I have been reading a lot of papers on Passive RFID, and there is a
trend which confuses me...
13MHz passive tags have a read range of up to about 3m (HF)
900MHz passive tags have a read range of over 10m (UHF)
2.5GHz passive tags have a read range of about 3m (uWave)
This suggests that the range increases with frequency for a while and
then drops off...
So my question, why does that happen?
- Is it that HF antennas are far less efficient than UHF and
mircrowave antennas, so low range (but good material penetration)
- UHF has the best trade off between antenna efficiency and material
- uWave performs worst near metals and liquids, and so range decreases
If I am right in any of the above then...
- is there a maximum on the graph of range versus frequency?
- at what frequency does the maximum occur
- what is the max range likely to be at that frequency (given typical
allowed tx power and antenna efficiencies at that frequency)