[TenTec] New and Improved Terminology (NVIS origins)

[Richards]

On 1/5/2011 5:38 PM, Steve Hunt wrote:
  Do some simple
> geometry and you'll see that at really low elevation angles - 5 degrees
> say - the ground reflections from an 80m quarter-wave vertical will
> extend out to at least 750ft from the base of the vertical. A typical
> ground radial system will do nothing about the conductivity there.

		Whoa....  I would need a much bigger yard for those !


> You can check this using EZNECs ability to have two different ground
> media. Set up Media 1 as salt water extending for about 50ft around the
> vertical, and Media 2 as rocky soil beyond.


		A local ham did that for me when I was researching
		vertical antennas....  but I think I will re-visit the issue
		with him and discover what might have gone wrong,
		or where we might have leaped along a bit.   He is the
		sort of engineer who would be just as curious about
		it as I am so he won't be offended or become defensive
		if I challenge our previous work.


Look at the elevation
> patterns - they will be those which pertain to poor ground. Now
> gradually extend Media 1 outwards until you get the elevation pattern to
> match that over salt water - that will tell you how far out the
> reflections are taking place and how extensive your radial system would
> need to be.


		Interesting point.    This has not come up before, and
		seems like a worthwhile distinction.   I wonder how or
		why this has been overlooked or not clarified in the
		literature.


> Please note - I'm not saying a good ground system doesn't help. I'm
> saying that the usual ground radial system predominantly affects antenna
> efficiency and not elevation pattern.
>
> Please also note that nothing I have said conflicts with the Exam
> Question answers.


		I see that.    It does, I think, contradict what Old Gordo
		says on his audio course.   He seems to have conflated
		the two concepts of efficiency and elevation - and given
		the test questions, his comments about increasing low
		angle radiation seem unnecessary, if not mistaken.

		Hmmmm . . .

This is what Mr. Gordo says verbatim on Disc 6 of his Extra Class Exam 
course:

		"What strongly affects the shape of the far-field low angle
		which we want?   the low angle elevation pattern of a vertically
		polarized antenna ?  Well, that would be the conductivity
		and dielectric constant of the soil in the area just below the
		antenna.   And the more conductive the soil is, the better
		the low angle of radiation.    How is the far-field elevation pattern
		of of a vertically polarized antenna affected by being mounted
		over seawater versus, let's say, ah, dry rocky ground?  Well the
		low angle radiation increases and that is what we want is low
		angle radiation.

		Now don't read too much into this... when they say it increases,
		it does not mean it goes up, it means that there is more signal
		coming out of it at a low angle.

		What is the main effect of placing a vertical antenna over an
		imperfect ground... the big effect there is it will reduce the
		low angle radiation and create high levels of radiation like
		NVIS  where you are bouncing signals and, as hams call
		it, "warming the clouds."

		So, if you are thinking of putting up a vertical antenna, on
		your roof, then have plenty of ground radials, usually two
		to three inch-wide copper foil strips, as the ground plane.
		If you are thinking of doing a vertical antenna in your back
		yard, relying on your soil as ground, unless you live right
		on the water, I recommend don't.  Put it up on a metal
		shed or up on a roof, with ground foil radials, and you
		will have a much better low angle radiation pattern."

		
So.... I guess I am re-thinking all of this and not so sure that Ol' 
Gordo is sufficiently precise when he says all this about improving 
vertical antenna low angle radiation.   Or am I still missing something ?

Thanks.


===============  Happy Trails - James - K8JHR  ============