Square loops are the easiest to make, but cover less area for a given perimeter than other shapes. This lowers efficiency. Make the antenna rotatable. The loop's deep nulls can be used to advantage in nulling out interference from power lines, TV sets and other stations. Use a larger loop.
Efficiency increases rapidly with loop size.
Even a 2 or 2.5 metre square loop should be noticeably more efficient than the 1.5 metre antenna presented here. Use more reduction on the variable capacitor to make adjustment easier. The first prototype had only one vernier drive on the capacitor's shaft. With this arrangement, getting the antenna tuned to the desired frequency was tedious because the tuning is sharp. If you routinely change frequency, a second drive is well worth the cost, particularly if 40 and 30 metres are the main bands of interest.