[Milsurplus] What is NVIS?

[Brooke Clarke]

Hi Robert:

NVIS is an acronym that stands for Near Vertical Incidence Skywave.  I think the term came about during the Vietnam era, although the mode has always been there.

HF antennas, like a 1/4 wave whip, have a pattern where most of the energy is radiated at low angels.  For HF the coverage is good out to maybe 90 miles, then there is a "skip zone" that may be many hundreds of miles where there's no signal, then the first bounce from the ionosphere, conditions permitting, allows reception.

If the antenna sends the energy nearly straight up (
Near Vertical Incidence) and the frequency is low enough, then it is reflected back down. The coverage is continuos from the transmitter site out many many hundreds of miles and does not require much power.  From Northern California I can work all of the state and Eastward a few states, but not anywhere near the Mississippi or East of there.  This frequency has many factors that cause it to change like time of day, location, etc.  To see an almost real time map have a look at: 
http://www.spacew.com/www/fof2.html - world map
or
http://www.w0ipl.com/ECom/NVIS/NVISprop.htm - localized to a single continent.  The North America map at 10 am today is showing frequencies between 3 and 8 MHz.  This mode does not work at 30 Mhz.

73,

Brooke Clarke, N6GCE

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>Date: Sun, 30 Jan 2005 23:13:33 EST
>From: WA5CAB at cs.com
>Subject: Re: [Milsurplus] tank radio skip?
>To: milsurplus at mailman.qth.net
>Message-ID: <c1.50f4c1de.2f2f0a6d at cs.com>
>Content-Type: text/plain; charset="US-ASCII"
>
>Group,
>
>What's "NVIS"?  
>
>In a message dated 1/30/2005 7:49:45 PM Central Standard Time, 
>kargo_cult at msn.com writes: 
>  
>
>>>A vertical antenna makes a very bad NVIS antenna.  I suspect the antenna 
>>>is indeed a NVIS antenna.
>>>
>>>      
>>>
>
>Robert Downs - Houston
><http://www.wa5cab.com> (Web Store)
><wa5cab at cs.com> (Primary email)
><wa5cab at houston.rr.com> (Backup email)
>