Consider the picture below is design of half wave length (λ) dipole antenna. I read some explanations saying that it is the best design to get the optimum transmit power. Half wave length (λ) is from tip to the tip of the two elements (you know it).
But my concern here is, what is the minimum or maximum distance of the gap as in the picture? Gap between the two elements which the feeder line is connected? Some say that it should be as small as possible. But unfortunately, I don't have mathematical justification for that reason even it make sense. I expect that the explanation is in λ. But if you really need the used frequency, then just put 2,100 MHz. If needed, the feeder line is RG6 75 ohm.