[TowerTalk] NVIS antennas Re: dumbing down

[Jim Lux]

At 01:33 PM 7/22/2005, ersmar at 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