Re: [TowerTalk] Vertical Antennas near salt-water

[Grant Saviers <grants2@pacbell.net>]

JIm,

Thanks for the clarification, it's very helpful.   Alternative modeling 
results will be very interesting.
Grant KZ1W

On 2/5/2016 18:10 PM, jimlux wrote:
> On 2/5/16 9:48 AM, Grant Saviers wrote:
> > Jim,
> >
> > EZNEC doesn't agree with your conclusion about azimuthal pattern.
> >
> > I modeled a 1/4 wl vertical with 100 1/4 wl radials on 180 degrees of
> > azimuth over average ground, elevated 0.05 wl and the F/B is about 3 db
> > at 20 degrees.   That's a little less than what 2 radials show as
> > directivity when elevated at the tide line at the same 20 degrees.
> >
> > At 5 degrees elevation the infinite salt water 180 degree ground plane
> > increases the gain over this model.   The difference is 9db. About what
> > experience validated with real VOB's.
> >
> > Can you provide alternative modeling results to compare with the EZNEC
> > 4.2 outputs?
>
> I will
>
> My comment was that the effect of the radial distribution is small 
> compared to the interaction with the ground properties.
> Hence, I suggested modeling the vertical in free space with various 
> radial configurations. That would tell you how much is "radial 
> distribution effect"
> Then, look at the soil properties effects.
>
> In most antenna systems, what you see is the "free space" pattern 
> multiplied by the "point source in the real environment" pattern.
> The big difference between that and a real model/real installation, is 
> that there are loss effects from real soil near the antenna.  In 
> general, those change the "gain" but not the "directivity"
>
>
>
> > Grant KZ1W
> >
> >
> >
> > On 2/5/2016 8:24 AM, jimlux wrote:
> > > On 2/5/16 8:02 AM, Grant Saviers wrote:
> > > > Roger,
> > > >
> > > >  From the link Dan AC6LA posted there are some long standing different
> > > > views of near and far fields from vertical antennas.  A discussion 
> > > > above
> > > > my pay grade as to whether NEC 4.2 analysis is correct for these 
> > > > models,
> > > > but it is validated in my experience.  I can offer an intuitive
> > > > explanation to part of your question.
> > > >
> > > > So why does a vertical at the edge of the sea radiate more energy
> > > > seaward than landward?  The relative conductivity is different by a
> > > > factor of 1000, 4 S/m for salt water vs 0.005 S/m for "average" earth.
> > > > So in that situation the return currents flow in the low resistance 
> > > > side
> > > > to a much higher value than the high resistance side. Further the
> > > > losses from a radiated field over salt water ground resistance
> > > > approaches that of copper.  I think that accounts for the directivity
> > > > gain.
> > > That's a very small effect.  You can model it by doing a vertical in
> > > free space with a variety of counter poise configurations. Start with
> > > a 90 degree bend dipole (e.g. 1 vertical, 1 radial) and then start
> > > adding more radials.
> > >
> > > Just not much change.. the direction of the main lobe changes a bit,
> > > but the azimuthal variation is probably less than 1 dB. After all, an
> > > ideal dipole has a gain of 2.15dB compared to an isotrope. An
> > > infinitesimally small dipole has a gain of 1.75.
> > >
> > >
> > >  Perhaps the more important factor is that the pattern starts to
> > > > look like a vertical over "perfect" ground which shows the elevation
> > > > lobe at a maximum value at the horizon, which is great for long 
> > > > distance
> > > > DX propagation if you look at the HFTA statistics re arrival angles.
> > > This is exactly what's going on and what's important.  You shouldn't
> > > be using NEC to model this kind of thing: you need a code that deals
> > > with reflections from partial conductors.  Jim Breakall did a model
> > > decades ago for terrain that modeled the surface as a series of flat
> > > plates.
> > >
> > > HFTA uses similar analysis, except it can't handle changing the soil
> > > properties over the profile.  Nor does HFTA do verticals, it's h-pol
> > > only.
> > >
> > > You need a different modeling code for this problem. Something more
> > > like used in the microwave fields, and you're going to need a very big
> > > grid, and lots of computational horsepower.
> > >
> > >
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> > >
> > >
> > >
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