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passive RFID read range

27/11/2007 - 10:35 von porterboy | Report spam
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
 

Lesen sie die antworten

#1 Wimpie
27/11/2007 - 16:09 | Warnen spam
On 27 nov, 10:35, porterboy wrote:
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



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
www.tetech.nl

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