Update magloop.html

Fixed area calc.

magloop.html

@@ -553,7 +553,7 @@
 
                 // Write loop area:
                 fctx.textAlign = "right";
-                fctx.fillText("A = " + (Math.PI * dia**2).toPrecision(3).toString() + " m\u00B2", win_width-8, 18);
+                fctx.fillText("A = " + (Math.PI * (0.5*dia)**2).toPrecision(3).toString() + " m\u00B2", win_width-8, 18);
             }
             
             const aside_canvas = document.getElementById("antennaSide2D");