http://elecraft.85.s1.nabble.com/K3-Diversity-Reception-and-Antenna-Directivity-tp5537531p5539796.html
> On Sep 16, 2010, at 9/16 9:59 AM, Jim Brown wrote:
>
>> A VERY large component of fading is due to multipath -- that is,
>> the arrival of more two or more wavefronts that travel different
>> paths, and
>> thus have slightly different travel times.
> Selective fading does not require multipath. The CCIR 520-2 profiles
> for Raleigh fading are all single path models. Raleigh fading causes
> selective fading.
>
>
http://www.itu.int/rec/R-REC-F.520-2-199203-W/en>
> Whenever you hear distorted AM signals on HF, chances are it is caused
> by selective fading. You can see selective fading in a waterfall by
> watching the fading occur as moving holes that sweep across the
> spectrum (very visible when you tune in broadband signal such as a
> Coast Guard weather FAX station on HF). You can also see it take away
> individual tones in an Olivia signal in a waterfall. Selective fading
> was also one of the primary impetus to switch from on-off keying to
> FSK in the early days of RTTY.
>
> The Watterson model for ionospheric propagation breaks up a path into
> a complex signal with in-phase and quadrature components:
>
>
http://ieeexplore.ieee.org:80/xpl/freeabs_all.jsp?reload=true&arnumber=1090438>
> Each component passes through a independent scattering function that
> have Gaussian random processes, both a Gaussian Doppler spreading term
> and also a Gaussian amplitude term.
>
> The modulus (i.e. "amplitude", or square root of power of the I and Q
> components) of a bivariate Gaussian random process happens to have
> Rayleigh distribution. See references here (the Rician distribution
> is more general in that the mean of the components need not be zero):
>
>
http://en.wikipedia.org/wiki/Rayleigh_distribution>
http://en.wikipedia.org/wiki/Rice_distribution>
> I.e., the amplitude of a signal whose I and Q components each have
> independent Gaussian statistics, has a Rayleigh distribution.
>
> As a consequence, a Rayleigh signal can cause selective fading without
> the need for a second path. Since the Rayleigh probability density
> function has finite probability of being infinitesimally small, the
> fade has a chance of being very deep.
>
> If you run a signal through an HF Channel Simulator (such as AE4JY's
> PathSim or cocoaPath) set to rayleigh fading parameters, you will see
> selective fading.
>
> Watterson's paper also considers both the cases of multi- paths and
> multi- magneto-ionic rays that are scattered by the ionosphere.
>
> Multipath signals have a time delay, multiray signals do not have a
> time delay between the rays.
>
> It is a fascinating paper that hams interested in HF propagation
> should read. Unfortunately, I have not found a free version on the
> web that I can reference, even though the research was done using tax
> payer's money at what is today NIST. But if you are an IEEE member,
> you can download the paper for free.
>
> 73
> Chen, W7AY
>
>
>
>
>
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