[TowerTalk] Re: Dipoles or G5RV
Thu, 1 Feb 2001 05:18:41 -0600
N4KG comment below.
On Wed, 31 Jan 2001 "Al Williams" <email@example.com> writes:
> > > NVIS has shown up in several posts. I'm a big fan of acronyms
> but I can't figure this one out. What is it?
> > >
> > > Ken AB5A
> > >
> NEAR VERTICAL INCIDENT SKYWAVE
> As the excellent article on p39 of Jan 1995 QST reports, NVIS
> basically straight up radiation without any skip regions i.e.
> coverage out to about 400 miles.
> I just modeled an 80m wire dipole over perfect ground. The EZNEC
> elevation pattern showed virtually no difference in gain at all
> elevation angles for
> heights of 16,24, and 33 feet. The gain for height of 63 feet was
> down a db or so from elevation angle of 90 deg(straight up) to 60 deg
> where the gain converged with the lower heights.
> PS Another helpful article in QST "Why an Antenna Radiates" , Nov
> 1992 p59
The unanswered question here is:
How well does the 'effective ground' approach 'perfect ground'?
A metal roof may actually come fairly close.
Over 'perfect ground', dipoles lower than 1/4WL
show some gain over a 1/4 WL high dipole.
REAL ground is quite lossy and I suspect
a 1/4 WL high dipole would be a better NVIS
antenna than a lower one due to near field losses.
Remember that most modeling programs do NOT
model real earth losses well. Anyone with NEC2
want to confirm this?
de Tom N4KG
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