passive RFID read range

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?

Ideas:

- 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 penatration

- uWave performs worst near metals and liquids, and so range decreases again...

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)

cheers

Reply to
porterboy
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Hello,

The main difference between UHF and HF is where you are in the field.

In LF and HF systems, the energy transfer takes place via magnetic induction. The radiated power from an LF or HF system is very low. These are near field systems. With a proper designed and installed system, field drops off rapidly with distance reducing interference between other RFID systems within the same frequency band.

Both TX antenna and tag antenna uses resonance to get a reasonable reading distance.

Main advantage of HF systems from a propagation perspective is that it penetrates reasonably through a variety of materials, but not very well through open metallic structures.

From an antenna perspective, the tag antenna has a rather high Q and it can be detuned by conducting materials (depending on tag design). One cannot increase the Q factor of both TX and tag antenna to increase the range. Above a certain value the data transmission will fail because of pulse distortion (insufficient available bandwidth).

UHF systems use EM radiation to transfer the energy. Advantage of UHF is that it does penetrate to open metal structures, but due to multi- path effects, the detection range is difficult to confine within a specified zone. You can use antenna diversity to get a better coverage of the detection zone.

Field drop off is not as fast as with HF (disadvantage for frequency reuse). It does not penetrate very well through water containing substances (absorption like in a microwave oven).

Advantage is that one can use highly directional antennas (especially at 2.45 GHz) to shape the detection zone more or less. As mentioned before, due to multi-path effects, the detection zone can have a really strange shape. One can use EM absorbers to reduce the effect of reflection. In some cases, one can use directional tag antennas (when orientation of tag to reader antenna is known).

Disadvantage (seen from the tag) is that the receiving antennas become less effective (proportional to 1/freq^2). That means with same chip sensitivity, you need 4 times more power flux density for detection when you double the frequency. In many cases chip sensitivity reduces with increasing frequency because of losses in the RF rectifying circuitry. Therefore actual required power flux density goes faster (down) then with 1/freq^2 (above 1 GHz).

When it comes to maximum range, the optimum frequency is below 900 MHz (assuming a certain maximum EIRP). As reducing the frequency increases the efficiency of the tag antenna. But when size is limited, other things appear. When you have to design a credit card antenna for 433 MHz, you need a certain Q factor (resonance) to extract all the power from the antenna.

I think you should familiarize yourself with tag characteristics and link budget calculation / EM wave propagation to have a well educated answer to your questions. There are many other factors (like commercial and political ones) that may overrule every technical aspect.

Best regards,

Wim PA3DJS

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Reply to
Wimpie

Great answer Wim, thanks!

I don't suppose you could point me to some reference material for above?

cheers

Reply to
donnacha.daly

Being just very busy with design of antennas and readers for RFID, I have to say the answer could not be more precise and correct.

Reply to
Otto Sykora

You could check

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,
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|pp=[v=d,t=pfp,i=53420,fi=,ps=0]|[0][0], Probably you can get some chip specifications without signing an NDA. Most vendor sites (like CAEN, FEIG) do not have much info available.

When you have a reasonable electronics background, the required knowledge to setup some link budget calculation for UHF is not that difficult (it can be done without differential calculus). For RF you need some more system related things like maximum Q factor.

Best regards,

Wim PA3DJS

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Reply to
Wimpie

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