[TowerTalk] [Bulk] Re: [Bulk] Vertical Antennas near salt-water

[jimlux]

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.