphere goes transparent. Below 7MHz the ionosphere acts in the   
   opposite mode and becomes reflective at night. Hence the ability to hear   
   far away AM radio stations at night.   
      
   Television frequences (above 50MHz in all places of the planet I know   
   of) are high enough that the ionosphere is largely transparent to   
   them. One of the reasons TV towers are so tall is to get the "ground   
   wave" signal to propogate as far as practical. Usually within a radius of   
   60-100 miles or so. Sometimes you will get "sporadic E layers" becoming   
   active in the ionosphere and these can reflect VHF television signals   
   often across vast distances. But this is a pretty rare phenomenon. With   
   the advent of digital TV transmission in the USA it's pretty hard to get   
   a decent distant signal off an antenna. Not even a snowy picture. It'll   
   be all or (almost always) nothing.   
      
   > But a flat earth would still provide that behaviour with a flat   
   > ionosphere above it.   
      
   A flat Earth would have as the only limiting factor the ability of the   
   transmitting station to overpower everyone else in getting a "ground   
   wave" signal to you. EHF, UHF, VHF, HF, LF, ULF, ELF frequency would   
   make no difference as long as there weren't significant obstacles   
   (like mountains) between you and the transmitter. You'd still also get   
   the ionospheric bounce phenomenon as well. But on a flat Earth it'd be a   
   real nuisance because if the signal comes in out of phase it will   
   interfere and tend to cancel out the ground wave signal. This type of   
   interference was common with the old analog TV signals (bouncing of   
   nearby structures or even hills or mountains) and was known as   
   "herringbone". See:   
      
   > https://educ.jmu.edu/~fawcetwd/archive/tvi-gifs/tvi8.gif   
      
   I guess this conversation and drifted a bit "High and Far" as well...   
      
   -)   
      
   Dave   
      
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