Re: NEC-LIST:Multiple port excitation

>I am working an a crossed dipole antenna, and am having problems
>with the two port voltage excitation.
>
>
>The question is how should I excite and load these two ports?
>
>The load Z that the antennas will be attached to a two port load
>that a complex Z.
>
>I want to make the two antenna feed points a conjugate match to this
>two port load.

Wayne,

Since your antenna configuration has two driving points that are
interacting, we can view (at a given frequency) this configuration as
a two-port that is described by a 2 x 2 y (admittance) parameter
matrix. The goal is to deliver the available exciter/transmitter
power to the twin-fed antenna.

Using NEC we determine the driving point and transfer admittance
parameters as follows:

        y11 = I1/V1 with port 2 short-circuited

        y12 = I1/V2 with port 1 short-circuited

        y21 = I2/V1 with port 2 short-circuited (should be equal to y12)

        y22 = I2/V2 with port 1 short-circuited

The following is from the NEC-4 Program Description/Theory
documentation for simultaneously matching the two antenna feedpoints:

Compute the maximum coupling (GMAX) as

        GMAX = 1/L * [1-sqrt(1-L^2)]

where L = Mag(y12 * y21)/[2 * Real(y11) * Real(y22) -Real(y12 * y21)]
and "Mag" and "Real" are the magnitude and real parts of the complex
number, respectively,

The load admittance (looking back toward the exciter/transmitter)
that we must have on port 2 is

        YL = [(1 - rho)/(1 + rho) + 1] * Real(y22) -y22

where rho = GMAX * conjugate(y12 * y21)/Mag(y12 * y21)
and "conjugate" indicates the conjugate of the complex number.

The input admittance at port 1 is given by

        Yin = y11 - y21 * y12/(YL + y22)

The load admittance needed for port 1 is conjugate(Yin).

This all looks messy but use of a spreadsheet or Mathcad alleviates
the pain. Just remember that the y-parameter values apply at a
SINGLE frequency only. Their functional dependence on frequency is
determined by the antenna geometry and the environment (ground
planes, nearby metallic objects, etc) in which it is situated.
Sincerely,

-- 
        __    __     ______       __   John Wood
       /  |\ / /\   / ____ \     / /\  Code 5551
      / | ||/ / /  / /\__/ /|   / / /  U.S. Naval Research Lab
     / /| |/ / /  / /_/_/ / /  / / /   4555 Overlook Avenue, SW
    / / |   / /  / ___   / /  / / /    Washington, DC 20375-5337
   / / /|  / /  / /\_| |\_/  / /_/_    (202) 767-2608
  /_/ / |_/ /  /_/ / |_||   /_____/\   (202) 767-3377 (FAX)
  \_\/  \_\/   \_\/  \_\|   \_____\/   e-mail: wood_at_itd.nrl.navy.mil
                             WWW: http://server5550.itd.nrl.navy.mil
NOTHING SPLENDID HAS EVER BEEN ACHIEVED EXCEPT BY THOSE WHO DARED 
BELIEVE THAT SOMETHING INSIDE THEM WAS SUPERIOR TO CIRCUMSTANCES. - 
Bruce Barton
-- 
The NEC-List mailing list <nec-list_at_gweep.ca>
http://www.gweep.ca/mailman/listinfo.cgi/nec-list