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The 71-degree phasing line should be: ft or m
The 71-degree phasing line is 90 degrees at Mhz
Cut the coax to the suggested length plus a few inches, in case your velocity factor is not quite right.
Leaving one end of the coax open, set your RF analyzer to Mhz and trim the coax until you see minimum Z impedance. You now have a length of 71 degrees at your desired operating frequency.