TT:
The distortion that is mentioned below and in (the poorly edited) QST
article is actually alleged multi-path interference. The explanation I've
read in a couple of sources is thus:
When an NVIS antenna is set up to communicate with another station within
the ground-wave zone (defined in the sources as within 20-40 miles or so from
the transmitter for NVIS signals up to about 10 MHz), the different path
lengths (longer ionospheric NVIS vs shorter ground wave paths) can cause
destructive interference of the combined signals at the receiving station. The
use of a low NVIS antenna results, so the theory goes, in a diminished ground
wave signal at the RX, thus allowing the RX to hear the NVIS signal
unencumbered by multipath interference.
My principal interest in NVIS antennas is for RACES and MARS work on HF.
I found a good now-out-of-print source on the technique in "Near Vertical
Incidence Skywave Communication: Theory, Techniques and Validation" by Fiedler
and Farmer. It was available from Worldradio Books, P.O. Box 189490,
Sacramento, CA 95818. Cost me $14 plus a coupla bucks P&H. The articles are
reprints from military and non-amateur researchers, and well as from hams, on
the technique. The measurements done in Thailand are mentioned in the book.
One final item. Here's a site
http://www.co.missoula.mt.us/acs/NVIS%20EZNEC.htm that includes a bunch of
EZNEC studies of various NVIS antenna configurations, including the military
AS-2259 (two crossed dipole inverted V wire antennas with feed point at 15
feet.)
FWIW.
73 de
Gene Smar AD3F
> At 01:33 PM 7/22/2005, ersmar@comcast.net wrote:
> >TT:
> >
> > FWIW - Here's a possible source for the eighteen-inch-above-ground
> > NVIS antenna fables: http://www.tactical-link.com/field_deployed_nvis.htm .
> >
> >73 de
> >Gene Smar AD3F
>
> Interesting..
> "
> NVIS was originally evaluated by U.S. Army Forces in Thailand during the
> Vietnam conflict in the mid-1960's
> "
>
> Yup.. that would be George Hagn's work, I suspect.
>
> Later it says:
> "...This will include multi-path distortion because of the extreme
> difference in the lengths of the two paths. Keeping antennas close to the
> ground will reduce the generation of a ground-wave signal. .."
>
> Comments anyone? They don't explain why in this article. Could it just be
> that close to the ground it's such an inefficient antenna overall that not
> much of anything is radiated? Or is it a pattern effect.. suppress the low
> angle signal.
>
>
>
>
> I ran a quick NEC4 model of a 40m dipole at 50cm above "average earth"
> (ideal conductor for the antenna though)
>
> As one might expect, it has a fair amount of gain straight up (9 dBi)
> Out to 45 degrees elevation the gain ranges from 2 to 6 dBi
> At 30 degrees, it's dropped off to -4.4 to +3 dBi
> At 15 degrees, it's -16 to -2.7
>
> So, fairly omnidirectional for high elevation angles.
>
>
> Now, compare that to the same antenna, but at 2 meters off the ground
> Same 9dBi straight up
> same 2-6 dBi at 45 degrees
> same -4.4 to 3 dBi at 30
> same -16 to -3 at 15
>
> Now go up to 10meters (about 1/4 wavelength)
> 7.61 dB straight up
> 4.75 to 7.13 dBi at 45 degrees
> -3.2 to 4.3dBi at 30
> -14 to -0.8 at 15
>
> Starting to get a bit more signal closer to the horizon, and a bit less
> straight up, and a bit more at 45 degrees, but we're talking about 1-2 dB,
> which is insignificant.
>
> Now go up to 20meters (half a wavelength off the ground) and things change
> a bunch
> At the zenith, -10dBi
> At 45 degrees, +2.7 to +6.7 dBi
> At 30 degrees, +0.62 to +8.1 dBi
> at 15 degrees, -8.5 to +5.1 dBi
>
> A big drop in straight up, about the same at 45 (although a 4 dB difference
> depending on az)
> A big gain at 30
> A huge gain (depending on direction) at 15 degrees.
>
> OK.. So, if we assume that for NVIS, you're interested in angles ABOVE 45
> degrees, going above 1/4 wavelength off the ground really starts to
> hurt. And, the low level radiation starts to come up.
>
> And, for those above 45 degree angles, the height of the antenna makes
> almost no difference, all the way from 50 centimeters (pretty close to
> 18") up to 10 meters.
>
> I might have made a mistake here, but it sure looks like it's pretty casual
> on antenna positioning for close in work.
>
> here's the model file:
> CM Dipole over groundplane ' Comment cards
> CM J. Lux 7 March 2005
> CE ' End of comment
> SY ht=20 'height of dipole above ground
> SY f=7.2 ' frequency
> SY l = 0.468243 * 299.8/f ' in meters
> SY tilt=0
> SY xsp=0 'x spacing
> SY ysp=0.6 'y axis spacing
> '
> GW 1 15 l/2 0 0 -l/2 0 0 l/200
> 'GM 0 0 0 tilt 0 0 0 0
> 'GW 2 15 xsp+l/2 ysp 0 xsp-l/2 ysp 0 l/200
> GM 0 0 0 0 0 0 0 ht
> GE 1 ' End of geometry
> '
> GN 2 0 0 0 13 .005
> 'LD 0 2 8 00 5.57 0
> EX 0 1 8 00 1 0
> ' Voltage source (1+j0) at wire 1 segment 5.
> 'EX 0 2 8 00 1 0
> '
> FR 0 1 0 0 7.2 0 '
> RP 0 31 61 1101 0 0 3 6
> '
> EN ' End of NEC input
>
>
>
>
>
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