A couple of quick things.
The model will be sensitive to ground type. Mininec ground or
"perfect" ground will not work. Use high accuracy ground that matches
your target location.
Model your tower as accurately as you can including any beams up top.
This will interact with that antenna.
At the bottom of the tower, add a load. This will simulate the
resistance from the bottom of the tower to ground at RF. Vary that
resistance widely and see if the impedance, etc varies with it. If it
does, then you are going to have to tune that antenna after you put it
up and will most likely not be able to do any better than get it
somewhat close. Knowing what the bottom of the antenna looks like at
RF is nearly impossible.
The Comtek figure is based on experience, and is more likely to be right.
73, Guy.
On Thu, Sep 2, 2010 at 1:55 PM, Natale Borghetti <i5nph@i5nph.net> wrote:
> I am considering to built a 4 share for 3.780 MHz by 4 sloping 1/2 dipole
> supported by 140 feet tower.
>
> I have just simulated the system on EZNEC...but my results...contradict..what
> comtek suggest (to tune the 1/2 dipole 5% LOWER then the target
> frequency)..because when ALL 4 dipole will be in place the 4 square resonant
> frequency will be higher...
>
> By simulation on EZNEC I saw exactly the opposite...seems the frequency of 4
> square is LOWER compared to the single Dipole....
>
> I am....making..some mistakes ????...or...wrong..interpretation ???
>
> Any 80 meter Guru..could Help on clarify the..strange results
>
> 73
>
> Natale
> i5nph
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