Thanks, Andrew. This makes me quite optimistic. I thought my reflector might be a bit small since all biquad plans I found online have bigger reflectors. So I built the biquad from the scrap box just to test if the reflector is big enough to perform well. I wrapped hard paper in aluminium foil and mounted the biquad on it. I didn’t want to spent the money for a copper-plated pcb which equals a container of Lindener Spezial without knowing if the concept will work. 
I think I’ll order the pcb.
Ahoi
Pom