Re: Why do the satellite use high frequency ?
时间：04-10
整理：3721RD
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Hi Hermin -- In empty space, the polarization and frequency have absolutely nothing to do with how far it will travel. In the far field it will always fade away with exactly 1/r**2. If it does anything else (even slightly!) we no longer have conservation of energy.
When you tune in an AM radio station, it is indeed vertical polarization, usually. That is because the AM radio station antenna (1/4 wavelength ≈ 40 meters) is vertical. This launches a surface wave over the earth. The wave is coupled partly to the earth and partly to the space above the earth. The earth is acting like an optical fiber, only it is flat. You can receive the surface wave maybe 100 kM away from the radio station.
At night (especially just as the sun sets), the absorbing D layer in the ionosphere disappears and the reflecting F layer is still nice and strong. Now, some of the AM radio wave that radiates up into the sky is reflected back to the earth at very long distances.
There is a ham radio band (1.8 MHz) just above the AM band. I have talked to stations all over the world on that band (nearly 200 countries, in fact), when conditions are just right.
You can do the same thing with the FM band too. But the ionosphere does not (usually) reflect at those VHF frequencies. There is a ham radio band just above the FM band too, 145 MHz. Many hams (not including me, at least not yet) bounce signals off the moon and talk to people all over the world.
Why don't we bounce signals off the moon at 1.8 MHz? Beause moon bounce requires very high gain antennas, and that would be difficult to build at 1.8 MHz. Even if you built such an antenna, the radio waves would be either absorbed or reflected by the ionosphere.
Thus the frequency makes absolutely no difference for radio communciation in free space. It is only when you introduce things like the earth, the ionosphere, the moon, air, etc., that choice of frequency becomes important.
When you tune in an AM radio station, it is indeed vertical polarization, usually. That is because the AM radio station antenna (1/4 wavelength ≈ 40 meters) is vertical. This launches a surface wave over the earth. The wave is coupled partly to the earth and partly to the space above the earth. The earth is acting like an optical fiber, only it is flat. You can receive the surface wave maybe 100 kM away from the radio station.
At night (especially just as the sun sets), the absorbing D layer in the ionosphere disappears and the reflecting F layer is still nice and strong. Now, some of the AM radio wave that radiates up into the sky is reflected back to the earth at very long distances.
There is a ham radio band (1.8 MHz) just above the AM band. I have talked to stations all over the world on that band (nearly 200 countries, in fact), when conditions are just right.
You can do the same thing with the FM band too. But the ionosphere does not (usually) reflect at those VHF frequencies. There is a ham radio band just above the FM band too, 145 MHz. Many hams (not including me, at least not yet) bounce signals off the moon and talk to people all over the world.
Why don't we bounce signals off the moon at 1.8 MHz? Beause moon bounce requires very high gain antennas, and that would be difficult to build at 1.8 MHz. Even if you built such an antenna, the radio waves would be either absorbed or reflected by the ionosphere.
Thus the frequency makes absolutely no difference for radio communciation in free space. It is only when you introduce things like the earth, the ionosphere, the moon, air, etc., that choice of frequency becomes important.
Signals for Satellites have to cross the Ionosphere and reach the the satellite.. So we use micro waves which would not get reflected by the stratosphere.. Micro waves are high frequency waves.. So we use high frequency waves for signal transmission..
for wave propagation where high frequecy signals are transmitted and received through antennas