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We propose to use IEEE 802.22 networks as a natural interconnection between energy-constrained networks, such as sensor networks, and the Internet. In this interconnection scenario, we propose Activecast, broadcast designed as a continuation of the IEEE 802.22 broadcast on sensor networks. Activecast effectively transmits a query to all active sensors in the network. Activecast minimizes the number of retransmitted messages while maintaining high reachability. Activecast requires minimum communication overhead, yet performs very well in the presence of various environmental constrains. Activecast can also be integrated to any sleep based sensor protocol, as it does not make any assumptions on the underlying wake up scheduling protocol and wake up times/durations of the sensors. We study the properties of Activecast by analysis and simulations while also considering irregular radio coverage of sensors. Index Terms--Broadcasting, energy-constrained networks, IEEE 802.22, sensor networks.