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

[Grant Saviers]

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?

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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