This chapter provides information on the methodology used to determine the operating range of the RFID system. It also describes the experimental methods used to test the selected RFID tags.
2.1. Determining the Operation Range of the RFID System
One of the most important parameter determining the quality of an RFID system is the so-called read range. It is determined by two parameters: read range forward and read range reverse. Read range forward
is the maximum range, at which the RWD reader is able to deliver the minimum power
to properly activate the RFID tag [
25]. Read range reverse
is the maximum distance at which the backscatter transmit power (power backscattered by tags)
is sufficient for the RWD reader to be able to read the RFID tag. The relationship between reading ranges and delivered/backscattered powers is described by the Friis transmission equation [
26]. The value of the power delivered to the chip is determined using the equation:
where
c is speed of light in a vacuum,
f - frequency,
- power delivered to RWD antenna terminals,
- gain of impedance-matched RWD antenna,
- gain of impedance-matched tag antenna,
- polarization matching factor of antennas,
- power transfer coefficient,
r - distance between the RWD and the transponder.
The value of the power transfer coefficient depends on the impedance parameters of the RFID chip and the antenna attached to it, as described by the equation [
25]:
where
is the chip impedance and
is the antenna impedance.
The maximum value of the power transfer coefficient is 1 and is obtained in the case of impedance matching of the antenna and the chip, when
= *
. The value of
≈ *
should be obtained for the operating frequency of the RFID system (for Europe about 865 MHz), so that it is possible to power the chip from the maximum distance. The maximum distance to properly power the chip can be determined from the equation:
where
is the minimum power that must be delivered to the chip for it to function properly.
In practice, determining the distance directly using equation
3 is very difficult. Therefore, for this purpose it will be easier to use the value of the power
, delivered to the transponder antenna. In terms of theoretical relationships, the
power is the
power with additional consideration of the influence of the transponder antenna parameters and can be described by the equation:
In addition, it is also possible to determine
antenna gain and
power as
radiated power, according to the equation (given in Watts):
The maximum value of power that can be radiated,
, is limited by the regulatory standards in force in the region (e.g. ETSI standards [
27]). The largest distance
will be achieved at
=
, according to the equation [
28]:
In the case of
distance, the factor that causes the limitation of the range of proper functioning of the RFID system is the value of reflected power
, received with the RWD antenna. One of the basic parameters of any RWD is sensitivity, that is, the minimum recordable value of
power (
). For communication to take place between the transponder and the RWD, it is necessary to meet the condition that
>
. The value of the received reflected power depends on the radiated power, losses in the transmission path and the ability of the transponder to reflect a wave of a given frequency (defined as the effective reflection area
). However, for simplicity, it is possible to determine the distance
based on an analysis of the received
reflected power, using the equation [
28]:
The value of
can be determined by analyzing the received power value
and the transmission path losses, according to the equation (values given in decibel scale):
where
is cable losses and
is the free space loss.
In a similar way, it is possible to determine the power
, using the equation (values given in decibel scale):
where
is the minimum value of
power at which the transponder feedback will be obtained.
In summary, in order to properly carry out communication between the RWD and the transponder, it is necessary to obtain an adequate value of both power and power, which is possible only at a certain distance (the smaller of and ). An analysis of the power transmitted/received by the RWD, taking into account the transmission path losses, will enable a preliminary determination of the maximum theoretical reading distance of a given RFID system. However, it should be borne in mind that in real conditions the obtained distances and may be smaller, due to the presence of interference and the possibility of antenna jitter, caused, for example, by contact with the ground, temperature or other atmospheric conditions. Therefore, it seems that it will be necessary to measure the distance of the correct reading, both under ideal conditions (maximum possible range) and real conditions (practical application). It should also be remembered that the gain value of antennas changes depending on the polarization coordinates. In RFID systems, the largest range of proper operation, will be obtained for coaxial alignment of RWD and transponder antennas.
2.2. Selection of RFID Tags for Experimental Studies
For the acorn prototypes, we used two types of RFID tags: a passive tag with industry-leading read sensitivity of up to
dBm MONZA R6-P [
29] and a semi-passive tag (otherwise called battery assisted passive (BAP)) with configurable sensitivity set to
dBm EM4325 [
30]. The passive tag MONZA R6-P was inductively coupled to a flexible dipole antenna (see red wire in
Figure 1). The semi-passive tag EM4325, together with the power supply, was placed on a dedicated PCB (bottom left corner of
Figure 1). Both tags were tuned to 868 MHz (for RFID purposes with a transmission power of up to 2 W in the European Union). The use of a flexible dipole antenna seems to be the optimal choice when considering the use of acorn tags to be spread by jays. It was decided to test three antenna configurations for the passive tag: the spiral configuration (see
in
Figure 1), the s-shaped configuration
and the straight line configuration
, and one straight line configuration for the semi-passive tag
.
The semi-passive tag was powered by a 3 Volt CR927 battery with a capacity of 30 mAh. According to the manufacturer’s documentation, the EM4325 tag consumes an average battery current in sleep mode of a maximum of 2.6 A, and therefore the CR927 battery should power the chip for approximately 16 months.
2.3. Laboratory Experimental Set-Up
Laboratory experiments were performed in the Microwave Vision Group anechoic chamber, which is shown in
Figure 2. For the measurements, the Voyantic Tagformance Pro system was used [
28] (see ③in
Figure 2) together with Tagformance UHF software ≤. The AARONIA HyperLOG 7025 broadband logarithmic-periodic antenna ②[
31] was coupled with a directional coupler (frequency band: 600-1300 MHz, TX-ANT insertion loss: 1.2 dB, ANTRX insertion loss: 6.5 dB, maximum input power: 250 W) ④. A measuring top ①was mounted in the chamber, on which the RFID acorns under test were placed. The quantities measured by the Voyantic Tagformance Pro system were the minimum power
to power up the RFID tag ( so-called power on tag forward) and the backscatter transmit power
( so-called power on tag reverse) for data transmission performed in the 800-1000 MHz band. Based on the measured powers, the read ranges were calculated according to equations
6 and
7.
Many leading RFID reader models have a sensitivity of -84 dBm (e.g. readers: UR1A, FR900, IMPINJ Speedway Revolution R420, YRI1 or UR4), which combined with an antenna gain of -4dBm gives a reader system with a sensitivity of -88 dBm = 1.58 pW. According to equation
7, the read range reverse
depends on the sensitivity of the RWD reader
. The read range reverse for the experiments in the anechoic chamber were determined for a reader sensitivity just equal to
=1.58 pW. The read range forward
described by the equation
6, on the other hand, depends on the maximal reader power
, whose value for data from an anechoic chamber was assumed to be
=2 W (the maximum allowed for RFID in the European Union).
Figure 2.
Laboratory experimental set-up.
Figure 2.
Laboratory experimental set-up.
The prototype acorn tags were pre-tested in an air environment, as indicated by ①in
Figure 3, and then the selected tag with the best performance was tested in a quasi-real environment. The quasi-real environment was a piece of soil (arranged in order: antenna-tag-soil), which can be seen in
Figure 3 ②.