A sequence OFDM modulated with normalization factor
The power of the non-zero elements is $|1+j|^2=2$, so the average power is

$$\overline{P}=\frac{12\cdot 2}{52}=\frac{24}{52}=\frac{6}{13}$$

So in order to get unity average power, you have to normalize by $\sqrt{13/6}$.