GIGO! Your error is on line 2 of the calculation. You are using .84, I don't know where that is coming from. Certainly, the VF of a piece of insulated house wire is not .84, it is closer to .95 (as others have mentioned). Change that line to .95 and you get this: 300 / 27.805MHz = 10.76m 10.76m * 0.95 = 10.22m 10.22 * .25WL = 2.555m for a quarter wave. 2.555m * 39.37" = 100.59" That's pretty close to the 100" that it should be.