Theory - Practice - my Quad
MY QUAD (2 EL. FOR 10-12-15 MT)

I built two quad (one to me, the other to Mario, ik7zcq).

I used "+ " ("Diamond") configuration which gives just 0,1-0,2 db more in the gain than "Square" configuration: I built it in this way only for mechanical reasons. I used square-aluminum boom 30*30*1 mm 180 cm length; into it I put a steel pipe just to make it stronger; the mast to boom attachment comes from a CB antenna (\$6)(see picture 1).
The spreader to boom attachment (one for each element) is home-made using a square-steel bar (35 cm length); on the tips there are 4 perpendicular pipe (25 mm diameter) where you'll insert poles (see picture 2).

Poles are 5 mt length (21 mhz); I use four bamboo poles for each element (\$1 each); each pole is 240 cm length so I made it longer using in the lower part a short aluminum pipe (25 mm diameter, 100 cm length).

I gave the poles a protective coat of boat-paint and then I covered it with PVC ribbon (thanks to Jan, G0KRL for info).
Loops are on copper-insulated multi-wire (1mm for my quad, 3mm for Mario's one).
Here are formulas for loop length:

L(Driven)=7658/F
L(Reflector)=7849/F
L(Director)=7430/F
L=Length for one side.

I never spent time tuning it (it's "no problem, no tune", and works fine!) but I know working some hours I can optimize Gain or F/B Ratio ecc.

I fed driven elements with 3 (one for each band) 1/4 lambda stub using TV 75 Ohm cable; on the Mario's quad i used RG 11 useful for high power;
over the outside of the feed line, near the feed point, there are some ferrite beads taped up and sealed against moisture: that's my balun (see picture 3); on the boom there's also the rele' box for antenna switch (see picture 4); then a common 50 ohm RG 213 goes down on the shack.
I suggest you do not solder the stub directly on the loop, but using a SO239: here it's still working fine but will be better how i suggest.