[TowerTalk] NVIS antennas Re: dumbing down

[Jim Lux]

>
>
>If you look at Hagn's actual measured data you'll see over
>30 mS or 50 mS/m soil (very good earth) FS drops several dB
>as antenna heights move below .05 wl.
>
>.05 wl would be about six feet on 40 meters and about a
>dozen feet on 80 meters.

Perhaps. I did say it was a quick model. I suspect that the better the 
ground, the more pronounced the effect. Or, more properly, there's some 
ground properties which have an optimum match to the fields from the 
antenna. For instance, if the "ground" consisted of a series of sheets of 
"space cloth" with an impedance of 377 ohms per square, you'd get the free 
space pattern of a dipole.

Just thinking out loud here, if you consider two cases: where the ground 
has zero conductivity, you'll get either the free space pattern (if 
epsilon   = 1) or a pattern similar to that with a perfect reflector 
(epsilon not =1).  With a very high conductivity, it will look like the 
pattern with a perfect reflector.




>I know from my own measured A-B comparisons if I add a good
>screen  below a dipole at 35 feet on 80 meters signal levels
>increase about 5-6 dB.

Is that the 80m signal straight up?  (or somewhere close, so the path is 
high angle, but far enough that you're not seeing diffraction or ground 
wave direct path).

>So how are you getting 9 dBi straight up? That makes no
>sense unless the model handles earth losses poorly. With an
>error or disagreement like that, I'd toss the models out.

It is an interesting anomaly, and one I intend to check out.  You should be 
getting at least 2.15 + 6 dBi.. 2.15 because it's a dipole, 6 dB because 
you've got field reinforcement with twice the voltage.  The ground IS 
reactive, so the phase of the "image" is different than that which you'd 
get with a perfect reflector, so maybe you're getting some additional 
directivity.

I did expect to see some difference between 50 cm and 2 meters.

I do need to check to make sure that some modeling assumption isn't being 
met (like segment lengths.. I only used 15 segments, and we're VERY close 
to the groung), but overall, I do trust NEC4's handling of ground 
losses.  After all, that's one of the things that differentiates NEC4 from 
NEC2, is that the handling of nearfield losses is much better.

> > 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.
>
>So what? Groundwave is zero degrees, not 15 and certainly
>not 45 degrees.  A horizontally polarized signal at 3.5 or 7
>MHz has virtually no ground wave signal at all.

I think you mean a horizontally polarized antenna doesn't radiate any 
ground wave? I can propagate a horizontally polarized signal parallel to 
the ground perfectly well (which most TV broadcast stations depend 
on).  Whether a horizontal dipole over ground does it well is really the 
question, and I readily concede that an idealized horizontal wire doesn't 
have any radiation at zero elevation. However, a "real antenna" over "real 
ground" that isn't perfectly flat may radiate at zero elevation angle.


 > 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.
>
>Then the model is behaving poorly because that makes no
>sense at all.



>When I run the model in EZnec4 over average soil using high
>accuracy ground, I get 7dBi (straight up) at 30 feet high,
>and -3.63 dBi at 3 feet high. That's 7.2% antenna efficiency
>at 3 feet of height.

My model, or your model of the same system? It's entirely possible that my 
model (or, more properly, which options got used) may have been normalized, 
etc.


>The EZnec numbers are very realistic. The gain values you
>listed that show no change as the induction fields are
>brought very close to a lossy media make no sense at all,
>and disagree with every measurement I've seen including the
>data from Thailand.


Indeed...   which is why I want to redo the model.


>73 Tom