Wideband Active Integrated Antenna for RFID Tag Applications
Christopher Yafrate and Daniel Schaubert
University of Massachusetts-Amherst, ECE Dept., Antennas and Propagation Lab.
INTRODUCTION
VARACTOR TUNED SPLIT PATCH
IMPEDANCE & RADIATION MEASUREMENTS
Split-Patch Resonant Frequency and |S11|2 vs. Bias Voltage
Antenna Substrate: Rogers 5880
Dielectric Constant: εr = 2.2
Dissipation Factor: tan δ = 0.0009
Thickness: t = 45 mil (1.143 mm)
DC-RF
Isolation
Resistors
Coaxial Feed Probe
Antenna Geometry:
Sub patch 1 Length: L1 = 10 mm
Sub patch 2 Length: L2 = 10 mm
Patch Width: W = 10 mm
W
Varactor Specifications:
Model: Micrometrics MHV500
Cj @ Vr = 0 V: 2.5 pF
Cj @ Vr = 4 V: 0.8 pF
Cj @ Vr = 20 V: 0.2 pF
Q: 2600 (min)
L2
L1
Hyper-abrupt Varactor
-22
5.4
-24
5.2
-26
5
-28
4.8
-30
4.6
-32
4.4
|S11|2
4.2
Resonant Frequency
4
0
2
4
6
8
10
12
Varactor Reverse Bias Voltage (V)
-34
14
16
4
2
0
-2
-4
-36
-38
-6
0
2
4
6
8
10
12
Varactor Reverse Bias Voltage (V)
14
16
Split Patch - H-plane Radiation Pattern
Varactor Tuned Split Patch Test Antenna
0o10 dB
-30o
Design Procedure:
• Parameter sweeps performed on L1, L2, W
• Varactor model selected based on simulations
• Optimal varactor loading position(s) determined
30o
0 V Bias
17 V Bias
60o
-10
-60
-20
-90o
90o
-20
-10
-120o
120o
0
-150o
10 dB
150o
180o
Split Patch - Resonant Frequency vs. Varactor Capacitance
Split Patch - Gain vs. Varactor Capacitance
9
5
4
8
7.5
2
7
Gain (dBi)
Resonant Frequency (GHz)
3
6.5
6
Measurement Observations:
• Linear relationship between varactor bias voltage
and patch resonant frequency
• Well matched across entire tuning range
• Measurements agree well with simulated results
1
FIXED FREQUENCY
ACTIVE INTEGRATED ANTENNA
0
5.5
-1
5
-2
4.5
4
0
0.5
1
1.5
Varactor Capacitance (pF)
2
2.5
-3
Proximity Coupled Feedback
0
0.5
1
1.5
Varactor Capacitance (pF)
2
2.5
HFSS Simulated Split Patch Radiation Pattern
Amplifier Bias Circuitry
Split Patch - H-plane Radiation Pattern
0o10 dB
-30o
30o
Proximity Fed
Plastic Packaged Monolithic Amplifier
2.5 pF Cap.
0.1 pF Cap.
0
-60o
60o
-10
-20
-90o
90o
• Oscillates and radiates at 5.8 GHz
• Cross-polarization minimized by symmetric design
-20
-10
-120o
120o
WHAT’S NEXT
0
Several antenna designs are investigated to determine
the optimal design. An optimal design is defined as
having the maximum attainable tuning bandwidth while
maintaining acceptable radiation efficiency across the
entire tuning range.
5.6
6
0
8.5
APPROACH
-20
o
ANSOFT DESIGNER & HFSS SIMULATIONS
Given these requirements, an active RFID tag is to be
designed capable of operating over a wide bandwidth
and in various environments. An electronically tunable
active integrated antenna is to be created by reactively
loading a microstrip patch antenna using a varactor
diode. The microstrip patch provides performance
robustness to objects behind the antenna while the
tunability provides the antenna with a wide operating
bandwidth. The patch is integrated into the feedback
loop of a series feedback oscillator.
-18
Gain (dBi)
Ground Via
Split Patch - Absolute Gain vs. Bias Voltage
6
5.8
|S 11|2 (dB)
Bias Voltage Pad
Resonant Frequency (GHz)
Radio Frequency Identification (RFID) tags are
gradually becoming a more efficient and effective
means of maintaining a large inventory. Because the
tags are used in a wide range of environments, they
must be capable of performing well regardless of their
environment. For example, an RFID tag placed on a
box containing a metallic body must operate with similar
performance characteristics as if it were placed on a
box containing a non-metallic object. It is also desirable
for the tags to operate over a wide range of frequencies
(UWB) in order to overcome multipath fading and
mitigate the effects of large signal interference.
Additionally, active RFID tags are desirable due to their
ability to operate over larger distances than their
passive counterparts.
Simulation Observations:
• Characteristic patch pattern achieved
• Pattern quality maintained through tuning range
• Operation limited by low efficiency at low tuning
frequencies (large capacitive loading)
-150o
10 dB
180o
150o
•
•
•
•
Integrate tunable patch into series feedback oscillator
Maximize oscillator tuning range and DC-RF efficiency
Adjust feedback coupling to optimize radiated power
Minimize design footprint and profile