Re: R: [amsat-bb] Reference/Source for patch antenna

[Anthony Monteiro <aa2tx@xxxxxxxxxxxx>]

Hi Domenico,
I don't recall the qst article but I think I
do remember complex math so lets see,

two impedances in parallel is the same as the inverse of the sum of their
inverses (or add the admitances and take the inverse) :

         Z = 50+j50 || 50-j50

         Z = 1/(1/(50+j50) + 1/(50-j50))

         Z = ((50 + j50) * (50 - j50)) / ((50 + j50) + (50 -j50))

         Z = ( (50^2) + (j50^2) + (-j50^2) + (50^2) ) / (50 +j50 - j50 + 50)

         Z = ( 2500 + 2500 ) / 100

         Z = 5000 / 100

         Z = 50

If you try this with 100 + j100 and 100 - j100, I bet you get 100 ohms!

Somebody check my math, did I miss something?

73,
Tony AA2TX

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At 11:50 AM 8/24/01 +0200, i8cvs wrote:
>Hi  Frederic,
>
>Get a look at QST June 1993 page 49 and carefully read AMSAT Journal
>March/April 1993 pages 19 to 20
>You will get many good dimensions to know expecially on page 20 ,columns 1
>and 2
>
>About the comment  of 50 + jXX  and 50 - j XX the theory of trasmission
>lines says that if the imaginary part of the impedance has the same value of
>the real part than the current is 45 deg out of phase with the voltage.
>
>More exactly,if the imaginary part of the impedance is negative - jXX the
>reactance is capacitive and the current leads the voltage by 45 deg
>
>If the imaginary part of the impedance is positive + j XX the reactance is
>inductive and the current lag  the voltage by 45 deg
>
>If you have two points in wich for the same frequency the first one has an
>impedance of 100 - j 100 ohm and the other one has an impedance of
>100 + j 100 ohm and you connect both in parallel it happens that the
>reactances cancel out ,the real part  of the impedance became 50 ohm but
>  more important the current is now out of phase with the voltage by 90 deg
>  that we need to get circular polarization.
>
>This is exactly wath W1GE did on its square patch described in QST October
>1995 duplicated by you.
>
>He selectet the feedpoint in the diagonal of the patch in order to get
>100 - J 100 ohm  to cause a 45 deg current leads and chosed the other
>  dimensions of the patch to set the resonant frequency below the operating
>  frequency to cause  a 100 + j 100 ohm and get a 45 deg  current lag
>
>The result is that the net impedance in the feedpoint is now 50+ j 0 ohm and
>the polarization is circular.
>
>You will understand this readind again QST october 1995 page 45 column 1
>
>To get LHCP it is necessary to find a feedpoint in the diagonal between the
>center and the corner in wich the impedance has a capacitive reactance
>- j 100 ohm and set the other patch dimensions  above the operating
>frequency  to get an inductive reactance of + j 100 ohm
>
>This metod of  self-phasing requires to use a network analizer to measure
>the - J XX and + J XX reactive components  and so i feel that it is easier
>to use the phasing and matching metod described in QST 1993 and AMSAT
>journal  March/April 1993 because we have in it the coefficients to get
>feedpoints  of 100+j0 ohm and 50+j0 ohm for any frequency.
>
>The same self-phasing metod is used  for the Quadrifilar Helix Antenna
>described by Walter Maxwell, W2DU in his wonderfull book "REFLECTIONS"
>Chapter 22 but in this case to get  the relationship of +/- X =R
>is very easy because the autor quoted the coefficients  to lengthen and
>shorten both helix to get it for any frequency.
>
>Best 73 de i8CVS Domenico

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