RFID tags may be powered by an on-board battery or by an external power
source. The external power is usually delivered via magnetic flux or radio waves.
The magnetic flux creates a uniform density zone, allowing tags to be powered
consistently within the zone. Radio waves, used by UHF and microwave tag,
create a zone of radio signals that is affected by reflected waves from surrounding
objects. The field density varies unpredictably within the zone. Therefore,
depending on the tag location within the zone, the tag may or may not receive
strong enough radio waves to power itself.