Metrol. Meas. Syst., Vol. 23 (2016), No. 2, pp. 163–172.

METROLOGY AND MEASUREMENT SYSTEMS

Index 330930, ISSN 0860-8229
www.metrology.pg.gda.pl

A METHOD FOR MEASURING THE RADIATION PATTERN

OF UHF RFID TRANSPONDERS

Piotr Jankowski-Mihułowicz, Mariusz Węglarski

Rzeszów University of Technology, Faculty of Electrical and Computer Engineering, Pola 2, 35-959 Rzeszów, Poland
(
pjanko@prz.edu.pl, +48 17 854 4708, wmar@prz.edu.pl)

Abstract
The operating principles of RFID antennas should be considered differently than it is applied in the classical theory
of radio communication systems. The procedure of measuring the radiation pattern of antennas that could be
applied to RFID transponders operating in the UHF band is seldom discussed correctly in the scientific literature.
The problem consists in the variability of the RFID chip impedance that strongly influences measurement results.
The authors propose the proper methodology for determining the radiation pattern with respect to an individual
transponder as well as an electronically tagged object. The advantage of the solution consists in the possibility of
using components of different measuring systems that are available in typical antenna laboratories. The proposed
procedure is particularly important in terms of parameter validation – the identification efficiency and costs of an
RFID system implementation can be evaluated properly only on the basis of real values of considered parameters.

Keywords: RFID, UHF transponder, antenna, radiation pattern.

© 2016 Polish Academy of Sciences. All rights reserved

1. Introduction

Intensive development of wireless systems that are widely used in various areas of life and
economy [1] is evident in contemporary communications. The antenna radiation pattern is one
of the basic parameters that are required to evaluate usefulness of a given radio communication
system. Typically, it is defined as the amplitude distribution of the electric field strength on a
spherical surface with a very large radius of curvature whereas the radiation source is located
in the centre of the sphere [2]. The measurements of the radiation pattern are carried out in the
far-field or near-field zone, regarding the purpose of a wireless communication system and a
type of antenna [3, 4]. In the first case the measurements are performed directly, whereas in
the second case transposition of the obtained results to the far-field region has to be applied –
for this purpose the two-dimensional Fourier transformation is used [3, 5, 6]. If it is possible to
establish a far-field in antenna laboratories, the pattern is measured in large anechoic chambers
[3, 7] or in their small equivalents [8]. Also GTEM (Gigahertz Transverse Electromagnetic
Cell) [8, 9] or reverberation chambers [10] can be used instead, in order to provide adequate
conditions. In any type of chamber, the measuring area has to be adjusted to geometrical
dimensions of antennas and their operating frequencies. If there is not enough space in order to
establish a far-field during tests, then compact products (anechoic chambers with parabolic
reflectors) are used as a substitute [3, 4, 11]. Dedicated polygon stands with the possibility of
suppressing unwanted electromagnetic wave reflections can also be applied [4, 12, 13] to the
considered tasks. Whatever alternative solution is chosen, it is necessary to use an advanced
equipment in order to make the measurement process feasible in the frequency- [3−5] and time-
domains [14].

_____________________________________________________________________________________________________________________________________________________________________________________

Article history: received on Dec. 08, 2015; accepted on Jan. 05, 2016; available online on May 16, 2016; DOI: 10.1515/mms-2016-0018.

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A typical hardware and software configuration of the above mentioned measurement

systems can be applied to determine the radiation pattern of most antennas that are commonly
used in radio communication systems (DVB-T, GSM, UMTS, LTE, WiFi and others) [15]. It
can also be adapted to new antenna constructions [16] as well as to new implementations of
common antennas in wireless communications [17]. But there is a problem in RFID (Radio
Frequency IDentification) systems operating in UHF band (860−960 MHz, the operating
frequency varies depending on world regions). It is impossible to determine the radiation pattern
of RFID transponders by using standard laboratory stands and measurement methods. The
problem consists in matching the impedance of the antenna and that of the chip. The complex
impedance of RF front-end varies during the transponder chip working and its value is
dependent on the electromagnetic field parameters (the electromagnetic field in RFID systems
is influenced by environmental conditions around marked objects). It is the reason why the
classical theory of antennas cannot be applied to solve the matching problem [18] and new
measurement methods have to be developed in order to determine the parameters of RFID
antennas.
The measurement process in which the nature of a UHF RFID transponder (the variable
impedance of the chip) is taken into consideration is seldom described in the branch literature.
In one of encountered solutions, the authors use a very expensive apparatus dedicated to the
intended aim only [19]. In another proposal, supplementary (e.g. movable) antennas are
implemented [20] but the described experiment is highly complicated and additional
measurement uncertainties have to be taken into consideration.
With regard to the endeavours made to solve the above mentioned problems, the authors
have worked out a universal method of radiation pattern determination and described it in detail
in the paper. The main advantage of the proposed research methodology is the possibility of
using a standard equipment of a typical antenna laboratory (such as an anechoic chamber,
antenna positioners, etc.). The measurement stands are supplemented with cheap and
commercially available RFID devices and some own control and data-acquisition software
procedures. An additional benefit of the proposed method is that the radiation pattern can be
determined just for a transponder as well as for a whole electronically marked object. The
second option is particularly useful when the efficiency of identification process in automated
systems and implementation or maintenance costs are considered [21].

