root-beer
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loucy said:Root-Beer: you should read carefully on the definition of "antenna gain"--the "gain" is relative to isotropic antenna, not absolute power gain.
root-beer said:Another confusing issue is the Antenna Reciprocity Theorem which states that
"reciprocity implies that antennas work equally well as transmitters or receivers, and specifically that an antenna's radiation and receiving patterns are identical"
see: http://en.wikipedia.org/wiki/Reciprocity_(electromagnetism)
I think that "reciprocity" is also widely misunderstood. Given the radiation patterns are the same for tranmitting as receiving, but signal gains are not equal.
The point actually hasn't to do with conservation of energy. It's just an application of basic optics laws to the described setup. But not of general validity. You'll find other optical setups, that can generate a spot above 6000K, or convert solar light to electricity and you don't have a principle temperature limit. I guess, you have been thinking about 2nd fundamental law of thermodynamics, but it doesn't show this way.it CANNOT reach more than the temperature of the Sun, some 6000K. This is again the principle of energy conservation which holds.
Root beer:
1) Friis assumes you are in the far zone of the antenna...
2) In addition to the r >> lambda far zone requirement we also need to consider r>>D and r>2D^2/lambda requirements.
3) As far as I can tell (playing with Friis in MATLAB) the only time you realize positive gain is when you are either VERY close.. or you have HUGE gain (which ultimately translates to HUGE D)... probably something could be said about the r/D ratio and break down of Friis...
4) Also, I think you should re-calculate your received power.. for 2 30dBi antennas operating at 2.4GHz at a distance of 1mile (1.6km) I get: Pr/Pt = 3.9e-5... good enough for a pretty good com link!!
:]
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