Measured focal maximum gain (TX/RX)  vs. cm from

"theoretic" ; the edge of Septum as I thought was the focal point ,

never heard about Phase-focal-point earlier, here called "0 cm".

(0 cm = 75.4 cm in my case , from bottom of the dish to the edge of septum front face , see photo).

Dish diameter 2.2 m and F/D 0.34 .Choke ring diameter 40cm and deep 9 cm .

The dB value is mean value from # 20 echo transmissions (each dot). 

In my case (small dish) the Septum should be 3-3.5 cm closer to dish than "theoretic" !

If I do the measurement / adjustment using  echoes, the dish gain comes in the equation

twice, its more critical  (positively) than using sun noise and so I get a sharper curve.

The vertical scale is dB in WSJT echo-test and horizontal scale is adjusted deviation from zero (0) cm.

Yellow line is "trend line" in Excel.

This is a "close up" measurement . Condition has to be very stable if one should be able to

resolve 0.5 dB out of -237 dB ! hi

  the old measurement 

Yellow line is "trend line" in Excel.