OK1SIP wrote:
this a simple, intuitive and maybe completely wrong approach:
Let's consider a travelling EM wave at any point, distant from
antenna. Both the E and H fields change in time. IMHO the power
density at that point should be constant in time.

From _Optics_, by Hecht: Equation 3.43 is the instantaneous flow of
energy per unit area per unit time, i.e. the instantaneous Poynting
Vector and is a cosine squared function. "It should be evident that
E^xH^ cycles from maxima to minima. At optical frequencies, S^ is an
extremely rapidly varying function of time ... Therefore, its
instantaneous value would be an impractical quantity to measure
directly. This suggests that we employ an averaging process."

The average power density (irradiance) is constant, not the
instantaneous power density which is a cosine squared function.

Why should they change "synchronously", causing the power density at
any point swing from zero to "maximum squared" ?

From _Optics_, by Hecht:
"Since Ey and Bz differ only by a scalar, and so have the same time-
dependence, E and B are *in-phase* at all points in space".
--
73, Cecil http://www.qsl.net/w5dxp



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