Wednesday, 18 July 2018

energy - Energies carried by electric and magnetic fields in plane waves


This is a newbie question. I'm trying to understand what I think must be a very basic concept with an obvious answer, but I want to be sure.


Do plane wave electric and magnetic fields of equal strength at the same frequency and same waveform carry equal amounts of energy?


In other words, if a dipole and a loop antenna of equal length encounter plane waves in free space at the same frequency and with the same waveform, and signal source power levels are adjusted so that the electric field to which the dipole is exposed is equal in strength to the magnetic field to which the loop is exposed, and all other variables are equal, is the current induced in each antenna equal? Please assume ideal antennas that convert all of the energy they receive into current and that have no other properties that affect the results.



Answer





Do plane wave electric and magnetic fields of equal strength at the same frequency and same waveform carry equal amounts of energy?



An analogy: -


If I put 377 V RMS across a 377 ohm resistor the power would be \$V^2/R\$ = 377 watts and the current would be 1 amp. If I calculated power using current it would be \$I^2R\$ = 377 watts.



signal source power levels are adjusted so that the electric field to which the dipole is exposed is equal in strength to the magnetic field to which the loop is exposed



If both antennas have equal sized apertures (aka capture area) and are designed to be resonant at the incoming frequency then no adjustment is necessary. One will convert volts/m (E field) and the other will convert amps/m (H field) and both will produce the same power/signal output.


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