NVIS performance vs quality of ground.

[WB4WZR]

I work QRP portable in the high forests of Appalachia using trees as supports. I can usually get antennas up higher than recommended NVIS heights with a slingshot.  Should I set my sights lower to gain NVIS operation?

I've seen notes that antennas designed for NVIS operation work best in valleys rather than peaks because valleys have better ground conductance.  Suppose an antenna must be set up in an area in which the ground conductance is unknown.  Assuming the usual NVIS range is acceptable and the antenna can be set up at a range of reasonable heights, should the antenna at low height for possible NVIS operation, at maximum height, or at some intermediate height?

Alternatively, if NVIS range is acceptable and no knowledge about ground conductance is available, then should I ever deliberately lower an antenna specifically to obtain NVIS operation?


Thanks,

Chris
NQ8Z

[K2DC]

Chris,

   There's little question that an NVIS antenna will work better over highly conductive ground than it will over average or poorly conductive ground.  But regardless of the ground conductivity, the optimum height for an NVIS antenna is a quarter wavelength above ground.  That will result in a single lobe pointed at zenith, and offer the highest gain and most complete NVIS coverage that the ground conductivity will support.

73,

Don, K2DC

[WB4WZR]

I thought the height was to be 1/8 to as low as 1/20 of lambda. 

I'd be lucky indeed to get a height of 1/4 lambda in my situation.

Thanks,

Chris
NQ8Z

[WB6BYU]

There are lots of recommendations.  Some make sense.  Some are based
on poor computer modeling (especially MININEC.)

"Best" has many interpretations, of course, and what meets the needs for
one operator might not for another.

From my experience and modeling, the antenna height really isn't as critical
as many people make it out to be.  While it's true that maximum signal at
90 degrees peaks at about 35 feet for "Very Good" ground, 45' for "Average",
and about 50' for "Poor", the actual differences are insignificant.  For a given
ground type there is barely 1dB of variation from 30' to 60' height on 80m.
It isn't worth worrying about.

My rules of thumb for NVIS antenna height, regardless of ground type:

1) you gain about 4dB going from 5' to 10', and another 4dB from 10' to 20'.
That's worthwhile.  You get about another 2dB going from 20' to 30', and
probably less than 1dB for any increase above that.  While antennas can
work at lower heights, 15' to 20' is a reasonable minimum on 80m if you can
do it.

2) the maximum height in feet is the band in meters, so 80' for 80m operation.
The peak is no longer overhead at that height, but there is still plenty of
signal going up.  Above about 30' is a point of diminishing returns where you
don't gain much from going higher.

[NH7O]

If the ground is poor, placing ground wires directly under a low antenna can help with NVIS losses.

[KA4NMA]

Chris, What part of the Appalachia?  I used to operate VHF contests from some of those mountians.

Randy ka4nma

[WB4WZR]

I'm an AT hiker, having completed the Trail about 1.5 times by sections.  I don't carry radio gear on the AT hikes, but go to AT shelters for short "radio hikes".  So they're representative of where I might want to set up, they're typically in the Northeast.

From the comments, the quality of ground doesn't appear to be as big as I thought it was. In fact, anyone working portable with long wire antennas on 40 m and below is highly likely to get NVIS mode regardless of what they do.  Looks to me like the NVIS thing is for people working above 40 m who can't get local  communications.

Chris
NQ8Z

[G7DIE]

You can't control the quality of the ground, so on this point we're stuck with what we've got. I'd venture the reason for mounting an NVIS antenna in a valley has everything to do with ground reflections, where any ground 'gain' will contribute to high angle radiation as opposed to on a peak with a sloping foreground where ground gain would contribute to low angle radiation

[WB6BYU]

Looks to me like the NVIS thing is for people working above 40 m who can't get local  communications.

