[TowerTalk] Re: Dipoles or G5RV

n4kg@juno.com n4kg@juno.com
Thu, 1 Feb 2001 05:18:41 -0600

N4KG comment below.

On Wed, 31 Jan 2001  "Al Williams" <alwilliams@olywa.net> 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
>  > >
>  As the excellent article on p39 of Jan 1995 QST reports, NVIS 
> propagation
> is
>  basically straight up radiation without any skip regions i.e. 
> continuos
>  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
> k7puc

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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