Electronmagnetic signals and properties

[medwatt]

Hello,
I want to know what factors determine that a signal (radio wave, microwave, infra red) cannot be use for broadcasting but only point to point communication. I know that it has to do with frequency and the energy associated with it but cannot seem to have a concrete reason because of those in the microwave range.
For example I know that infra red which has highest frequency of the other two mentioned is only utilised in point to point communication as they cannot pass through walls and so it cannot be transmitted uni directionally.
Microwaves which start from 2 GHz can sometimes be used for broadcasting as in the case of FM or Television where the signals can penetrate through walls and at the same time for point to point between communication towers. Why then when considering the ku band ground station antennas send microwave signals to satellites which are in geostationary orbit using point to point communication (12GHz) and the satellite can then broadcast at 11GHz.
I really want to know the demarcation line where broadcasting is no longer an option but point to point !
Thank You

 

[davenn]

hi there :)

terresterial TV and FM radio is transmitted at much lower freqs than 2GHz, in bands from 50 MHz to ~ 700 MHz is the main region

Microwave communications freq's from 1GHz and up become more line of sight because those frequencies have a higher absorption rate as the freq increases. It only takes a wall or 2 or a few wet trees to stop or at least severely attenuate a 2.4GHz signal.

We use microwave links for line of sight point to point links because they offer very narrow beanwidths when used with dish and other directional antennas. In my activities, I have experimented on freq's up to 47GHz and as an example at 24GHz and using a 1 metre diameter dish, the -3dB beamwidth of the radiated signal is only 5 - 6 degrees VERY narrow. This has great advantages in point to point systems as there is very little power wasted as there is little spreading out of the signal.

Why then when considering the ku band ground station antennas send microwave signals to satellites which are in geostationary orbit using point to point communication (12GHz) and the satellite can then broadcast at 11GHz.

not to sure what you meant there, can you make your question/comment clearer :)

I really want to know the demarcation line where broadcasting is no longer an option but point to point !
Thank You

there is no real demarcation. any signal in the microwave bands can have a broad beamwidth ... omni-directional, or it can be point to point ... it depends on the type of antenna used.

Dave