Zeros of linear combinations of Dirichlet $L$-functions on the critical line

Let $F$ be a linear combination of $N\ge 1$ Dirichlet $L$-functions attached to even (or odd) primitive characters with the same modulus. Selberg proved that a positive proportion of non-trivial zeros of $F$ lie on the critical line. Our work here is to provide an explicit lower bound for this proportion. In particular, we show that the lower bound $2.16\times {10}^{-6}\mathrm{/}(N\log N)$ is admissible for large $N$.