[Elecraft] Re: Band condx Questions from a Liberal Arts Major

[Jack Smith]

Ionospheric reflection seems to be generally N-ray fading, not two-ray 
fading. Two-ray fading gives you a Rician distribution when looking at 
the probability density function, and N-ray fading is Rayleigh 
statistics with a Rayleigh PDF.

I've looked at this  recently but not nearly in the detail I would like 
to, but what I have seen is closer to Rayleigh than Rician fading. The 
standard references, including both of Kenneth Davies' books on 
ionospheric propagation say Rayleigh fading as well.

Rician fading is more common in microwave where a direct and reflected 
path will exist due to abnormal atmospheric conditions or mid-path  
reflections. (Paths are normally designed not to have a reflected path, 
but sometimes they occur.)

I've put some data up at 
http://www.cliftonlaboratories.com/signal_statistics.htm but as I say, 
it needs quite a bit more work and analysis to verify Rayleigh versus 
Rician fading. Over  the years, I've collected a lot of signal data at 
VHF and UHF, going back to when we drove a chart recorded from the 
speedometer cable and there's no doubt whatsoever that in the urban 
environment, fading follows Rayleigh statistics once you are more than a 
kilometer or so from the transmitter.

Jack K8ZOA


Jim Brown wrote:
>> The fading on SSB and CW modes can tell you a lot about what the 
>> band is doing. 
>>     
>
> Yes, but not necessarily what you think. Most of the "in and out" 
> fading we hear on HF is the result of multipath. That is, the signal 
> travels over two paths that differ a bit in time, which puts them 
> out of phase. When they out of phase by some odd multiple of 180 
> degrees, they cancel, and by an even multiple of 180 degrees, they 
> add. 
>
> The time interval of the fading depends on the time difference of 
> the arrivals and the wavelength of the signal. We're used to hearing 
> this kind of thing on mobile signals, and also when an aircraft 
> reflects a VHF or UHF signal. Those VHF/UHF wavelengths are short, 
> so the fading has a fast characteristic (an interval often less than 
> a second). Down on 160M, it's common for the fading interval to be 
> tens of seconds or a minute. 
>
> The DEPTH of fading (that is, the weak part of the fade) depends on 
> perfect cancellation of the signal, which happens when the two 
> arrivals are almost precisely equal in strength. 
>
> 73,
>
> Jim Brown K9YC
>
>
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