Rick wrote in newsan.2007.04.09.16.49.43.321645
@reply.in.gp:


My experience seems to be that the performance of NVIS dipoles (at or
under around 30 feet high for 80 meters) doesn't noticeably change with
orientation. In other words ... and despite what modeling programs say
... BVIS dipoles are pretty much omnidirectional.

I don't know that modelling programs are in conflict with the the
observation that dipoles low over real ground are nearly omni directional
at high elevations.


I haven't scientifically tested that but it does seem to me that since
most NVIS radiation goes approximately straight up (definition of
NVIS),
it really shouldn't matter much how the antenna is oriented.


I wrote an article on the design of an antenna for local contacts on 40m,
it is at http://www.vk1od.net/7MDipole/7MDipole.htm . It may be of
interest.


Have you all found that to be true, or false?

If I do want near-omnidirectional performance, in the real world
(versus
in a software program's modeling), am I better off with an inverted vee
that's up 30 feet in the center, vs a dipole that's 30 feet up along
its
full length?

I think you are on the wrong tram about the modelled performance.


How about an inverted vee that's up 50 feet in the center and 10 feet
at
the ends, versus a dipole that's 30 feet up along its full length?


Mounting a half wave dipole low over real ground modifies the pattern
from the free space pattern, the the whole pattern is deformed by the
presence of the ground and nulls previously off the end are not nearly as
pronounced.

When you rig the dipole as an inverted V, the pattern is further deformed
and the nulls are even less pronounced.

Ask yourself the question "is a dipole horizontal over ground (at any
height) omni-directional at the zenith?". If you found a modelling
program that, used properly, says otherwise, tell us about it.

NVIS isn't strictly about the zenith, but an antenna that is omni at the
zenit, and doesn't have deep nulls (dipole low over real ground), is
close to omni at high elevations. The exact variation will depend on
frequency, mounting height, ground parameters, and leg angles if an
inverted V. If you could rotate such a dipole, I would defy you to
reliably measure the out-of-omni above 50 deg elevation with an S meter.

Owen