Understanding NVIS 

Thanks!

My pleasure - thanks for the feedback!

Thanks!

Thanks, appreciate the feedback!

My pleasure - thanks and 73! Paul, KO4LZ

Thanks - appreciate the feedback!

Actually, quite a few of the "professionals" that work on this (such as myself, KO4LZ) are amateur radio operators as well :)

Thanks!

Thank you!

Thanks for the feedback!

"Amazing" is a pretty good description of NVIS performance in mountainous areas :)

Thanks!

Really appreciate the feedback!

Good question. When it comes to the ionosphere, it's not easy to define the difference between refraction and reflection. Both cause a charge in direction due to changes in the propagation medium. For reflection to occur, this change has to be very large (air and a solid object), whereas in refraction it's a somewhat more gradual change. Reflection also requires a large (compared to wavelength) and ideally a flat/smooth object, and the change of direction is usually specular (incident angle = reflected angle). None of these apply to the ionosphere. That said, a lot of people imprecisely talk about signals "reflecting" off the ionosphere, and I'm sometimes guilty of this as well :) Depending on the antenna pattern, height, etc. some of the transmitted signal can (does) travel straight up. NVIS is "near" vertical incidence because a purely vertical incident signal would simply come straight back down and not be useful for communication. Purely vertical incidence is useful when making measurements of maximum usable frequency (e.g. ionosondes) but not for much else :)

Polarization is usually not preserved once the signal is refracted from the ionosphere, so the best way to deal with (potential) polarization mismatch would be to use a pair of cross-polarized antennas such as the "turnstile" vertical dipole pair shown in the presentation. Jamming is all about efficiency, and it's a lot harder to jam if you can't concentrate power towards the receiving antenna. Yes, you could simply radiate skywards at a high angle but this would distribute the jamming power over a wide area and thus the J/S (jamming to signal) level would also be very low. And NVIS stations can be DF'ed - I actually hold a patent on one methodology (US11550021B2) :) Thanks!

Thanks, Chris. I didn't appreciate that.

Tnx es 73, Paul, KO4LZ

Incidence angle from vertical (i.e. the angle at which the signal was refracted from the ionosphere)

Good question. I think to warm up water vapor you need energy in the low GHz range, so most HF signals probably wouldn't be able to actually warm the clouds :)

Thanks for the feedback - I guess it depends on how you define "good" :) I've personally worked non-NVIS stations with a NVIS antenna and vice-versa, so it can be done, but results are better with similar (vertical) radiation patterns. Thanks again - 73, Paul, KO4LZ

I've heard "en-vihs", "nih-vihs" and plain old "N -V -I -S" Most of the people I work with pronounce it "nih-vis" and that's why I used that pronunciation in this video.