Discussion in 'Antennas, Feedlines, Towers & Rotors' started by M0AGP, Dec 19, 2020.
Amplifying on Mike's "Yep" reply, here's a quote from the EZNEC Help:
This is why I was long ago dreaming of lying 360 1 wavelength radials. It would be an equvalent of one's own salt lake 2 wavelength across. It is farely possible for 10m band The higher the frequency- the more sense and the easier to make. But now I think - let the worms live in quiet.
Nick, the problem is that ground reflections take place for many wavelengths. Radials that extend even two to three wavelengths out from the vertical radiator wouldn't make much difference.
I think that depends partly on the elevation angle of interest. Or perhaps I've misunderstood Mike's intent.
I created a "two media" AutoEZ model of a simple qtr-wave vertical similar to EZNEC sample Vert1.ez. The inner medium is almost perfect ground (conductivity 100 S/m) and the outer medium is "extremely poor" ground (conductivity 0.001 S/m). The radius of the boundary between inner and outer is defined as "X" wavelengths. Variable "X" was incremented from 0.25 to 8 wavelengths.
Here's an animation of the elevation pattern as "X" is incremented. The value of "X" for each frame of the animation is shown in the lower-right corner. The reference blue trace was with a single medium (infinite extent) of almost perfect ground.
AutoEZ format model attached. By changing the number of segments in the vertical wire from 50 to 20 this model can be used with the free Demo versions of EZNEC and AutoEZ.
As far as I remember I chose 360 1 WL radials for 7 dg of elevation. MMANA demonstrates it as well.
Curios enough - the shorter the vertical - the better the effect of the dence radials. Because the area of reflection is closer to the center
What is unic for shirt verticals - their "near" and "far" grounds mingle or overlap, as Dan pointed out in his post.
I'm wondering if anyone can reconcile the differences in that clip of EZNEC Help text with the information below about h-pol vs. v-pol Earth reflections from F.E. Terman (published long ago).
This is very, very interesting. I may be misinterpreting the result, but there seems be an implication for radial length. If radials are trying to imitate near-perfect ground, and focussing on the 10 degree radiation angle then a radial field with an infinte number of radials (a disk of metal) than is 1/4 wave in radius has about a 5dB loss compared to a perfect radial field 8 wavelengths in radius.
Is that a fair interpretation?
And then there is the Fresnel zone effect, which I guess EZNEC doesn’t know about. True?
Maybe I am wrong here...
Dan - What impact (if any) would you attribute to your AutoEZ animated graphic with respect to the EZNEC Help file comments about two media grounds, below?
\\Using Two Ground Media
The ground may be broken into two "media", each having its own conductivity and dielectric constant (relative permittivity). The second medium can be at a different height, but must be at the same level or below the first medium. The media can be arranged in parallel slices or concentric rings. One use of two media would be to model an antenna on a lake surrounded by land. A very important thing to understand is that the second medium is used only for far field pattern calculations, and is ignored for all other purposes. Be careful when using two media, and keep the following in mind:
Even if you place the antenna over the second medium, EZNEC will always use the ground constants and height (z = 0) of the first medium for calculation of the impedances and currents.
The second medium is used only for far field calculations. Near Field and Ground Wave calculations assume that the first medium is of infinite extent, and ignores the second medium.
The effect of the second medium is taken into account only in a very simplified way. The vertical pattern is generated by tracing "rays" direct from the antenna and reflected from the ground. When a second medium is used, the ground reflection "ray" is determined by whichever medium it strikes the top of. The "ray" does not penetrate either medium, and diffraction or similar effects aren't considered. Because of this, a highly conductive inner medium and normally conductive outer medium is not a good model of a ground radial system, and shouldn't be used for this purpose.
Buried wires in EZNEC Pro/4 will always be treated as though they're immersed wholly in the first medium. //
You are correct in that I was using "almost perfect" ground as a substitute for "very dense radial field". I failed to make that clear.
At a 10 deg elevation angle a 1/4 wavelength radius "disk of metal" is about 6.6 dB down from an infinite disk of metal (about -1.7 dBi vs 4.9 dBi). Here's how the two scenarios converge at a 10 deg elevation angle, using easier-to-read linear scales for Gain (Y axis) and "disk of metal" radius (X axis).
At a 5 deg elevation angle the convergence looks like this.
And at a 2.5 deg elevation angle.
I believe that is correct. Except when using plane wave excitation the NEC engine does not take into account the receiving antenna.
Revised AutoEZ model attached showing how to set up the test cases to produce the above charts.