1/4 wave matching transformer:
Loop design >>> 
1005/F in (MHZ) = FEET
or 306.402 / F in (MHZ = Meter

The matching transformer,an electrical quarter wave length of 75 OHM  coax cable,can be computed by dividing 246 by the operating frequency in MHZ,then multiplying that number by the velocity factor of the cable being used.
Thus, for operation at 13.555,   246/13.555 mhz=18.14 feet.
¡ In Meter? 75/13.555= 5.53 3.
If coax with solid polyethylene insulation is used a velocity factor of 0.66 must be employed.
Foam-Polyethylene coax has a velocity factor of 0.80 assuming RG-59/U is used,

The length of the matching transformer becomes 5.53 3 X 0.66= 3.65 1 meter.