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.

Cheers

DC

DO NOT, repeat, DO NOT, think about this any more... Only madness and
physical decline will follow you down such a twisted path... There is
no hope for those of us who have already fallen into this trap... Save
yourself while you can..

DO

DC wrote:
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.

Cheers

DC

On 6 Dec 2006 01:32:55 -0800, "DC" wrote:

Basically I have been told that shorter wavelength waves attenuate and
scatter more easily.

Hi OM,

As a generality, this is true; but generalities that are this
all-encompassing don't actually explain anything. Scattering is
dependant upon what is doing the actual scattering, and this
dependency is related to its size in terms of the wavelength.

So working on this principle

This is your first mistake because the generality is NOT a principle.
We will skip the remaining presumptions.

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

Light (as in illumination) will serve this purpose too, because it is
shortwaved (by a million fold or more) and yet it confounds many of
those presumptions that I discarded above.

Attenuation can by one of two ways. The energy can be consumed along
the way, or it can be diverted (scattered) which to the observer
amounts to the same thing - less light (or RF or waves or
what-have-you).

Scattering is a little more sophisticated in that the path along which
the energy was directed, changes (and sometimes, so does the
wavelength). Scattering will also offer other opportunistic modes
such as diffraction, reflection, refraction (a whole list of shuns).
You should note that a partial reflection of light, to the observer,
will appear to be attenuation. Further, through diffractions the
original wave may appear to become multiple sources of the same
energy, but with different phases. When those phases combine, the
observer may yet again perceive an attenuation.

To tie this all together, wavelength will determine if the scattering
body is suitable large/small to lend an effect to the observer. This
is why through your list of wavelengths there appeared to be unusual
distinctions. Those distinctions were the properties of the medium or
the interfering bodies (scatterers) - not the source wavelength, per
se. These properties fall into the studies of chromatography,
spectroscopy, and so on (which have their equivalents at all
wavelengths, but are suitable more for the optical wavelengths).

One excellent example is in the InfraRed region where water vapor
selectively scatters, absorbs or transmits various wavelengths in a
small band of frequencies. Water vapor will also do similar tricks in
the 2GHz region.

73's
Richard Clark, KB7QHC

"DC" wrote:
"Basically I have been told that shorter wavelength waves attenuate and
scatter more easily."

That is only part of the story. To pass through an opening, the wave
must be small with respevct to the opening to avoid severe attenuation.
An example may be evident when you are tuned to an AM broadcast on a car
radio and you enter a bridge, tunnel, or parking garage. The AM signal
may disappear. If you tun-in an FM signal, it may be strong. The FM
wavelength fits through pores in the structure.

In a tunnel, the diameter must be at least 1/2-wavelength not to
severely attenuate attempted propagation of a wave through the tunnel.

Best regards, Richard Harrison, KB5WZI

"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