Чтение общенаучной литературы. Кытманова О.А. - 41 стр.

UptoLike

Составители:

Рубрика:

few, specialized, stations operate in this hand Their transmissions are almost

wholly reflected from the ionosphere and are thus guided round the Earth in a

"channel"' formed by the ground and the atmosphere. This gives them reliable

worldwide coverage.

In the low frequency (LF) band the ranges are not &o large because of absorption

by the ionosphere Nevertheless LF stations can cover considerable areas by day

and night Many radio sets have a long wave or LF band But few stations broadcast

on this although their signals are fairly good

Medium wave or medium frequency (MF) stations behave quite different. After

dark MF stations which cannot be heard by day are received at great strength often

causing serious interference with other transmissions Also their signals tend to fade

I hose effects are caused by the ionosphere.

All radio transmissions have two components or parts the ground wave,

which travels along the ground and the sky nave, which travels up to the

ionosphere. In day light the sky wave of an MF station is almost completely

absorbed by the ionosphere and the station's receiving area receives only the

ground wave which gives a steady signal over reasonable distances. But because

the ionized layers are produced by the Sun’s action, they are affected in density

and height both by seasonal changes and by the change from day to night. When

darkness falls there is much less absorption allowing the sky wave to be reflected

back to Earth. The reflected waves which return to Earth within the ground wave

area add to or subtract from the strength of the signal causing fading. They are also