2. UHF RFID system

In typical radio communication systems, the input impedance of a transceiver (TX) or a

receiver (RX) is constant and equals to e.g. 50 Ω, 75 Ω. The impedance of antennas or signal
paths in measurement instruments has to match the input value of the TX/RX circuits. Yet
another problem appears in the RFID systems operating in the UHF band – the radiated
electromagnetic wave of power density S (Fig. 1) is an energy medium supplying passive
(without any built-in energy sources) or semi-passive (with an auxiliary battery) transponders.
The carrier wave of frequency f0 is used to transmit energy by matched antennas, but it should
be noticed that the impedance matching of a transmitter and a receiver known from classical
theory is valid only for the Read/Write Device (RWD) and its antenna (50 Ω). Unfortunately,
matching of the transponder antenna with the chip cannot be considered in the way that is
typical for radio communication systems.
The internal structure of an electronic chip is designed to be supplied by the minimal voltage
UT that is induced at terminals of the transponder antenna. As a consequence, the complex
impedance (ZTC) of the chip front-end is continuously changed. The part (ZTCR) of the
impedance that represents a rectifier and voltage regulator is strongly influenced by the
electromagnetic field. On the other hand, the electromagnetic field parameters are dependent

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on the orientation of marked object and its location in the operating space where both energy
and communication conditions have to be established in order to ensure proper working of the
system [22]. The conditions are described by the Interrogation Zone (IZ) that constitutes the
basic parameter of RFID systems. It is defined as the space in which the two main tasks of an
RFID system can take place: a) energy can be conveyed from the RWD to transponders and b)
data can be transmitted in both directions [22, 23]. Since the amount of conveyed energy is very
small, the backscatter communication is used for transmitting data in the direction from the
transponder to the RWD. In this process, a battery-less device communicates by modulating its
reflections of an incident RF signal. The modulation is realized by step changes of the chip
impedance (ZTCM switching). The communications principles are implemented in the protocol
of Electronic Product Code EPC Class 1 Gen 2 [24], standardized in ISO/IEC 18000-6.

Fig. 1. A generalized block diagram of an UHF RFID system.

3. Proposed measurement method

The radiation pattern of the tested transponder antenna (AUT – Antenna Under Test) can be
measured in the far-field that fulfils the condition:
2d 2 , (1)
r>
λ
where: r means the distance between the AUT and a radiation source (an antenna of RWD in
an RFID system); d is the maximal dimension of the AUT; and λ denotes the wavelength).
The main requirement for measurements to be carried out in a proper way is to maintain a
constant value of the impedance ZTC (ZTCR without modulation) while polar diagrams of the
radiation pattern (according to θ and ϕ angles of the spherical coordinate system) are being
determined. The measuring procedure has to be performed when the transponder is placed
inside the interrogation zone of an RFID system (the energy and communication conditions of
transponder operation are met). The test conditions can be controlled only at the IZ boundary
(Fig. 1). In the developed method, the authors propose to perform it by changing the power PR
supplied to the terminals of the impedance-matched RWD antenna.
The suggested procedure arises from the Friis transmission equation. The dependency on the
power PT received by the transponder antenna in a given RFID system can be described by the
formula:
G G λ 2τχ
PT = PR R T 2 , (2)
(4π r )
where: GR means the gain of the impedance-matched RWD antenna; GT – the gain of the
transponder antenna (the chip and antenna impedance matching is assumed); χ – the
polarization matching factor for a given arrangement of radio communications antennas; τ –
the coefficient of power transfer from the antenna to the chip.

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If the transponder changes its location inside the IZ, then the antenna impedance (ZTA) is
constant at a given frequency f0 but the chip impedance (ZTC) varies as a function of the power
PT. This dependency can be expressed by the following equation [22]:
4 Re ( Z TA ) Re  Z TC ( PT ) 
τ ( PT ) = 2 2
, (3)
{Re  Z TA + Z TC ( PT ) 
 } + {Im  Z TA + Z TC ( PT ) 
 }
where Re describes the real and Im – imaginary part of the complex impedance.
Power received in the transponder antenna is equal to the minimal value PTmin if the terminals
of the impedance-matched RWD antenna are supplied with the minimal power PRmin. It enables
the transponder to be properly supplied (in given environmental conditions) according to the
relation:
GRGT λ 2τχ
PTmin = PRmin . (4)
(4π r )2
The (4) dependency is crucial for the IZ boundary determination where the impedance ZTC
is equal to ZTCR = f(PTmin) [22]. This impedance is obtained on the basis of a communication
protocol in the task process where the transponder sends its unique identification number (UID
– Unique IDentifier) as an answer to the Query command from the RWD [24]. The requirement
of maintaining a constant value of the chip impedance that is met when the condition PT = PTmin
is true for any variation of θ and ϕ angles at the constant distance r, is fundamental for the
authors’ method of measuring the radiation pattern (Fig. 2).

