----- Original Message -----
From: "Ford Peterson" <ford@cmgate.com>
> 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. But I do know that
> the characteristic impedance of a 1/4wL monopole, properly constructed, is
> about 38 ohms. That's a ~10:1 impedance ratio. What am I missing here?
> Is there any intrinsic loss attributable to coupling a relatively low
> impedance system to the high impedance of free space? E.g. is there any
> benefit to using a higher impedance system?
>
Good question, Ford. The impedance of free space describes the ratio
of the electric field to the magnetic field for a propagating plane wave.
IOW, if the electric field in a plane wave is 377 volts/meter, the
magnitude of the magnetic field will be 1 amp/meter. This is an instrinsic
(read: unchangeable) property of empty space. As far as I know, there
is no direct relationship between radiation resistance and efficacy of
coupling into free space impedance. One relates to current distribution
and near field conditions, while the other relates to far field properties
of the propagation medium. The analogy between impedance matching
on transmission lines and impedance matching between propagation
mediums does hold, however. If a plane wave in traveling in free space
(Zo = 377 ohms) encounters a propagation medium with different
properties (e.g. a different Zo), there will be a reflection and partial
transmission in the same way that a wave traveling down a transmission
line gets reflected when it hits a impedance discontinuity (a plane wave
encountering a large metallic reflector is analogous, for instance, to a
short circuit on a transmission line). Those blue pyramid shaped things
that you see lining the walls of anechoic chambers used to test aircraft
for radar cross section are an example of an attempt to match the
characterstic impedance of free space to that of a lossy dielectric
(carbon loaded foam). There is a good explanation of all this (much better
than mine) in W8JK's textbook on Antennas.
Mike, W4EF...........................
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