The effect of open orbits on helicon propagation is investigated theoretically and experimentally. At the high field limit, the damping of helicons is found to be very sensitive to the amount of open orbit current, whereas the dispersion is only slightly affected. The low rf surface impedance of a copper single crystal is measured at 4.2°K in a magnetic field up to 55 kG. The field direction is oriented along or within a few degrees of the [001] axis. The result is compared with the calculated rf susceptibility based on the knowledge of several known open orbits. The standing wave pattern is observed whenBis parallel to the [001] axis. The ``effective Hall coefficient'' obtained from the dispersion relation agrees with other dc measurements. The attenuation whenBtilts away from the [001] axis is interpreted to be the damping effect caused by the two‐dimensional open orbit current. The average damping effect is smaller than that calculated from open orbits of known order.