NVIS techniques, Part 5

Keypounder continues his updates on NVIS, a subject he first wrote about 5 years ago, which NC Scout published at the Brushbeater site.  This article is being posted as the 6th of what looks to be now  at least 7, more likely 8 articles on NVIS.

As NC Scout stated 5 years ago-

“…. I will re-iterate that these skills, along with Land Navigation, are among the most perishable and most difficult to learn- under duress, near impossible. So for those of you who feel you’ll do it when ‘the time comes’, you’ll be sadly mistaken.  Please folks, try this at home.”

Part One of this series on NVIS operation focused primarily on the basics of NVIS; what it is, why it is, how it works, and listed some of the major factors involved in successful NVIS operation, briefly touching on these factors. Link here: https://www.americanpartisan.org/2021/05/nvis-techniques-part-one

Part Two of this series on NVIS operation looked at HF listening and transmitting techniques, some specific to NVIS. Link here: https://www.americanpartisan.org/2021/05/nvis-techniques-part-2

Part Three discussed how to decide which HF radio to purchase. Several common civilian amateur radios will be reviewed in some detail, and general characteristics desirable in an NVIS station specifically was discussed. Link here:https://www.americanpartisan.org/2021/05/nvis-techniques-part-3/

Part Four reviewed NVIS antenna characteristics in detail, and discussed different types of operation and a brief discussion of the implications of these differences on antenna selection.  Link here: https://www.americanpartisan.org/2021/06/nvis-techniques-part-4/

Part  4 1/2, was an amplification and further discussion of simple NVIS antennas in more detail, including the effects of height on antenna performance in response to questions.  Link here: https://www.americanpartisan.org/2021/07/nvis-techniques-part-4-1-2/

Part 5 is a discussion of advanced NVIS antennas  primarily for fixed locations and beginning to touch on NVIS operation in non-permissive environments.

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Advanced NVIS Antennas

So far, we’ve covered the simple and easy to erect NVIS antennas, all dipole variants, suitable for man-portable operation.  Now we are going to examine some more advanced antennas that offer advantages for fixed permanent or semi-permanent locations.

First up are the family of double dipoles. These can be deployed either in parallel, spaced about half to 5/8 wavelengths apart, or in line. As one might expect from the examples seen to date, the flat-top versions maximize gain and minimize low-angle ground wave off the ends of the antenna, at the price of more effort to erect. The Inverted vee types take less time to erect and take down, but have significantly more low angle radiation. The Vee antennas are lower noise, with less end-fire vertical RF than the Inverted Vee, but more than the Flat-top.

However, if you space two dipoles, both fed in phase, a bit over ½ wavelength apart from feedpoint to feedpoint and in line, regardless of the type (Inverted Vee, Flat-Top or Vee) the vertical radiation off the ends is significantly attenuated, while providing broad coverage off the broadside of the antenna. If you have a preferred direction for comms, or if you have a particular direction that concerns you with regard to being DF-ed, then this antenna offers some advantages if you have the space for it.

Here is the broadside pattern:80M_2el_inline_InvV_broadside Here is the elevation plot off the ends of the dipole:80M_2el_inline_InvV_offends Here is the azimuth pattern at 6 degrees of elevation off the horizon:80M_2el_inline_InvV_AZ_6degEL

Note the significant reduction in vertically polarized RF off the ends of the antenna; there is an extremely deep notch off the ends of this in-line array, a significant plus for local noise reduction.  Even where the vertically polarized RF is at a maximum (the 4-lobed red line pattern) the vertically polarized RF is down over 20, approaching 23 db of attenuation.  The average is much less;  30 db reduction of vertically polarized RF reduces the ERP by a factor of a thousand, reducing the received signal by over 5 S units from the peak overhead gain.

Advantages of the dual in-line dipole are:

  • Low amount of low-angle vertically polarized radiation- much less ground wave than other antennas;
  • Deep notch off the ends allows nulling of a single local noise source, improving S/N ratio.
  • A broad pattern of broadside horizontally polarized high angle RF, covering a wide area while reducing POI and improving S/N ratio over the basic NVIS antennas;
  • About 5dBi of gain, a bit more than 2.5 dB better than a single inverted Vee.
  • Easier to deploy in mature forest than a cross dipole or a loop
  • Coax feeds are lower profile than other antenna types that use window or parallel feeders.

