Are you asking if you can use the coax you have as the loop itself? if so it should work, but the characteristics as coaxial cable don't really matter. I would just be a fat copper wire.
do you need a fixed frequency? or tuneability? Wide bandwidth is fairly easy to attain if you are willing to re-tune when changing frequencies, but quite difficult if you don't want to adjust the cap.
Are you looking to transmit? or just receive? a circumference of 7 feet isn't really enough to build an effecient magnetic loop for frequencies below 20 MHz, but for RX only you would be fine.
for 80 meters, a good magnetic loop for transmitting would have a circumference around 30 feet (diameter around 10 feet) and need several hundred Pf of tuning cap rated for about 100 volts per watt of input power.
Of course, this data goes for transmitting, To just receive, don't bother with a single turn loop, use a 2 to 5 turn loop (or more if you want it to be smaller) and a tuning cap from an old am radio. you will still need to tune as you move across the band