Fig. 2. A block diagram of the proposed method for measuring the radiation pattern.

Since the chip impedance is complex (≠50 Ω), the radiation pattern is determined wirelessly.
It means that signal paths of measuring devices do not have to be connected to the transponder
chip by wires and the radiation plots can also be drawn for electronically marked objects. The
test procedure is performed in an anechoic chamber equipped with an AUT positioner, a linear
polarized RWD antenna with a mast, and a digital controller.
A small anechoic chamber can be chosen for the test purposes [8] according to the
dimensions of transponders and their operating frequency band. The amount of power conveyed
to the tested transponders can be controlled by the RWD and an output attenuator. The
normalized radiation pattern (in dB) is determined on the basis of the following dependency:

FTn dB (θ ,φ ) =  PR dBm (θ ,φ )  − PR dBm (θ ,φ ) , (5)

min

where: PR dBm is the measured power in dBm (by using a scope probe, a spectrum analyser, etc.)
whereas the „min” index refers to the minimal value of this parameter.

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4. Experiment

Two samples of semi-passive transponders operating in the UHF band have been designed
to verify the elaborated method. Both transponders are equipped with an AMS SL900A [25]
chip and an omnidirectional antenna in the first case or a directorial antenna in the second one
[26]. Two different laboratory stands have been prepared in order to perform the test procedures
of the evaluated radiation pattern. The common components of radio communications test sets
(anechoic chambers, positioners, etc.) that are typically dedicated to measure radiation patterns
of standard antennas have been used to build up the new stands for the method proposed by the
authors. In addition, commercially available and relatively cheap RFID devices (comparing
with the equipment for common antenna tests) have been implemented in the stand with the
aim of adjusting the antenna laboratory to the proposed research. Moreover, own control
procedures have been designed in order to adapt the apparatus to the scheduled tests. To
evaluate the obtained results, the radiation pattern has been measured twice: using first the
authors’ method and then the classical method, in the same experimental conditions.

Fig. 3. A UHF RFID transponder with an omnidirectional antenna: a) sample #1;

b) an HL3DEM antenna model.

a) b)

Fig. 4. A diagram of the antenna radiation pattern at 866 MHz – sample #1: a) comparison of plots obtained
with the numerical calculation and the authors’ method; b) comparison of plots obtained with the numerical
calculation and the classical method.

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The verification process of the method starts from designing antenna models of the tested
semi-passive transponders. The numerical model of the omnidirectional antenna (Fig. 3a) is
created in HyperLynx 3D EM (HL3DEM) software (Fig. 3b).
In the second step, the radiation pattern is measured and plotted at the specially prepared
laboratory stand equipped with the designed software tools. Comparison of the measured and
calculated characteristic diagrams are plotted in a V-vertical (X−Z) and H-horizontal (Y−Z)
plane (Fig. 4a).
The measurement set-up for the authors’ method is presented in Fig. 5 and consists of an
MVG EQ7922-01 anechoic chamber, a one-axis positioner, a designed directional antenna
(with linear polarization) connected with RWD (Feig ID ISC.LRU2000) by duplex rotary step
attenuators (Telonic Berkeley 8052S), and a 6 dB directional coupler (Pasternack PE2238-6).
The process parameters and transmitted power are supervised by a spectrum analyzer
(Tektronix RSA 3408B) with a built-in protocol decoder EPC Class 1 Gen 2. The system is
controlled by the software tool RFID(UHF)SysAntPat implemented in LabView.

Fig. 5. The first test stand – sample #1.

