Why is long wave reception better in mountainous areas compared to VHF?

[tushi81]

in mountains part of the country, reception is better on long wave than vhf. why?

 

[Born2bwire]

The attenuation that a wave experiences in a medium is related the the distance that it has to travel through the medium in terms of its wavelength. The longer the wavelength, the shorter a given depth looks electrically. Assuming that the absorption spectrum is fairly consistent across the general radio frequencies, then we would expect that longer wavelengths would attenuate less over a given distance and set of obstacles.

In addition, the electrical size of a scatterer, the size in wavelengths, also dictates the strength of its perturbation to the wave. A long wavelength signal is going to be much more resistant to scattering off of a given obstacle when compared with a shorter wavelength signal if the material properties do not change drastically.

This means that higher frequencies rely more and more on line of sight as they are affected to a larger degree by buildings, trees, and hills that appear in the way.

At very low frequencies, you can propagate out by a surface wave. This means that the signal is coupled to the surface of the ground. While this is not a very good signal for long distance propagation, it does facilitate reception when not in line of sight since the wave follows the ground and can "dip" down into the valleys. The higher frequencies of TV and FM radio make surface waves less useful. The surface wave will attenuate as it propagates, so the shorter wavelengths again will die out faster than the longer wavelengths of say AM radio. There is the ability for radio to bounce off of the ionosphere and back down to the Earth. This facilitates true long distance propagation, where the signal bounces back and forth between the ionosphere and the Earth. AM can do this on a limited ability, usually best during the night and shortwave is famous for this. FM and TV do not have too much success with this.

 

[tushi81]

The attenuation that a wave experiences in a medium is related the the distance that it has to travel through the medium in terms of its wavelength. The longer the wavelength, the shorter a given depth looks electrically. Assuming that the absorption spectrum is fairly consistent across the general radio frequencies, then we would expect that longer wavelengths would attenuate less over a given distance and set of obstacles.

In addition, the electrical size of a scatterer, the size in wavelengths, also dictates the strength of its perturbation to the wave. A long wavelength signal is going to be much more resistant to scattering off of a given obstacle when compared with a shorter wavelength signal if the material properties do not change drastically.

This means that higher frequencies rely more and more on line of sight as they are affected to a larger degree by buildings, trees, and hills that appear in the way.

At very low frequencies, you can propagate out by a surface wave. This means that the signal is coupled to the surface of the ground. While this is not a very good signal for long distance propagation, it does facilitate reception when not in line of sight since the wave follows the ground and can "dip" down into the valleys. The higher frequencies of TV and FM radio make surface waves less useful. The surface wave will attenuate as it propagates, so the shorter wavelengths again will die out faster than the longer wavelengths of say AM radio. There is the ability for radio to bounce off of the ionosphere and back down to the Earth. This facilitates true long distance propagation, where the signal bounces back and forth between the ionosphere and the Earth. AM can do this on a limited ability, usually best during the night and shortwave is famous for this. FM and TV do not have too much success with this.

thanks

 

[tot]

you could test this pretty easy right.
just get a wall of concrete or something and a dipole.
then set it to oscillate at various frequencies on one side of the wall.
Then on the other you put a Electromagnetic Radiation Detector and compare the levels for different wavelengths.

this would work right?

 

[sophiecentaur]

I think the term 'diffraction' could be brought in, here. This describes what happens when a wave is restricted by an obstacle, edge or aperture. The longer wavelengths are diffracted more and tend to 'go round corners' more, whereas the shorted wavelengths are diffracted less and tend to propagate more 'line of sight'. This is in addition to surface absorption effects.

 

[tot]

oh yea, i did not think about that
lambda=v/f
VHF frequench = 30 MHz
wavelength = 10 m

longwave frequency = 30kHz
wavelength = 10000m!
10km! that is huge.

have any diffraction experiments been done with massive wavelengths like this?

 

[sophiecentaur]

Every field strength survey for radio and TV reception is a diffraction experiment. There are vanloads of such records.
I think that the RF spectrum of solar radiation shows this before and after an eclipse, too.