What is the approximate physical length of an air-insulated, parallel conductor transmission line that is electrically 1/2 wavelength long at 14.10 MHz?
Why A half wavelength in free space is 150/f(MHz) meters, so at 14.10 MHz that is 150/14.10 = 10.64 meters. An air-insulated open-wire line has almost no dielectric loading, so its velocity factor is essentially 1.0 and the physical length matches the free-space electrical length. In feet the same calculation is 492/f(MHz) = about 34.9 feet.
Watch out The choice near 7.0 meters is what you get if you apply a velocity factor of about 0.66, which belongs to solid-polyethylene coax, not open-wire line in air.
Half wave in meters = 150 / f(MHz); air line VF = 1, so no shrink factor.
14 MHz is the 20 meter band, so a half wave is about 10 meters: 10.6 m. Air line has no coax shrinkage.