Abstract: Radio-frequency identification (RFID) systems use readers to query and locate passive RFID tags in stores, warehouses, and other environments. A signal from the reader powers up the tag, which modulates and backscatters the signal toward the reader. Unfortunately, the maximum permitted RF signal power, self-interference at the reader, tag sensitivity, and channel loss limit the range at which readers can detect and locate tags. Using multiple readers simultaneously circumvents these limits. When used together, each reader transmits a signal to a tag in turn, and all of the readers listen for each of the tag's responses. The readers that are not transmitting do not experience self-interference and so can detect responses at lower power levels (longer ranges). Because the readers are at different locations, they measure different angles of arrival (AOAs) for each response. These simultaneous measurements can be used to locate each tag faster and with higher fidelity.

Abstract: A solar heater including a housing having a transparent front wall for passing incident solar radiation, a radiation absorbent collector element positioned in the housing and arranged to accept incident solar radiation passing through the front wall, means for passing a fluid medium to be heated through the housing in heat transfer relationship with the collector element and a transparent heat trap disposed in the housing between the collector element and the front wall. The heat trap is made from a cellular structure containing a multiplicity of adjacent cells having walls which are integrally formed with the walls of other cells in one continuous piece from a thermoformable material. The thermoformable material used for the heat trap may be glass or a clear plastic composition which is opaque to infrared radiation emitted from the collector element.

Abstract: A solar heater including a housing having a combined transparent front wall and heat trap, a radiation absorbent collector element positioned in the housing and arranged to accept incident solar radiation passing through the front wall, and means for passing a fluid medium to be heated through the housing in heat transfer relationship with the collector element. The combined front wall and heat trap are integrally formed in one piece from glass or a clear plastic composition which is opaque to infrared radiation emitted from the collector element.

Abstract: A solar air heater including a housing having a transparent front wall and an inlet and outlet for establishing a flow path for a gas such as air to be heated and a porous radiation absorbent collector plate positioned across the flow path and arranged to accept incident solar radiation passing through the front wall wherein a radiation trap is placed inside the housing either adjacent to the front wall, directly on top of the collector plate or between the collector plate and the front wall. The radiation trap has a cellular structure such as honeycomb and is made from a material which is light-transmitting but opaque to infrared radiation emitted from the collector plate in a direction toward the front wall. When the radiation trap is placed adjacent to the front wall the trap can additionally serve as an air buffer layer. In this instance, however, the radiation trap must be maintained in at least firm mechanical contact with the front wall and preferably the trap is actually bonded to the front wall.