I recently read an article from https://www.fcc.gov/media/radio/am-stations-at-night
It says:
During daytime hours when ionospheric reflection does not occur to any great degree, AM signals travel principally by conduction over the surface of the earth. This is known as "groundwave" propagation.
My queries are:
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Do the AM signals refer to “shortwaves” or the medium waves? I think it can’t be medium waves as they don’t undergo ionospheric reflection and instead undergo ground wave propagation.
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Do the shortwaves do undergo ionospheric reflection at daytime also but cover smaller range when compared to night time and at the same time during daytime, shortwaves undergo ionospheric reflection to cover a range larger than that achieved by medium waves via ground wave propagation?
Best Answer
"Medium wave" generally refers to the band of frequencies between 300 kHz and 3 Mhz (100 to 1000 m wavelength), and this includes the AM broadcast band (530 - 1600 kHz in the US).
"Long wave" refers to lower frequencies, 30 to 300 kHz (1 - 10 km), while "short wave" (also called "HF") refers to higher frequencies in the range 3 to 30 MHz (10 - 100 m).
All of these frequencies can be reflected by the ionosphere under the right conditions, with the effect tapering off toward the upper end of the SW band.
The ionosphere, created mainly by UV radiation from the sun, is lower and denser during the day, and higher and thinner at night, which is why it allows longer propagation distances at night.
Here's a more complete listing of the commonly-named bands: $$ \begin{matrix} \text{Band Name} & \text{Frequencies} & \text{Wavelengths} \\ \hline \text{ELF} & 3 - 30 \text{ Hz} & 100,000 - 10,000 \text{ km} \\ \text{SLF} & 30 - 300 \text{ Hz} & 10000 - 1000 \text{ km} \\ \text{ULF} & 300 \text{ Hz} - 3 \text{ kHz} & 1000 - 100 \text{ km} \\ \text{VLF} & 3 - 30 \text{ kHz} & 100 - 10 \text{ km} \\ \text{LF (also LW)} & 30 - 300 \text{ kHz} & 10 - 1 \text{ km} \\ \text{MF (also MW)} & 300 \text{ kHz} - 3 \text{ MHz} & 1000 - 100 \text{ m} \\ \text{HF (also SW)} & 3 - 30 \text{ MHz} & 100 - 10 \text{ m} \\ \text{VHF} & 30 - 300 \text{ MHz} & 10 - 1 \text{ m} \\ \text{UHF} & 300 \text{ MHz} - 3 \text{ GHz} & 1 \text{ m} - 10 \text{ cm} \\ \text{SHF} & 3 - 30 \text{ GHz} & 10 - 1 \text{ cm} \\ \text{EHF} & 30 - 300 \text{ GHz} & 1 \text{ cm} - 1 \text{ mm} \\ \end{matrix} $$
ELF is interesting because the "resonant frequency of the Earth" falls into this band. If you think of the space between the Earth's surface and the ionosphere as a waveguide, the path length (circumference) is roughly 40,000 km, corresponding to a frequency of about 7.5 Hz. This frequency also penetrates fairly well into seawater, which is why the military has used this band to communicate with submerged vessels.
EHF is also known as "millimeter wave", and at these frequencies (and up), you can start to do useful imaging. This is the kind of wavelength used for security scanners, etc.