More excellent comments.
Yes, I've been thinking about all of this in terms of wave theory, which
I'm pretty sure is valid for HF ;)
But just to clarify, I'm not trying to justify or promote HFTA. It just
happens to be the only piece of software I have that models complex
interactions of reflected and diffracted waves. How well it does that I
can't comment on, but I did find it interesting (and hopefully valid)
that it showed different degrees for diffraction for different shapes of
terrain features at the same height and distance.
My basic hypothesis is still the same, though, at least until someone
can help me wrap by mind around an explanation of my error, that an HF
signal (a wave of energy) could react differently to an asymmetric
terrain feature when traveling from opposite directions along the exact
same path as long as the asymmetry was significant relative to a wavelength.
73,
Dave AB7E
Michael Keane K1MK wrote:
> At 01:34 PM 11/3/2007, David Gilbert wrote:
>
>
>> Yes, the single ray (for the sake of argument) that travels from A to B
>> is monochromatic, but it isn't infinitely thin in terms of it's
>> interaction with surroundings.
>>
>
> Ahhh, there's a conceptual issue. In terms of geometric optics and
> ray tracing, a ray is assumed to be infinitely thin
>
>
>> That single "ray" is energy traveling
>> with a spatial density (in terms of it's interaction with surroundings)
>> that is a function of it's wavelength.
>>
>
> That's a ray bundle or a wave.
>
>
>> A "ray" passing within a
>> wavelength or few of a physical feature is going to be influenced by
>> that feature in a manner that is a function of its conductivity,
>> dielectric constant, physical shape, and the incident angle.
>>
>
> That's physical (or wave) optics.
>
>
>> My
>> proposition is that since the shape of a feature (relative to a
>> wavelength) as seen by the ray isn't necessarily the same going in both
>> directions, the paths taken aren't necessarily the same.
>>
>
> HFTA is based upon a computational method known as the Geometrical
> Theory of Diffraction (GTD) which attempts to combines geometrical
> optics (ray tracing) with diffraction at surfaces (wave optics).
>
> This is done by shooting out lots of individual rays and then uses a
> diffraction kernel (mathematical prescription) that describes how a
> single incident ray reflects/diffracts into arbitrarily many
> (depending on the resolution) diffracted rays at each
> interface/boundary. HFTA then sums up all the refracted and
> diffracted rays going in a certain direction at infinity to compute
> the gain/loss in that direction.
>
> In the GTD, a ray is unaffected by an obstacle that it passes near,
> only by obstacles that the ray intersects.
>
> 73,
> Mike K1MK
>
> Michael Keane K1MK
> k1mk@alum.mit.edu
>
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