Paul Butzi |||

NVIS Thinking

After giving my newly revised presentation on NVIS propagation and NVIS antennas last night I was left unsatisfied with the last part of the presentation covering several antenna designs from an NVIS perspective.

Part of that dissatisfaction stems from my growing suspicion that looking at NVIS antennas from a vertical pattern and gain perspective is just the wrong way to go about it. The reason for that suspicion is that the difference between a highly optimized antenna design and a less tuned design will be about 2-3dB of gain, and the difference between a compromised performance’ antenna and the highly optimized design will be perhaps 8db. That works out to about half an S unit of signal strength in the first case and 1.2 S units in the latter. I now suspect these differences in performance are swamped by other considerations.

The highly optimized antenna designs are also sensitive to construction, ground quality, and a whole host of hard to control factors. In the real world, such a design may easily lose quite a bit of performance and end up not outperforming a simpler, more forgiving design.

So if vertical pattern/gain is not the metric to use to judge NVIS antenna designs, what should the metrics be?

The following come to mind:

  • cost
  • ease of setup and tuning
  • low SWR across entirety of 80m, 60m, 40m (with 160m and 30m a plus)
  • band agility - being able to quickly and effortlessly shift from 80m to 60m to 40m (with 160m and 30m a plus)
  • size of antenna and area consumed when set up.
  • ease of construction
  • sensitivity of design to minor construction errors
  • portability
  • flexibility of design in terms of being adaptable to different environments - poor ground quality, lack of supports, etc.

And then those metrics need to be applied to three different situations

  • permanent installation, where construction effort, cost are amortized over a long period of use and ease of setup and tuning are investment that will pay off thru better performance over the long haul
  • emergency deployment, where the antenna must be brought to the site and set up, but little will be known about ground quality in advance, and elaborate antenna suspensions will not be practical.
  • portable use, similar to emergency deployment but the antenna must be even lighter and easy to erect with whatever can be found on site.

One thing that’s clear to me is that there isn’t going to be the One NVIS Antenna Design to Rule Them All. A loaded fan dipole with one feedpoint height might make sense for emergency deployment but is probably wrong for a permanent installation. A honking big low altitude set of horizontal loops might be great for a permanent installation but would be madness as a portable solution. And a random length end-fed wire might be fine for portable use but too compromised for anything else.

More thinking on this is definitely needed.

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