Are there power, penetration or cost downsides? I can hardly find anyone discussing such systems, let alone manufacturing them.
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There are two main drawbacks to making a passive RFID transponder that operates at 2.45 GHz. The first is that the read range is about a third the range of passive UHF RFID. This is because the length of radio waves sent to and from the tag is 2.7 times shorter than that of 915 MHz waves (that is, for passive UHF RFID systems).
Another issue is that 2.45 GHz waves do not penetrate materials as well as passive UHF transponders. As the frequency of radio waves increases, they behave more like light, which bounces off any solid object. UHF waves penetrate materials better than super-high-frequency (SHF) radio waves, and HF waves penetrate better than UHF waves. A good academic paper on the pros and cons of passive SHF tags can be found here.
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