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
_______________________________________________
Elecraft mailing list
Post to: Elecraft@mailman.qth.net
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com
_______________________________________________
Elecraft mailing list
Post to: Elecraft@mailman.qth.net
You must be a subscriber to post to the list.
Subscriber Info (Addr. Change, sub, unsub etc.):
http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/subscribers.htm
Elecraft web page: http://www.elecraft.com
