On 10/10/2014 04:44 AM, gareth wrote:
It is sometime since I studied the matter, but there is a formula
that describes the energy assocaited with an antenna, and it
is made up of two parts, for the near field and for the far field.

That part for the far field has the length of the antenna in
terms of fractions of a wavelength.

That part for the near field shows the local stored energy
which is indicative of the antenna system being an oscilatory
store of energy which must dissipate as heat if it is not
reflected back down the feeder.

I cannot, for the moment, locate that formula. Can anyone
else whilst I search through the various EM tomes here?




Hello, and are you looking for the formula for the E & H fields for a
small dipole or a small loop or some other antenna? "Bible" antenna
reference books such as Kraus or Jasik provide these formulas. For
example for a small loop (magnetic dipole) we have for the radiated
(far) E and H fields:

E (phi) = (120 * pi^2 *N *A * I * sin(theta))/(r * lambda^2)

H (theta) = (pi * N * A * I * sin(theta))/r * lambda^2)

where

r is the distance from antenna, I is the uniform loop current, N
is the number of loop turns, A is the loop area and lambda is the wavelength

The small loop is assumed to lie in the x-y plane of an x-y-z orthogonal
Cartesian coordinate system where theta and phi are the angles measured
from the z-axis and x-axis, respectively.

Be advised that the above E and H formulas only apply in the far field;
the general expressions for E and H regardless of the distance from the
antenna are somewhat more complicated. Hope this helps and 73s from N4GGO,

--
J. B. Wood e-mail: