Hi.  NVIS stands for Near Vertical Incidence Skywave.  It means that the
antenna is designed for minimum ground-wave and low-angle radiation and
reception and is instead aiming more or less straight up.  A dipole or loop
that is up only 10 or 20 feet will probably behave this way on 80 meters,
especially if the ground under it is conductive.  But high-angle propagation
is what you usually get on 80 meters during the daytime, anyway.  

As you go to higher frequencies, a straight-up signal will tend to go
through the ionosphere and into space instead of being refracted somewhere
useful.  Occasionally 20 or even 10 meters will have some near-vertical
propagation; in these rare cases you will be working stations 200 miles or
less away from you rather than the normal 1,000 miles or more.  You will
have much more frequent openings on 20 meters and higher if you try to get a
low angle of radiation instead of a high angle.  The only place where
high-angle coverage would make sense is receiving satellites on  10 or 2
meters.  And vertical antennas are, by their orientation, not radiating
straight up.

Hope this helps.  73 --

Lloyd Rasmussen, W3IUU, Kensington, Maryland
Home:  http://lras.home.sprynet.com
Work:  http://www.loc.gov/nls
 
> -----Original Message-----
> From: For blind ham radio operators [mailto:[log in to unmask]]
> On Behalf Of Shaun Oliver
> Sent: Thursday, April 10, 2008 7:04 AM
> To: [log in to unmask]
> Subject: NVIS antennas
> 
> Hi all, what is the best hight for an NVIS antenna for 75 and 40 meters?
> also, can I use an NVIS setup on 15 and 10 meters? all teh documentation
> says I can't but, I don't necissarily believe it. providing conditions
> are favourable, you could load up your old coffee can and get somewhere.
> Failing that, I might just mount a couple of whips on the fence with
> some kind of ground system in effect. While I'm here, has anyone had the
> need to construct a dummy load? if so, what would be the best materials
> for such an undertaking? I have here a coffee tin I aquired from work
> yesterday to house the thing in. any tips and tricks would be greatly
> appreciated. unfortunately, alot of what's on the net is diagrams and
> not at all well described.
> thanks in advance.
> 
> --
> Shaun