[TowerTalk] Vertical Antennas near salt-water

[Grant Saviers]

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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