On Sun, 16 Nov 2014 21:11:49 +0100, Helmut Wabnig [email protected] ---
-.dotat wrote:

I have seen people talking about NVIS antennas for DX.
w.

They may actually have a point. The problem is the assumption that
when bouncing RF off the ionosphere, the angle of incidence is equal
to the angle of refraction. In other words, to do DX, you need a low
angle of incidence.

I got the clue long ago, when I noticed that spinning a beam (yagi)
antenna, often resulted in little or no change in signal strength. It
wasn't all the time, but it did happen often enough for me to notice.
The explanation offered by Eric Nichols, KL7AJ is that sometimes, the
signal appears to be coming from directly overhead. I've uploaded a
copy of his Dec 2010 QST article (and added a text layer to make it
searchable):
"HP Ionospheric propagation may not happen the way you think it does"
http://802.11junk.com/jeffl/HF-Circular-Polarization/
http://802.11junk.com/jeffl/HF-Circular-Polarization/QST_Dec_2010_p33-37.pdf
"The answer is rather simple, once one recognizes that those
signals are circularly polarized. Actually it’s coming from
straight overhead."

I built a copy of his setup using junk parts and tested it with WWV
15MHz. I would agree that the signal is certainly circular polarized,
but I'm not 100.0% sure that it's always arriving from directly
overhead.

Please note that NVIS is limited by the maximum usable frequency of
the F layer and is usually used only on 80 and 40 meters during the
day, and 160 and 80 meters at night:
http://en.wikipedia.org/wiki/Near_vertical_incidence_skywave

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558