|
Il Vecc et al,
SW (shortwave) and LW (longwave) are broadcast using amplitude modulation (AM) in the kilohertz bands rather than frequency modulation (FM) in the megahertz bands.
One of the properties of AM broadcasts is their ability to be heard 'over the horizon' whereas FM is usually 'line of sight'. The lower the broadcast frequency the further away the AM signal can be heard because the radiowaves bounce off the ionosphere and are reflected back toward earth. This is why ham radio enthusiasts operating in the low frequency shortwave bands can pick up signals from the other side of the earth.
FM broadcasts have an inherently higher energy by virtue of their wavelength and penetrate straight through the atmosphere and into outerspace. The advantage is that you can have lots of shared FM frequencies so long as the transmitters are out of line of sight, over the visible horizon.
At night the ionosphere is relatively smooth because it is not being hit directly by ionising radiation from the sun and therefore acts as a much better internal reflector, hence your better AM reception at night. Whether you have anything worthwhile to listen too of course is another issue .....
As an aside, the top secret 'over the horizon' radar is nothing more than focused shortwave transmissions in bursts with a frequency low enough to be internally reflected but high enough to resolve aircraft. It too works much better at night though the defence forces will never admit that fact.
Hope this helps.
As always,
Chris
__________________
Own - 1970 1750 GTV
Want - 1969 Duetto (in addition)
|
'84 manufacture ~ '85 MY Spider Graduate 
Quote:
) and listen in to the SW.
)
Linear Mode