Disadvantages are:

  • Requires more feedline than a single antenna or the side by side double dipole below, adding cost and weight;
  • Requires an impedance transformer for best performance;
  • Requires an area more or less in a straight line, with trees or other supports properly spaced,  for a full wavelength on the band of interest;
  • Not suitable for multiband use.

This antenna is a good choice for a semi permanent or permanent installation, especially if a flat-top configuration, which gives another dB or two, is employed. This requires only 5 lifting points at most if you elect to set up a flat-top array, as the center hoisting point can be shared.

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Next is one of the classic NVIS antennas, often cited in the literature on the subject, the Shirley. This was one of the first advanced antennas I modeled because it was a classic, and I was surprised at what I found. Here is the pattern of the Inverted Vee version of the classic Shirley antenna- This pattern is off the ends of two side by side Inverted Vee antennas:

80M_Shirley_InvV_offends

Note that this setup reduces the low angle vertically polarized RF off the ends only about 15 dB at low angles. The horizontal Rf off the sides looks more like the pattern off the ends of the double in-line above, see the pattern here:

80M_Shirley_InvV_Broadside

 So, the Shirley limits the range of the horizontally polarized RF, but the vertical RF off the ends looks like the pattern from the single Inverted Vee, down only about 15 dB from the peak, about the same reduction from the peak overhead gain as the inverted Vee.  You can improve this by going to a flat-top Shirley, but not by much. This antenna radiates distinctly more vertically polarized RF than the dual inline. While the overhead gain of both the double inverted Vee antenna arrays is about the same, a tad over 5 dBi, the reduction in vertically polarized groundwave RF of the double in-line antenna array is much higher than the classic side by side array. The Shirley poses a significant risk of being DFed by ground wave. So why is this antenna still published in books on NVIS?

When the Shirley was first developed and used, there was little concern about the possibility of DF being employed against the users; the Shirley was developed by Major Shirley, a British Army officer during the Malayan crisis around 1950. The Malayan guerillas did not commonly use HF DF and did not have airstrike or artillery capability, so the operational advantages of the Shirley far outweighed the negatives. In the 21st century, those assumptions are no longer prudent for those operating in non-permissive environments. We’ll get a lot further into this when we review operating in non-permissive environments in one of the upcoming posts, but there is a lesson here. Not all classic solutions are still applicable, and you should not make the same mistake I did at first by ignoring the implications of modern technology, even when selecting something as simple as your antenna.

It is possible to set up a resonant dual-band broadside NVIS array, a multiband Shirley which does not require bandswitching and which can be used for cross-band simultaneous operation. I did a fair amount of modeling of this option including the feedline arrangements required. The radiation pattern for this is the same as for a single band, so there is no particular advantage to setting one up if you do not contemplate multi-band ops. It requires multiple supports (10 for an 80-40 setup; 12+ for a 160-80 setup) and is not a low profile installation if erected in an open area. If erected in a wooded area it can be difficult to get the elements properly arranged with proper spacing needed for best efficiency.  With all that said, if you have a fixed location where the topography limits the potential for ground wave DF, AND you need the operational flexibility for instantaneous multi-band operation, then the Shirley might be worth a look, but I would make sure that I checked to see that I was not propagating ground wave if I did elect this option.  Caveat Operator.

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Now we move to a variant of the Shirley, the Jamaica, an adaptation of the Shirley, essentially a classic “Lazy H” with doublet elements a full wavelength long spaced a half wave or a bit more apart.  The Jamaica significantly reduces low-angle vertically polarized RF off the ends. It does require a balanced tuner, however, and ladder or window feedline. Here is the broadside pattern:

80M_Jamaica_FT_Broadside

Here is the pattern off the ends:

80M_Jamaica_FT_offends

Here is a horizontal slice taken at 6 degrees elevation- note that the pattern is more complex than with a shorter antenna: