Texas Instruments | DN004 Folded Dipole Antenna for CCC25xx | Application notes | Texas Instruments DN004 Folded Dipole Antenna for CCC25xx Application notes

Texas Instruments DN004 Folded Dipole Antenna for CCC25xx Application notes
Design Note DN004
Folded Dipole Antenna for CC25xx
By Audun Andersen
Keywords
•
•
•
•
1
• Folded Dipole
• PCB Antenna
• 2.4 GHz
CC2500
CC2550
CC2510
CC2511
Introduction
This document describes a folded dipole
PCB antenna design that can be used with
the following products from Texas
Instruments, CC2500, CC2550, CC2510
and CC2511. Maximum gain is measured
to be 7.4 dB and overall size requirements
for this antenna is 46 x 9 mm. Thus this is
a compact, low cost and high performance
antenna.
Figure 1: CC2500 Folded Dipole EM.
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Table of Contents
KEYWORDS.............................................................................................................................. 1
1
INTRODUCTION............................................................................................................. 1
2
ABBREVIATIONS........................................................................................................... 2
3
DESCRIPTION OF THE FOLDED DIPOLE DESIGN .................................................... 3
3.1
IMPLEMENTATION OF THE FOLDED DIPOLE .................................................................... 3
4
RESULTS........................................................................................................................ 5
4.1
RADIATION PATTERN .................................................................................................. 5
4.2
BANDWIDTH ............................................................................................................. 12
4.3
SPURIOUS AND HARMONIC EMISSION ......................................................................... 13
5
CONCLUSION .............................................................................................................. 13
6
GENERAL INFORMATION .......................................................................................... 14
6.1
DOCUMENT HISTORY................................................................................................ 14
7
IMPORTANT NOTICE .................................................................................................. 15
2
Abbreviations
CC25xx
CAD
DXF
EM
LOS
PCB
CC2500, CC2550, CC2510, CC2511
Computer Aided Design
Data eXchange Format
Evaluation Module
Line Of Sight
Printed Circuit Board
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3
Description of the folded dipole design
Since the impedance of the folded dipole is matched directly to the impedance of the radio no
external matching components are needed. Dependent on which regulatory limits the
application should comply with, two filtering capacitors might however be needed. To ensure
compliance with ETSI requirements for receiver spurious emission, C121 and C131 in Figure
3 must be included. The optimum value of C121 and C131 depends on which ETSI standard
to comply with. For EN 300 440, 2.2 pF is recommended and 1.5 pF is recommended for EN
300 328. The required filtering for compliance with FCC is dependent on which regulatory
part to meet, part 15.247 or 15.249, and the duty cycle of the transmitted signal. Including
C121 and C131 reduces the output power but ensures lower harmonic emission in TX and
reduced VCO leakage in RX. Typically a board with two 1.5 pF capacitors will have 2 dB less
output power than a board without C121 and C131.
3.1
Implementation of the folded dipole
It is important to make an exact copy of the antenna dimensions to obtain optimum
performance. The easiest approach to implement the antenna in a PCB CAD tool is to import
the antenna layout from either a gerber or DXF file. Such files are included in the reference
design, “CC2500EM_FD_Reference_Design_1_1.zip” available at www.ti.com/lpw. The
gerber file is called “CC25xx_Folded_Dipole.spl” and the DXF file is called
“CC25xx_Folded_Dipole.dxf”. If the antenna is implemented on a PCB that is wider than the
antenna it is important to avoid placing component or having a ground plane close to the end
points of the antenna. If the CAD tool being used doesn’t support import of gerber or DXF
files, Figure 2 and Table 1 can be used. Since the pinout of CC251x differs from CC2500 it
might be necessary to make a slight modification on the routing of the antenna feed lines. If
this is done it is important to keep the same distance between the chip and the antenna.
Figure 2: Antenna dimension.
L1
L2
D1
D2
44.7 mm
21.0 mm
1.5 mm
0.9 mm
H1
H2
H3
H4
2.4 mm
3.1 mm
6.0 mm
2.8 mm
Table 1: Antenna dimension.
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Figure 3: Schematic for CC2500 folded dipole reference design.
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4
Results
All results presented in this chapter are based on measurements performed with the CC2500
Folded Dipole EM placed on SmartRF04EB.
4.1
Radiation pattern
Figure 4 shows how to relate all the radiation patterns in this section to the orientation of the
antenna. The radiation patterns were measured with 0 dBm output power.
XY plane
XZ plane
YZ plane
Figure 4: How to relate the antenna to the radiation patterns.
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Figure 5: XY plane horizontal polarization.
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Figure 6: XY plane vertical polarization.
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Figure 7: XZ plane horizontal polarization.
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Figure 8: XZ plane vertical polarization.
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Figure 9: YZ plane horizontal polarization.
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Figure 10: YZ plane vertical polarization.
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4.2
Bandwidth
The bandwidth of the antenna was measured by using a simple test program. This program
steps a carrier programmed for 0 dBm output power in 1 MHz step, from 2.3 GHz to 2.8 GHz.
The measurement is performed with 0° in Figure 5 directed towards a horizontal polarized
receiving antenna. Figure 11 shows that the folded dipole ensures more than 0 dBm output
power across the whole 2.4 GHz ISM band at 0° in the XY plane.
Figure 11: Bandwidth of folded dipole antenna.
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4.3
Spurious and harmonic emission
Table 2 shows measured output power and harmonic emission for different power settings
between +1 dBm and -6 dBm. These measurements were performed with C121 and C131 as
1.5 pF.
Programmed
output power
+1 dBm
FF
0 dBm
FE
-1 dBm
F9
-2 dBm
BB
-4 dBm
A9
-6 dBm
7F
Measured output power and harmonic emission
2.44 GHz
4.88 GHz
7.32 GHz
9.76 GHz
100.0 dBµV/m 48.3 dBµV/m 49.3 dBµV/m
37 dBµV/m
Below noise
98.4 dBµV/m
44.4 dBµV/m 47.3 dBµV/m
floor
Below
noise
floor
Below noise
97.4 dBµV/m
43.7 dBµV/m
floor
96.4 dBµV/m
48.3 dBµV/m Below noise floor Below noise
floor
94.1 dBµV/m
44.1 dBµV/m Below noise floor Below noise
floor
92.0 dBµV/m
46.8 dBµV/m Below noise floor Below noise
floor
Table 2: Measured output power and harmonic emission.
The field strength values from Table 2 can be converted into an Effective Isotropic Radiated
Power (EIRP) using the following formula:
⎛
E 2r 2 ⎞
⎟[dBm]
PEIRP = 10 log⎜⎜1000
30 ⎟⎠
⎝
Where E is the field strength in V/m and r is the distance in m between the transmitter and
receiving antenna. The results in Table 2 are based on measurements with 3 meters between
the CC2500 Folded Dipole EM and the receiving antenna.
5
Conclusion
Table 2 shows that the folded dipole antenna described in this document can meet both ETSI
and FCC regulations. When using +1 dBm output power, either duty cycling or frequency
hopping must be used to ensure compliance with FCC limits. For more information about
SRD regulations for license free transceiver operation in the 2.4 GHz ISM band please
download Application Note AN032 from:
http://focus.ti.com/analog/docs/techdocsabstract.tsp?familyId=368&abstractName=swra060
Table 3 lists the properties of the folded dipole antenna. The range test was performed
outdoors with free line of sight (LOS). The range was measured with 250 kbps and 1% PER
without C121 and C131 mounted.
Gain in XY plane
Gain in XZ plane
Gain in YZ plane
LOS Range
Antenna size
5.6 dB
7.4 dB
6.5 dB
300 m
46 x 9 mm
Table 3: Folded dipole properties.
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6
6.1
General Information
Document History
Revision
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Date
2006.12.06
Description/Changes
Initial release.
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7
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