In the third step − executed only for comparison reasons − the radiation pattern is measured
by means of the classical method (Fig. 4b) under the same environmental conditions (anechoic
chamber, positioner). The comparison measurements are made for the transponder antenna
which is connected directly to one of the VNA ports (Keysight N9912A FieldFox). The
impedance mismatch between the AUT and the signal paths in VNA (ZTA ≠ Z0 where the
characteristic impedance Z0 of a signal line is 50 Ω) leads to divergent results due to a large-
signal reflection (Fig. 4b). This phenomenon results from a special antenna construction that is
adjusted to the RF front-end of UHF RFID transponder. The voltage reflection coefficient Γ at
the input terminals of the transponder antenna can be expressed by the following equation [2]:
ZTA − Z0
Γ= . (6)
ZTA + Z0
On the basis of [27], the measured antenna impedance ZTA in this case is 64.9 + j300 Ω at
f0 = 866 MHz. This value is adjusted to the SL900A chip, where the measured chip impedance
ZTC is equal to 14.9 ‒ j342 Ω at the frequency 866 MHz for the passive mode (τ = 0.477). The
ZTC value is obtained at the measured sensitivity PTmin = −13.1 dBm. On the basis of (6), the
reflection coefficient is close to unity (|Γ| = 0.935) for the transponder antenna connected
directly to one of the VNA ports (Fig. 4b) and it is the reason of huge discrepancies in the plots.

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The similar verification procedure has been carried out for the second sample #2. In this
case, the development board of the autonomous semi-passive RFID transponder with a
directional antenna of the UHF band (Fig. 6a) has been tested.

Fig. 6. A development board of the autonomous semi-passive RFID transponder:

a) sample #2; b) an HL3DEM model of the UHF directional antenna.

Comparison of the measured and calculated characteristic diagrams is plotted in a V-vertical

(X−Z) and H-horizontal (Y−Z) plane (Fig. 7).The proposed measurement set-up for the second
experiment is presented in Fig. 8 and consists of a TDK anechoic chamber, a one-axis positioner
(Dream Catcher ME1300), a directional antenna (with linear polarization) connected with the
RWD (IDS R903) by duplex rotary step attenuators (Telonic Berkeley 8052S), and a 6 dB
directional coupler (Pasternack PE2238-6). The process parameters and transmitted power are
supervised by the RWD. The system is controlled by the software tool RFID(UHF)SysAntPat
implemented in LabView.
On the basis of [26], the measured antenna impedance ZTA in this case is 42.2 + j363 Ω at
f0 = 866 MHz. This value is adjusted to the SL900A chip impedance at the frequency
f0 = 866 MHz for the passive mode (τ = 0.68). As in the first case, the reflection coefficient is
also close to unity (|Γ| = 0.969) for the transponder antenna connected directly to one of the
VNA ports. The impedance mismatch between the AUT and the signal paths in VNA leads to
divergent results due to a large-signal reflection (Fig. 7b).
The measurements with the authors’ method are performed at the distance of r = 0.35 m in
both arrangements. At this distance the signal of wave reflected by the transponder can be
detected for all positions of the antenna rotated by the positioner (0−360º with step 5º). The
results of measurements and calculations obtained in both experiments are convergent for the
V as well as H plane of the radiation pattern. It confirms usefulness of the developed method.
The minor discrepancies are caused due to the use of an infinite plane for the dielectric layer in
the prepared HL3DEM numerical models. The impedance mismatch between the transponder
antenna and the signal paths in the classical method with VNA precludes the radiation pattern
of the RFID UHF transponder from being determined. Moreover, the authors’ method can be

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also adapted to determine (in a non-classical way) the power gain GT of antennas which
impedance matches that of the transponder chip.

a) b)

Fig. 7. A diagram of the antenna radiation pattern at 866 MHz – sample #2: a) comparison of plots obtained
with the numerical calculation and the authors’ method; b) comparison of plots obtained with the numerical
calculation and the classical method.

Fig. 8. The second test stand – sample #2.

5. Conclusion

The radiation pattern of classical antennas operating in common radio communication

systems is determined by dint of the standard equipment that is available in most specialized
research RF laboratories. Unfortunately, the RFID antennas operate in the way that not always
can be described by the classical theory of antennas. It is the reason why some wrong
characteristics appear in the literature and it is hard to find measurement procedures that are
properly described and where the variability of chip impedance in the UHF band is considered
correctly. In view of that, the authors have proposed a universal method for determining the
radiation pattern. No expensive re-built experimental stand is necessary to apply the method in
a traditional antenna laboratory as only standard RFID devices, which are commercially

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available, are needed as a supplement. The idea of the measurement procedure is described on
the basis of the communication process between the RWD and the transponder in an RFID
system of the UHF band. The elaborated conception has been verified and developed in
experimental tests on two examples: omnidirectional and directorial antennas, designed
especially for commercial RFID chips. The research has been performed in the authors’ RFID
laboratory.

Acknowledgements

This work was supported in part by the Polish National Centre for Research and
Development (NCBR) under Grant No. PBS1/A3/3/2012. The work was developed by means
of the equipment purchased in the Operational Program Development of Eastern Poland
2007-2013 of the Priority Axis I Modern Economics of Activity I.3 Supporting Innovation
under Grant No. POPW.01.03.00-18-012/09-00 and the Program of Development
of Podkarpacie Province of The European Regional Development Fund under Grant No. UDA-
RPPK.01.03.00-18-003/10-00.

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