On unboundedness of maximal operators for directional Hilbert transforms

We show that for any infinite set of unit vectors $U$ in $\ZR^2$ the maximal operator defined by $$H_Uf(x)=\sup_{u\in U}\bigg|\pv\int_{-\infty}^\infty \frac{f(x-tu)}{t}dt\bigg|,\quad x\in \ZR^2,$$ is not bounded in $L^2(\ZR^2)$.