Circular of square shaped loop radiator ?
pa0nhc 20181105 / 20200612
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            A circular shaped loop :
You could make a maximal efficient circular shaped loop by using VERY THICK  coax, for instance :

39mm Cellflex LCF114-50 (170 Euro / 10m).

50mm Cellflex LCF158-50 (200 Euro / 10m).

A circular shaped , full size MONO BAND loop for 80m, made with 20m 39mm Cellflex LCF114-50, costs 340 Euro. 
Its diameter will be 6.4m.

Rem : For an  800W matching transformer, stack two 101 mm #31 ring cores, and wind with PTFE silver plated AWG9 wires.

            A square shaped loop :
Using "loopcalc.exe", i compared the performance of square shaped loops with a circular shaped loop.

            A square shaped loop compared with a circular shaped loop :
1. On 40m a 10m circumference (1/4 lambda loop) has only 5% (0,2dB) higher losses unnoticeable difference.

2. On 80m a 10m circumference (1/8 lambda loop) has max. 1.4dB higher losses, nearly unnoticeable difference.

My budget was very limited. As i have to get the best price / performance-ratio, i made my copper loop square shaped.  
And with varying pipe diameters : the thickest copper pipe at the place with the highest currents running => on top..