berkeman said:The basic concept/trick (discovered by Marconi?) is that when you use a tuned antenna, you radiate real energy in the form of an EM wave. And a tuned receiving antenna will generate a terminal voltage that tracks the EM wave.
The simplest antenna is a tuned dipole or tuned monopole. The tuned dipole is basically a balanced transmission line that terminates in two opposing elements that are each a quarter wavelength long at the frequency of transmission. The input impedance of a quarter wave dipole antenna is around 73 Ohms real, which means that all of the energy that goes into it from a 73 Ohm transmission line will get radiated away into space as an EM wave. A tuned monopole is just a quarter wave antenna above a large ground plane, and it has half of the 73 Ohm real input impedance.
Once you realize that you can radiate real energy (as opposed to just having reactive energy trading off for a non-resonant LC type structure), then you can work on modulation schemes on the carrier frequency. I think probably simple AM was the first modulation scheme, with audio frequency voice information amplitude modulating the RF carrier that was resonant with the transmitting and receiving antennas.
As for range, there are lots of variables, including multipath reflections and whether the frequency band you are using is mostly line of sight, or can follow the earth's curvature. Basically more power into the antenna gives longer range, as well as a better (quieter) receiver gives better range through sensitivity. You can also make antennas with very high directional gain, like the parabolic antennas used for microwave communication with sattelites, for example.
berkeman gave a very nice concise description of radio communication.beserk said:Excuse my ignorance.
What it a tuned antenna?
As mentioned, an antenna is most efficient at radiating energy when the antenna is at resonance for the RF frequency of the transmission. And the same holds for the receiving antenna -- it convert the EM radiation flowing by it most efficiently to an output voltage (at the terminals of the antenna) when the antenna elements are tuned to resonate at the RF frequency of interest.beserk said:Excuse my ignorance.
What it a tuned antenna,terminal voltage,real energy ?
Is range dependent on power comsumption ?
thanks for help