A 500 pf cap is much too large for a mag loop. If you get a configuration and frequency where you can use 500 pf, the loop efficiency will be very low, probably less than 1%. Also the tuning will be just about impossible without a very large gear reduction.
The spacing on most 500 pf caps is not sufficient for a mag loop even at QRP levels. The cap voltage varies with all the design parameters, but with a low loss loop it's possible that the cap voltage can be over 1 KV at some frequencies at 5 watts.
The capacitor is the most critical item in a mag loop design. A poor cap can cause more loss than all the other things put together, and this is often overlooked.
Your 83 cm-sq loop is too small for decent efficiency below 15 meters.
Here is an easy to use mag loop calculator that will let you play with the loop parameters to estimate performance.http://www.aa5tb.com/aa5tb_loop_v1.22a.xls
Don't forget to estimate the resistive loss of the wires connecting the capacitor to the loop plus some additional amount for the path thru the cap, and plug that number into the calculator. To give you a feel for that number, the ac plus dc resistive loss (mostly ac) of 4 inches of #12 wire at 14 MHz is about 0.016 ohms.