Following formula finds the better spacing when coupling Yagi antennae, optimizing overall gain. Phi is the -3 dB angle.
lambda
d = --------------
2 sin(Phi/2)
Here is an example for my 25JXX70
693.96 693.96
Phi E plane = 23°, thus d = --------------- = ----------- = 1740 mm
2 sin(23/2) 0.3987
693.96 693.96
Phi H plane = 23.8°, thus d = ----------------- =---------- = 1683 mm
2 sin(23.8/2) 0.4124
It is very important to simmetrically feed both antennae, to
avoid phase losses.