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/archi...-gifs/tvi8.gif

I guess this conversation and drifted a bit "High and Far" as well...

;-)

Dave