comparison between AM and FM

The ionosphere is a layer of charged particles, including free electrons, which is caused mostly by ionising radiations including UV from the sun and cosmic rays. This makes it an electrically conductive plasma.

It is largely (entirely?) due to these electrons that refraction of an incident radio wave can occur, due to absorption and re-radiation of the waves.

The density of the free electrons determines the frequencies of radio waves that can be refracted; higher density allows higher frequencies to be refracted.

The relationships between the cut-off frequency and the electron density in the area of incidence is:

f_cut-off = 9x10^-3 √N

where:
f_cut-off is the frequency at or below which refraction can occur at vertical incidence
N is the free electron density

Also, since we don't usually beam radio waves straight up, the maximum usable frequency is given by:

f_max = f_cut-off / sin(α)

where
f_max is the maximum usable frequency
sin(α) is the angle of incidence relative to the horizontal horizon.


This is why things like the solar cycle and other solar events (the increased emission increases the electron density) make a big difference to 'skip' and interference, and gives radio amateurs hours of fun during these events.

Please refer to the carrier (frequency) of the radio waves and not AM and FM waves.
Please consult https://en.wikipedia.org/wiki/Radio_spectrum for definition of the radio spectrum
HF radio wave propagation includes refraction from the ionosphere. The enables long distance radio links. HF refraction in layman terminolgy is if the radio wave a "bend" back to earth due to different propagation speeds

The ionosphere is influenced by solar radiation. Solar activity causes the ionosphere to reflect higher frequencies, and at the same time NOT reflect lower ones. At night, when the sun isn't shining, lower frequencies, such as the AM broadcast band, are reflected.

wa1kij said:

The ionosphere is influenced by solar radiation. Solar activity causes the ionosphere to reflect higher frequencies, and at the same time NOT reflect lower ones. At night, when the sun isn't shining, lower frequencies, such as the AM broadcast band, are reflected.

Click to expand...

How? when Sun isn't there, there wouldn't be any ions. So they should not reflect back. Yet, AM channels work even in night.

ElectroEnthusiast said:

How? when Sun isn't there, there wouldn't be any ions. So they should not reflect back. Yet, AM channels work even in night.

Click to expand...

The ions don't immediately disappear when the Sun sets. I imagine that the recombination/capture rate becomes greater than the ion generation rate, thus lowering the density following the 'darkness' as it moves, but they are not all going to recombine in a few hours. It's a big atmosphere... and it moves about... a lot.

To add to FoxyRick you will find that HF (not AM waves) optimal propagation frequencies change at night (Refer to MUF LUF ....)

Here is a nice explanation of klystron's point from NOAA/USA: **broken link removed**

John