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

[Jim Brown]

>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