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For all you hip millennial AM radio listeners, you may have noticed something.
A lot of AM radio stations shut down at night and a lot of other ones can be heard from
further away and there’s a good reason for that.
AM radio waves travel further at night.
Now here’s some science talk.
You might actually have to learn something this episode.
Broadcast radio is essentially split into two types: AM and FM.
AM radio waves look like this, FM waves look like this.
Except this isn't to scale, this is.
You see, AM radio waves are hugely longer than FM waves.
The wavelength from a station broadcasting at 1000 AM, for example, is 300 meters long
while a station broadcasting at 100 FM only has a wave 3 meters long but with wavelengths,
much like with stab wounds, smaller is better, at least in some ways.
You see, the station 1000 AM broadcasts at 1000 kHz which translates to 1,000,000 hertz
which means that this wave oscillates 1 million times per second but the station 100 FM broadcasts
at 100 mHZ which translates to 100,000,000 oscillations per second.
I won’t go too deep into the science of this because I want views, but essentially
at the higher frequencies FM radio transmits in a way that is higher quality.
So that brings us to nighttime.
Between 40 and 600 miles of altitude, there’s a layer of the atmosphere called the ionosphere.
In this region, all the radiation coming from the sun gives off electrons that stick around
for a bit and bounce lower-frequency radio waves, including AM signals, back to earth,
but FM radio waves just go right through.
Without the bounce from the ionosphere, FM radio can only be picked up within line-of-sight
of the broadcast tower which, since the earth supposedly curves, is only about 30-40 miles.
At night though, the sun disappears, but you probably already knew that.
Without the sun there’s less radiation giving off electrons so some of the ionosphere disappears,
but not all of it.
Above 100 miles the ionosphere remains and it keeps bouncing the radio signals back to
earth, but since the bounce happens higher up, the signal gets back down to earth further
away.
This is why AM radio has a significantly longer range at night.
The difference is so significant that an AM radio station in New York with a transmission
distance of about a hundred miles during the day could, in the right conditions, potentially
be picked up by a skilled operator in Europe at night.
But this presents a bit of a problem.
There are only 116 AM radio frequencies in the US as decided by the much loved FCC starting
at 540 kHz and increasing in increments of 10 to 1700 kHz but there are thousands of
AM radio stations.
During the day this isn't a problem because their signals only reach a hundred or so miles,
but if they all broadcasted during the night the frequencies would just be a mumbled jumbled
mess of sound, so they don’t.
Certain frequencies are given clear-channel status which allow some large stations to
broadcast their signal at full power all the time, but those transmitting on the same frequency
during the day are required to shut down or reduce power overnight.
For example, on 1000 AM in the US, there are two clear channel stations—KOMO in Seattle,
Washington and KMVP in Chicago, Illinois.
At night, all the nearest stations, like WCCD in Parma, Ohio, will shut down while those
slightly further, like KSOO in Sioux Falls, South Dakota, will reduce their power significantly
so they can only be heard in the immediate area.
This leads to a weird situation where the broadcasting hours of a station change depending
on the the time of year.
When these daytime only stations can broadcast is directly tied to when the sun is up so
WCCD, the daytime station in Parma, Ohio, for example, is allowed to broadcast for 15
hours a day in the summer when days are long but in the winter, when the sun rises later
and sets earlier, they only have 9 hours of daylight during which they can be on air,
but between the clear channel stations, KOMO in Seattle and KMVP in Chicago, the range
is so significant that at night, anyone from coast to coast can tune into 1000 am and listen
to radio.
If you want a surefire way to make your millions, you should start a radio station but if you
want to start a radio station you’ll probably need a better understanding of how radio waves
work and there’s no better place to gain an understanding of scientific concepts than
brilliant.org.
As I’ve been learning in their solar radiation course, that same radiation that bounces AM
radio back to earth is also one of the abundant and renewable sources of power.
Shockingly, you’ll learn complex concepts like the Shockley Queisser limit simply with
their straight-forward explanations, simple graphics, and thought-provoking puzzles which
help you think like a scientist.
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