NEC-LIST: Backscatter from RFID tags - modeling problem

I have been trying to come up with a means to model the amount of backscatter
energy emitted from 900MHz and microwave RFID tags.

For those who have not come across this subject here is a brief explanation.

A reader emits an energy field which impinges on one or more RF tags in the
environment. The tags reflect some of this energy back to the reader which
detects the varying refelectivity of the tag. In effect the tag modulates the
incident energy with its data by instantaneously changing the load impedance
on the tag antenna. The propagation is thought to follow the rules of radar
scattering.

Here is the problem.

When the tag load is matched to the antenna, according to accepted theory the
tag will absorb half the energy and re-radiate the other half. When the
antenna terminals are short circuit, the tag will reflect 100% of its energy
and when high impedance will be almost transparent and therefore will reflect
almost nothing.

The reader sees only the differential change in radar cross section, which
means that the reflected incident carrier must be removed from the equation
when considering the energy received back at the reader.

I believe that when the tag switches between the matched condition and short
then the resulting reflected signal is equivalent to a 50% AM or Phase
modulated double sideband suppressed carrier signal. If one starts out by
saying that the the antenna aperture Ae when it is matched, is equivalent to
the radar cross section, then vary this as the load impedance changes, one can
use the Radar equations from Reference Data for Radio Engineers, Howard Sams
and co to calculate the received signal strength and just subtract 3dB for the
carrier.

Is this true?

Now, what happens when the differential match varies between high impedance
and low impedance and is never matched. It would seem that the reflectivity
would vary from nothing to 100%, the result being the equivalent of 100%
modulation. But, how do you remove the carrier from the reflected energy to
arrive at the true differential receiver power. What would be the actual power
level received at the receiver for 100% modulation state (ie. antenna
shorted).

I have used the equations for coefficient of reflectivity to arrive at some
result but this only seems to work when the differential state varies from
matched to shorted. When the antenna is matched, rho = 0 and when shorted
|rho| = 1. Therefore delta rho is 1 and delta reflectivity is 0.5 (when
subtracting the carrier). When you go from short to open delta rho is 2 but
one cannot have 200% modulation..... what do you do with the carrier
energy..?

I have seen a number of different treatments and they are all different
yielding variances of up to 6dB.

I hope someone out there has some ideas.

Chris
G4HKP

Chris Turner IEng MIIE
RFIP Solutions Ltd
Tel: +44 20 7575 1656
Mob: +44 7812 174133
Fax: +44 20 7575 1529
chris.turner_at_rfipsolutions.com

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