[TowerTalk] NVIS (not exactly towers, but HF)

[jimlux]

On 6/10/20 4:49 AM, John Simmons wrote:
> Jim,
> 
> I noted that your two links are modeled using NEC2. Let's see the 
> results using NEC4.
> 

I'm not sure there would be a huge amount of difference with a dipole at 
reasonable height analyzed over a narrow band.

NEC4 will give better answers if you have a wire *very close* to the 
ground or buried. Or if your antenna configuration is such that you have 
short segments or acute angles.

But for "dipole more than 0.05 wavelength above ground" NEC2 is probably 
pretty good. For that matter, so would an analytical approximation using 
the Fresnel equations and calculation of the reflection coefficient, 
particularly for computing a far field pattern.

The feedpoint impedance is going to be pretty close - NEC calculates 
"gain" of the antenna as driven from a 1 Volt source, so the actual feed 
point impedance does have an effect.


Where NEC4 is particularly useful is when you want to know the feed 
point impedances to high precision.



> -de John NI0K
> 
> Dan Maguire wrote on 6/9/2020 10:23 PM:
>> Jim Brown wrote:
>>
>>>>> The differences jump out at you when field strength for multiple 
>>>>> mounting heights are plotted with the same amplitude scale.
>> Like this:
>>
>> https://forums.qrz.com/index.php?attachments/upload_2020-6-8_6-33-57-png.670446/ 
>>
>>
>>  From this recent qrz thread:
>>
>> https://forums.qrz.com/index.php?threads/single-ground-radial-wire-placed-beneath-a-dipole-how-effective-is-it.709471/page-6#post-5498150 
>>
>>
>> Dan, AC6LA
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