[Milsurplus] NVIS Rant

[gl4d21a at juno.com]

NVIS lovers:

Let's see if we can apply some Physics and other facts to this NVIS discussion and make it a little more realistic.  First, the portions of the ionosphere of interest are anywhere from 90 (bottom of E layer)to 300 (top of summer F2 layer) kilometers above the surface of the earth.  Let's translate those to miles, since we are more familiar with them from driving around on the surface.  So, the lower end of the E layer is at 54 miles, and the higher end of the F2 is up at 180 miles.  These are all well known, confirmed and regularly measured heights.  Vertical incidence sounders and critical frequency F sub C have been under observation for a long time.

Another concept which is useful to keep in mind here is the concept of Maximum Usable Frequency (MUF) and Lowest Useful Frequency (LUF).  Notice the difference in the two middle words.  On the high end, as you go up in frequency, propagation quits quickly and thoroughly due to loss of ionospheric reflection.  Signals just aren't there.  On the low end, as you go lower in frequency, absorption increases, and mega multiples of power can be used to get through when signal levels start dropping.  A daytime phenomenon seen at LF up through 6 MHz or so.  See the MUF map (URL in a previous post) at spacew.com for real time information.  I'm going to incorrectly use the term ground wave for surface wave, ground wave, etc., since the variations of interest here are small.  For the cases under consideration it makes little difference except to the purist.

Now, the NV in NVIS supposedly means Near Vertical.  How near is Near?  Well, somebody has to decide, and I'm going with practical grammar.  I submit that anything beyond 45 degrees is nearer horizontal than vertical (NHIS?), so I offer that as the absolute limiting case.  Soooo, the longest range which you can possibly attribute to NVIS simply using geometry is (drum roll) 360 miles.  Forget describing 500 mile NVIS contacts, unless your geometry or ionosphere is something which the mathematicians or Ionospheric Physicists missed somehow.  And, using 15 or 20 degrees off vertical as an angle which might more honestly be called *Near* Vertical, the maximum ranges are more on the order of 40  to 130 miles.  The rabid promoters of NVIS apparently either never did this simple geometry exercise, or prefer to play fast and loose with the English language.  NVIS out to maybe 100 miles, 150 if you insist on stretching it.  Beyond that it is a gross misnomer, particularly since I used (daytime summer) maximum F2 height and not average F height.  How near is Near?  Horizontal is near vertical?  Only 90 tiny degrees away!  Sounds like advertising on one of the Border Blasters.

Brooke said that the NVIS term came about during the Vietnam conflict, "---ALTHOUGH THE MODE HAS ALWAYS BEEN THERE".  Yes! Yes! Yes!  It has ALWAYS been the principal mode of propagation on short haul HF.  "Noooooo! no-no-no" you respond, "what about ground wave?  I talk to my local buddies on 75 meters (3.7 to 4 MHz for you non-hams) GROUND WAVE!"  (Except during radio blackouts.  There's a clue there.)  Well, I should really have dug out the texts, but simply listening on 75 meters here last week provided suitable confirmation.  I observed local stations all within a 20 mile radius, running ~100 watts into doublet antennas, who were unable to hear each other in the morning on several occasions until the F layer recovered enough to support ionospheric propagation.  Solar flare = ions, electrons, all that.  See any propagation handbook.  Ground wave off those horizontally polarized antennas just wasn't there.  Certainly not the 90 miles mentioned previously (for a good vertical), and in the local case, not even 15 miles.  Longer range stations were heard, due to the fact that the ionosphere will reflect a glancing angle with a lower ion density than is required for NVIS.  And, that also confirms this was not an absorption event, which is an ionospheric phenomenon and does not affect ground wave.  So if not ground wave and not NVIS, what's left?  Unless you want to claim reflection off airplanes and flocks of low flying birds, nothing.  By process of elimination, "---the mode has always been there".

Now, if you want to look at antennas actually optimized for NVIS, forget the lossy configurations of wires on the ground described elsewhere.  Go to W4RNL's web site and look up "Cloud Burners".  Antennas which actually improve upward radiation, rather than reducing efficiency in all directions.

By the way, among the DF papers I have previously mentioned on here is a 1940s description of low HF propagation experiments in Panama by some government agency, I forget which, but it was for the Army, to address this very problem.  Short range (tactical) jungle communications at low HF, likely for the Pacific campaign.  Equipments used are mentioned on here often, but I made no record of the models.  Low number SCR something or other.  Same conclusions reached.  Horizontal polarization works better than vertical, ground wave difficult to generate from a mobile platform, etc.  Apparently the results were either never accepted or understood, or instantly forgotten.  Another case where something is reinvented every generation.

Bottom line is that unless you can deploy an efficient vertical radiator (read: at least nearly a quarter wave high with adequate radials or better yet, a properly implemented half-wave vertical) then ground wave success on low HF is likely to elude you.  Broadcast band verticals exhibit efficiencies in the same league as the horizontal doublets described above, but like the full size doublets, they tend to resist being mobile.  And those German frame antennas?  If I understand the description, they are loops in the horizontal plane?  Which have an upward null?  What am I missing?

Hope I didn't exceed Ralph's allowed bandwidth.

73,
George
W5VPQ

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