This is a direct quote resultant from a phone call to Bencher.
"The 75 Ohm Matching Line for 20 meter operation (P/N 290-08)
should be from RG-11 75 ohm coax, 11 ft. 4 in. (345.4 cm) if
solid dielectric, 13 ft. 5 in. (411.5 cm) if foam type."

KB4RDV
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Tnx for info , it is much appreciated

Frank GM0CSZ / KN6WH