antenna-theory.com

[RF1204]

Youtube video https://www.youtube.com/watch?v=bD4guW5NFs8 talks about some phenomn wich I never ear, and also is not well explained by the autor because what talks about the space inaccessible self- sustained Electromagnetic Radiation Field he do not divide from the accessible AC-high frequency circuit accessible at antenna terminals. Help if any one take a look and post to understand about what he talks. I have posted and repeat here my answer. Regards.

*As the Power outs to space as electromagnetic , we have spatial density of power (W/m^2) due to linear density of Electric field(V/m) and linear density of Magnetic field(A/m). At any spheric surface at R distance of the antenna the Integration of that density of power have always the power gived to the antenna, independent of the radiation pattern. If the pattern is isotropic is coincident with that spheric surface of integration, else have many areas of almost nothing density of power and other areas with increased W/m^2 relative to isotropic.
*The radiation pattern to an antenna (intensification of density of power in some directions of space) is due only to physical positioning of metals at some fractions of wavelenght wich give construtive reflections in that desired
directions.
*The free space Impedance to the electromagnetic radiation is Resistive with value close to 377(Ohm).Wikipedia good explication. This means The Electric field is in absolute values 377 times greater the Magnetic field (E=377H). That´s why the Polarization of antennas is physically correspondent to E. In parabolic antennas and satelites frequencies this is very discriminative and useful:2 different signals in the same frequency.

Finally my conclusion. The amazing ,simple and incredible transformation done in one Receive antenna: your Y-watts per square meter at receive site (from one distant TX) are transformed into Z-Electrical watts available at Rx antenna terminals, by only mechanical construction of the antenna!
The inaccessible self-sustainable far-field Electromagnetic radiation is transformed into a mensurable (high frequency) AC circuit, by a passive set of ressonant irons!
The mechanical ressonant (to the EMR field frequency) surface area of the RX antenna have a constant relation with the Electromagnetic inaccessible radiation. That K in Parabolic 12GHz antennas can be as 1.3m^2 mechanical to 1m^2 electromagnetic. To dipoles is much lesser.
By Lavoisier and 2-thermodinamics law the efficiency of any Energy transformation is always less than 100%. Power is energy by time.
If we have at RX site Y-density of power W/m^2 and antenna area 1m^2 we have available Z-watts of Power (closer but less than Y). Z is electrical high frequency AC power available. Only when we close the circuit with one load at antenna terminals we can take that power into useful signals. To take the maximum of that power the load must be conjugate of the antenna impedance. Again antenna impedance is something related to antenna mechanics. If we do not have load we do not have current intensity, only AC voltage, related to the power available and AC high frequencies circuit laws.That voltages can go to double, half etc when the load is diferent from the conjugate impedance, but power available is that. As I work with 50-Ohm antennas and cables, my spectrum analyser is also 50-Ohm and I read electrical AC high frequencies powers.
If I receive signals from 2 Tx in fase I expect 2 times Y-W/m^2 and I read 2 times watts in spectrum analiser (as expected by Lavoisier-law) and not 4 times.