Bert Modderman Engineer
Here is a discription of a 9 dBd double quad and its calculations
L = 0,6 lambda -> this shood be considered a minimal length and can be longer.
If you bought a bend one like me, be extremely carefull to straighten it. I told the seller that I did not want this because I want a straight one. The seller said just bend straight again like this. The seller took the semirigid and showed me how. And he broke it!
13 cm example
L = 8 cm
23 cm example
L = 16 cm
There are many ways to connect the semirigid to the reflector. If the reflector is cupper it could be soldered.
Good luck building!