You have quite a challenge there.  It will make a great cloud burner, but 
will be good for local work, and quiet.  The disadvantage of coax, is though 
you can tune it, that only means the transmitter is happy.  Very little of 
the reflected power will get to the antenna, except on a resonnent 
frequency.  For a loop, I believe that means about 250 feet of wire for an 
80 meter loop.  That means 62 feet on a side.  That thing is so close to the 
ground, you won't have enough feed line for an apropriate length.  Their are 
many fine books on antenna theory, and one would be helpful.  Effecient 
radiators are not random in their creation.