"DC" wrote in news:1165397575.751044.20090@
73g2000cwn.googlegroups.com:

Hi guys,

I've been reading up on the different types of radio waves etc and am
getting really confused between the concept different wavelength waves
travelling different distances before being attenuated etc.

The more I think about it all the more confused I am becomming and I
was wondering if someone would be able to help me with this....

Basically I have been told that shorter wavelength waves attenuate and
scatter more easily. So working on this principle I know that medium
waves (MW eg AM radio) are longer in wavelength than VHF (ie FM radio),
so why is it that in car parks etc (i.e. under a roof) you can still
pick up FM on the radio but not MW. Surely FM being shorter in
wavelength (and consequently higher freq) will be scattered more? Is
this to do with diffraction instead? Longer waves diffract more
readily, so as FM is longer it diffracts more and is therefore heard
'around corners' (sorry for putting it like that!), whereas MW would
not be?

So, shortwave is also longer in wavelength than FM so perhaps this is
better for transmitting over long distances ie for international
transmission, but MW is even longer in wavelength so why is this not
used instead?

Lastly, SHF and EHF are used for satellite transmissions, but these are
very small wavelengths for radio waves so surely here the scattering
and attenuation would be so large that this would impede its path from
the satellite to the ground? Or again are we just using SHF and EHF as
diffraction effects would be minimal?

If anyone could shed any light on this topic for me it would be really
appreciated.

Radio waves of different wavelengths do behave differently.

Long waves of 600 meters or more couple well to the ground and propagate
along it for very long distances. Waves of medium length, from about 100
to 600 or even 1000 meters also couple to the ground but do not travel as
far in that mode. They also reflect of the ionosphere's E and F layers
when those are present, though they are absorbed by the D layer in
daytime.

Shorter waves, from 100 meters on down to about 10 meters often bounce
off the F layer for very long distances. These, too can bounce off the E
layer. Waves shorter than about 60 meters do not get absorbed so badly
by the D layer and thus propagate in daytime. The F layer's contribution
depends heavily on energy from the sun. When the sunspot cycle is at a
low, like now, propagation on 10 meters is rare (and usually attributable
to E layer activity). Generally there is a maximum usable frequency
between any two paths. Transmission on a shorter wavelength that that
frequency (in other words a higher frequency) will result in the wave
simply travelling into outer space.

VHF frequencies normally propagate on line of sight paths, but there are
exceptions. Sometimes the E-layer will get a patch that's very hot and
the MUF between points about 1200 miles apart will rise up to as much as
250mhz. Sometimes an inversion layer can bend paths back along the
surface. And just plain brute force can scatter a signal forward through
the shrubbery for quite a distance if you use the right antenna and the
right modes (about 300 miles reliably on 144mhz).


--
Dave Oldridge+
ICQ 1800667