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 ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
