Your shack seems to be on a high building, so a short vertical antenna would give you better results than such a loop.
If there is no way to install a vertical, only a loop, then do it! You will certainly make some 100 km distance with 20W!

I would use a single turn loop because the resonance curve will be very sharp for a 2 turn loop, so the mechanical construction must be very stable. 7m x 3m is not so bad. You may need 10 nF or so, for the resonance (a very rough guess, without any calculations).
Instead of 6mm^2 wire i would use a RG213/RG214 cable, or many of them! For the C is would use WIMA-FKP-1 2kV 2.2 nF types and solder a few of them in parallel. It is most important to keep the wires as lossless as possible, even a 1 cm piece of wire with 1mm diameter will cause significant losses. Smaller C values could be switched in parallel (they only have to carry a small part of the total current so the losses are small) to find the exact resonance
Like in my images of the 1m diameter loop i would use a big ferrite toroid to couple the TX/cable to the loop. Galvanic decoupling may be helpful to prevent EMC problems and it is the easiest way to match the system.
In my loop, the most lossy thing is the capacitor pack. They become warm at > 25W.

Yes, it is a good idea to start TXing in WSPR since your signal will be to weak for large CW distances and you will see reports giving an idea about the signal strength and direction...

Good luck!

73, Stefan

Am 30.04.2015 23:16, schrieb IK1WVQ Mauro:

73 to all,

I can made a square TX loop for 470KHz of 7 * 3 meters (see picture) in the terrace of the building where I live (6 floors).

the questions are:

- it is enough a 6mmq copper electric wire ??  (20W is the output power of my tx).
- single turn or two turns spaced 10cm max ?? (two turns =  less pF required to take the resonance).
- Is this a good idea to start to trasmit on 470KHz WSPR ??