NVIS is for relatively local communications on 160m, 80m, 60m and 40m when
the ionosphere is cooperative.  Sometimes it is, sometimes it isn't, and to
cover distances of ~100 miles you have to choose the right band for current
conditions.  Here in the Pacific NW we haven't seen the critical frequency
reach 40m much if at all for the last 5-6 years, and even now 80m goes
out for most of night.  That's not to say you can't make contacts on those
bands, but that they will tend to be further out.

The military NVIS radios that I remember covered 2 to 12 MHz, but rarely
does the Critical Frequency reach the 30m band.  (When it does, 10m will
be hopping:  a rough rule of thumb is that the Critical Frequency is 4 times
the MUF, so 40m will support NVIS when 10m is open for longer distances.)


Looking at the plots for typical soil types, there is a maximum of about
3 - 4 dB difference among them for the same height.  If an antenna works
significantly better in a valley it may be because the trees tend to be
taller in better soil so it is easier to get the antenna up higher.

As long as you are using a horizontal wire dipole on the lower bands,
it probably will have a pattern suitable for NVIS.  Then you just need
to be operating below the Critical Frequency for the ionosphere to
support it by reflecting signals straight back down.

Antennas that are NOT particular good for NVIS include verticals and
wires with a "long wire" pattern:  for example, an 80m OCFD operated
on 40m has an overhead null (as is the case if it is fed at one end or
as a 1/4 - 3/4 wave dipole), while the same length of wire fed in the
center will have a maximum straight up.

[AC6LA]

This topic came up a few days ago on a different forum.  For a private email with another member, I prepared a few visuals which might be of interest here.  These are for an 80m dipole mounted 30m above ground so the max radiation is not straight up but they give an idea of how the ground quality affects the performance.  The three ground types are Very Poor, Average, and Very Good per the EZNEC definition of each.

First the polar patterns:
   http://ac6la.com/adhoc/80mDipoleTOA1.gif

A better way to look at things in this case is with a rectangular rather than polar pattern:
   http://ac6la.com/adhoc/80mDipoleTOA2.gif

Plots created with AutoEZ (see my homepage).

Dan, AC6LA
http://ac6la.com/

[WB4WZR]

An 80 m dipole mounted 30 m above ground is somewhat more than I can reasonably achieve with a slingshot in trees!

How about 10 m above ground?

Thanks,

Chris
NQ8Z

[WB6BYU]

My data might correspond exactly with Dan's, but gain straight
up (dBi) would be something like this for 30' and 35' heights:


ground        30'        35'

very good    7.3dB   7.6dB
average       5.8dB   6.3dB
poor            5.0dB   5.5dB
bad            4.3dB   4.8dB

Total difference among ground types is 3dB.

[AC6LA]

>>> An 80 m dipole mounted 30 m above ground is somewhat more than I can reasonably achieve with a slingshot in trees!

[grin] Well obviously you need a bigger slingshot!

Okay, seriously.  Here's the polar pattern with the dipole at 10m for Very Poor and Very Good.
   http://ac6la.com/adhoc/80mDipoleNVIS.gif
Average ground would be in between.  As WB6BYU just said, you get roughly 3 dB more gain with the better ground conditions.

And just to round out the topic, here's how the pattern changes as the antenna is raised from 10m (1/8 WL) to 80m (1 WL).  Average ground was used for this scenario.  This is an animated gif, similar to a lot of the banner ads you see on this site.  With most browsers you can press Esc to stop the animation, F5 to restart.  The height above ground is shown in the lower right corner as H=xx.  For this example I froze the outer ring at 8.18 dBi which is the max gain at 50m, the highest of the 8 patterns.  That lets you compare the absolute gains as well as the pattern shapes.  Also, the green dot marker is automatically placed at the max gain point for each separate pattern.  If you look in the upper right corner you can see how the take off angle gets lower as the dipole is raised.
   http://ac6la.com/adhoc/80mDipoleHeight1.gif

And here's the same scenario but  in rectangular format.
   http://ac6la.com/adhoc/80mDipoleHeight2.gif

All these charts were created with AutoEZ, http://ac6la.com/autoez.html .

Dan, AC6LA
http://ac6la.com/