[Milsurplus] Final NVIS post, - promise!

[gl4d21a at juno.com]

Group:

There has been some misunderstanding of my earlier post on NVIS.

I agree that NVIS exists.  No problem there.

What I object to is the misuse of terminology to call *ALL* ionospheric propagation of low HF by the very specific title Near VERTICAL Incidence Skywave, when simple geometry shows the take-off angle cannot be NEAR vertical, and in many cases is nearer horizontal.

Maybe one of the problems is one of reference.  Hams measure radiation angle from the horizontal, so a low angle is near the horizon.  Radio Physicists and mathematicians usually measure angles referenced to the normal, so a small incidence angle is near vertical.  I'm going with ham use, as I haven't had an ionospheric physicist check in here.

For the technically inclined, yes, there can be multiple bounces, and over salt water, they can produce signals of significant amplitude.  Here again, we can look to the results of years of propagation research for answers.  Specifically, the Critical Frequency, F sub C is the highest frequency which will reflect a(n absolutely) vertical incidence signal.  It is measured continuously at many locations.  The maximum usable frequency (MUF) is calculated from it and is always a higher frequency, but is also path dependent, as higher frequencies will be reflected at lower (nearer horizontal) incidence angles.  In other words, the ionosphere reflects better at glancing angles than head on.  Ground bounces account for 20 dB or more of path loss.  So, a two bounce NVIS signal will arrive slightly later than, and 20 dB or more weaker than an obliquely propagated one bounce signal.  Anyone with access to the Navy's "PROPHET" propagation prediction and analysis program can see the family of modes graphically illustrated, and also see a table of the relative signal strengths (or path loss) for each one.  Other programs may also produce this information, but I have not used them enough to claim to be familiar with them.  Frankly, I have fewer occasions to run PROPHET analyses these days too, so I'm getting a little rusty on it.

I might also point out a slightly misleading comment about quarter wave vertical antenna radiation patterns.  An overhead null exists, and there is significant signal radiated from say 10 degrees elevation angle up to 70 degrees or so, but below 10 degrees, the specifics of the antenna installation (e. g. number and quality of radials) and nearby terrain have the controlling effect, so blanket statements about the relative signal level at unspecified low angles can quickly get one in trouble unless all the parameters are carefully specified.

Another discussion was about loops near a reflector.  Again, be careful.  Non-resonant loops behave differently than resonant loops.  Non-resonant (small with respect to a wavelength) loops in the horizontal plane over a reflector still exhibit an overhead null, very similar to a vertical radiator, while larger loops at higher frequencies (think one wavelength on 20 meters for example) mounted in a horizontal plane and spaced above a reflecting earth can have a vertically directed lobe, depending on spacing.  Unfortunately, that is way above the critical frequency, and while someone in the right place in outer space will get a good signal, it's not going to help your terrestrial DXCC score much.

Hope this enlightens,
73,
George
W5VPQ

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