I took a look at the pattern of a vertical at C5 with respect to
coupling into the ionosphere, and I agree with W4ZV that C5 is very
similar to D4. I also agree with NM7M's assessment that North America is
not in the direction where the coupling loss is highest.
This coupling loss is just another loss that goes into the equation (not
that we really have an equation) that determines how strong a signal
will be in going from Point A to Point B. For the C5 to East Coast path,
it's 6000 to 7000km and it's essentially all over water.
Thus the coupling loss may not be a big factor on a path this short,
even in the direction of highest coupling loss (toward the Caribbean).
But on a longer path, where every dB really counts, I have no doubt that
coupling loss takes its toll and is very important. And yes, most of its
impact is to a narrow range of azimuthal angles. On the other hand, if
those angles are toward a target area, then it becomes very important.
Along these lines, W4ZV made a good point about all the other factors.
In addition to all the normal variables in propagation on our higher
frequencies, 160m also has the Earth's magnetic field and
weatherrelated effects tied to its propagation characteristics. I think
it can be very difficult to separate out all these variables to say that
X is what caused Y. All we can do is try to understand all of this and
apply it to make our chance of success as high as possible  whatever
the scenario is.
Carl K9LA
