From: Doug McKean (email@example.com)
Date: Mon May 08 2000 - 18:53:06 PDT
The ARRL handbook used to have a graph that showed
exactly this response of height versus impedance.
I'm really rusty on this but I believe from a
theoretical side, it's varying by way of a Bessel
function until a max height is reached then levels
From an intuitive side, if it would appear to follow
as a function of wavelength possibly by the reflected
wave off the ground plane causing varying degrees
of interference with the antenna. A max height is
reached where it is no longer a dominate factor.
I would very much agree that near field modeling
could get quite complex. - Doug
Ron Miller wrote:
> TO: Don Degroot@boulder.nist.gov
> In discussing EMI the following conversation came up.
> Can you shed some light on this discussion?
> Ron Miller
> One of my thoughts below:
> IMPEDANCE OF FREE SPACE VS HEIGHT NOT CONSTANT
> I have seen dipole antenna impedances versus height above the ground
> which go from zero ohms at ground level to about 400 ohms at 1/4 wavelength
> and back again to low impedance(~50 ohms) at 2/4, and 400 again at 3/4 and
> ripple like that with height until they meet an assymptote at 377 ohms.
> >From these impedance graphs I surmise that the near field impedance
> requires a more complex model which would probably come from transmission
> line equations. For our EMI case these equations would likely take the form
> a coupling structure to the 377 ohms of free space.
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