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dervast
Oct23-06, 08:27 AM
Hi to everyone. I have heard that we usually prefer to modulate signals at high frequencies so the wave length is small compared to the length of the antenna. Is there any rule or equation that implies this requirement?
berkeman
Oct23-06, 10:53 AM
To be an efficient radiator, an antenna will generally be a half-wave dipole or a quarter-wave monopole. That is, a dipole antenna will have two opposed elements, each a quarter-wavelength long. A monopole will have a single radiating element a quarter-wavelength long, over some ground plane or set of ground elements.

http://en.wikipedia.org/wiki/Antenna_%28radio%29
Averagesupernova
Oct23-06, 12:00 PM
Hi to everyone. I have heard that we usually prefer to modulate signals at high frequencies so the wave length is small compared to the length of the antenna. Is there any rule or equation that implies this requirement?

Modulation has little to nothing to do with the antenna. The carrier frequency and the antenna are related as berkeman pointed out.
dervast
Oct23-06, 02:46 PM
Modulation has little to nothing to do with the antenna. The carrier frequency and the antenna are related as berkeman pointed out.
I agree with u.. the antenna has nothing to do withthe modulation scheme. I was talking about modulation describing that we modulate signals in high frequencies
Averagesupernova
Oct23-06, 03:21 PM
So you are saying that the reason we use certain frequencies in the spectrum for the carrier is to keep the antenna size in check? The main reason we use the ones we do is propagation of the wave. Some signals we want to go over the horizon, some we don't. Some signals we want refracted off the ionosphere, some we don't. However, there ARE cases were antenna size is a consideration. I would not say it is the main reason. Throwing the word modulation in there has thrown me off.
Ouabache
Oct23-06, 04:52 PM
....wave length is small compared to the length of the antenna. Is there any rule or equation that implies this requirement?
As Berkeman has implied, the antenna length of efficient radiating elements is not larger than the wavelength of the transmitted signal.