[TowerTalk] Antenna Software...

Jim Lux jimlux at earthlink.net
Mon Feb 12 17:48:44 EST 2007

At 11:09 AM 2/12/2007, StellarCAT wrote:
>I know there is an antenna group as well - I will post there
>also but I believe some here are able to respond to my
>I was wondering - is there anything that prevents one from
>designing an antenna for 80 with hi-Q coils such that the
>resonant frequency is around 3.52 - 4% (to be used as a
>reflector at the low end)... and then have a shunt capacitor (if
>needed vacuum variable) at the feed point to "tune" it up the
>band to as high as 3.9 + 4% (to be used as a director at the top
>end)? Obviously the Q of the cap is very high and it should (?)
>be able to handle the high currents I would think...

Nope.. it's been done before.

>The idea is to have 2 elements with motor driven caps -
>basically a "steppir" for 80 that would follow one around AND
>allow "instant" direction change.

Sure... it's been done.   In fact, in the wireless comm world there's 
a bunch of people doing essentially the same thing using a circular 
array of slightly long 1/4 wave monopoles with varactors at 2.45 GHz 
to do adaptive beamforming. google for ESPAR  (Electronically Steered 
Parasitic Array Radiator)..  terribly fashionable over the last few 
years.. PhD and MS theses with "genetic algorithm optimized ESPAR" 
"adaptive ESPAR" "MIMO systems with ESPAR" and on and on and on

>Will EZNEC model this very well (the lower cost $80 version)? It
>would consist of both lumped inductance as well capacitance.

I would think so.  So would the free 4nec2 (which is what I use for 
antenna modeling).  Both are based on the underlying NEC engine, 
which supports lumped components quite well.  Those who use EZNEC 
like the user interface, although 4nec2 is pretty straightforward 
(bearing in mind that I've used the original NEC and ECAP with 
punched card decks, so EVERYTHING is a better interface than that).

The limitations on the modeling accuracy would be in your knowledge 
of local soil properties and the like. 

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