Posted by
Kok Chen on
Sep 16, 2010; 6:11pm
URL: http://elecraft.85.s1.nabble.com/K3-Diversity-Reception-and-Antenna-Directivity-tp5537531p5539759.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/enWhenever 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=1090438Each 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_distributionhttp://en.wikipedia.org/wiki/Rice_distributionI.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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