Topband: Antenna Impedance vs efficiency

[K9AY]

> Lately I've been pondering the notion of the impedance of free space.  I'm 
> told that the characteristic impedance of free space is 377 ohms.  I have 
> no idea where that comes from, or how it is measured.

> Ford-N0FP
________________

First, it's not measured.  The impedance of free space (377 ohms) is a 
mathematical construct, obtained by solving Maxwell's equations for an ideal 
isotropic radiator (point source), integrated over a spherical surface 
(radiation equal in all directions -- in three dimensions), with propagation 
at the speed of light.  The result is 120 pi = 376.9911 ohms.

Ohm's Law says ... R = V/I ... In this case, V is the E-field vector, and I 
(the current vector) is computed from the H-field.

A useful analogy is that the feedpoint impedance of an antenna (in free 
space) is the result of transforming the free-space impedance through the 
antenna's structure.  It's not quite this simple, but I find that it helps 
bridge the gap between our 'reality' and the abstract concepts of 
displacement currents and fields.

Where things get difficult is that we don't have antennas in free space --  
we operate in half space with near-free-space above and irregular dielectric 
properties below, along with many objects that disturb the fields of the 
specific structure we call the antenna.

73, Gary
K9AY