Re: AM radio waves bounce off the ionospher, what do FM radio waves bounce off

Radio waves are the same thing as light waves. But the wavelength of radio 
waves is much longer. An AM radio wave has a wavelength in the range from 
13 to 60 metres (shortwave) or around 200 to 500 metres (broadcast band). 
By comparison, visible blue light has a wavelength about 1/2000 of a 
millimetre, a microwave oven uses microwaves with a wavelength of a few 
centimetres, and FM radio operates with a wavelength around 3 metres.

When light of any sort passes into a material, there are three basic things 
that can happen. It can just go through, perhaps getting bent a little 
(transmission). It can get soaked up, making the material a little bit 
warmer (absorption). Or it can simply 'bounce off' the material 
(reflection).

There are two main rules for something to be a good reflector of light. It 
must be a good electrical conductor, because moving electrical charges are 
what helps the reflection process to occur. And it must have a structure of 
a size that more or less matches the wavelength of the light. So a polished 
metal surface is a good reflector for visible light. but microwaves will 
reflect off a sheet of metal flywire (the holes are too small for the 
longer wavelength waves to 'see').

It is much the same with radio waves. AM waves bounce off the very large 
structures involving charged particles in the ionosphere. The shorter 
wavelength FM waves are better able to see the 'holes' between the ions, 
and do not really reflect off the ionosphere except for a faint 'ghosting'.
So FM communications are pretty nearly 'line of sight'.

But a metal object of a size 3 metres or larger makes a good reflector of 
FM waves. A satellite dish can reflect and focus TV waves onto an antenna -
- they have similar wavelength to FM.

I live in a hilly area with fairly marginal TV reception, and we are not 
connected to cable or anything. We are also on an approach flight path for 
Melbourne Airport. Often when there are planes flying over we get a very 
strong flickering of the picture on our TV screen. An aeroplane is a very 
good reflector of FM waves, and the reflected waves from the moving 
aeroplane interfere with the direct waves from the TV tower.

John.