Find the physical length of a quarter-wave or half-wave coax stub or matching section, accounting for the cable velocity factor.
A coax stub or matching section is shorter than free space because the signal travels slower in the cable. Physical length in feet = (246 for a quarter wave, 492 for a half wave) times the velocity factor, divided by the frequency in MHz. Always confirm the velocity factor of your exact cable.
Physical length in feet is 246 times the cable velocity factor, divided by the frequency in MHz. For RG-213 (velocity factor 0.66) at 7.15 MHz that is about 22.7 feet. A half-wave section uses 492 instead of 246.
It is how fast a signal travels in the cable compared to free space, typically 0.66 for solid-dielectric coax and 0.80 to 0.85 for foam types. The dielectric slows the wave, so the physical cable is that fraction shorter than a free-space length.
Quarter-wave sections transform impedances (for example 75-ohm cable to match a loop or a vertical), half-wave sections repeat an impedance, and shorted or open stubs make traps and filters. The physical length must use the cable velocity factor.