Design Note DN041
Using CC253X or CC254X with Dipole PCB Antennas
By Espen Wium
Keywords
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Half wave dipole
RF
Antenna
Efficiency
Gain
TRP (Total Radiated Power)
CC2530
CC2531
CC2533
CC2540
CC2541
Introduction
Many RFICs today use differential ports to
transmit and receive RF power. This
makes the use of balanced antennas an
attractive proposition, as one can do away
with the balun normally needed to convert
the differential RF power at the IC ports to
single ended signals that can be fed to an
SWRA421
SMA connected antenna or RF instrument.
This document describes a simple half
wave dipole antenna that is easy to
integrate on a PCB and works well with all
CC253X and CC254X RFIC’s from Texas
Instruments.
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Design Note DN041
Table of Contents
KEYWORDS.............................................................................................................................. 1
1
INTRODUCTION ............................................................................................................. 1
2
ABBREVIATIONS ........................................................................................................... 2
3
THE HALF WAVE DIPOLE ANTENNA IN BRIEF .......................................................... 3
4
DESIGNING A HALF WAVE DIPOLE ANTENNA ON A PCB ....................................... 3
5
SIMULATING THE ANTENNA ....................................................................................... 3
5.1
STEP 1: CLOSE TO IDEAL ........................................................................................... 3
5.2
STEP 2: ADD SUBSTRATE ........................................................................................... 4
5.3
STEP 3: ADDING CURVATURE ..................................................................................... 5
5.4
STEP 4: ADDING A GROUND PLANE............................................................................. 6
6
DESIGNING THE PROTOTYPES .................................................................................. 7
7
PROTOTYPE TESTING AND OPTIMIZATION .............................................................. 7
7.1
ANTENNA LENGTH TUNING ......................................................................................... 7
7.2
FILTERING HARMONICS .............................................................................................. 8
8
CONCLUSION .............................................................................................................. 10
9
GENERAL INFORMATION........................................................................................... 11
9.1
DOCUMENT HISTORY ............................................................................................... 11
APPENDIX A: CTIA REPORT................................................................................................. 12
2
Abbreviations
BTM
EIRP
FCC
PCB
SMA
TRP
Below Threshold of Measurability
Equivalent Isotropic Radiated Power
Federal Communications Commission
Printed Circuit Board
SubMiniature version A connector
Total Radiated Power
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Design Note DN041
3
The Half Wave Dipole Antenna in Brief
The half wave dipole is one of the simplest and most commonly used antenna structures in
the RF world. It can be made from a piece of wire, or a trace on a PCB. The basic half wave
dipole antenna is a wire or trace with a length of lambda/2 and a slot in the middle where the
antenna is fed. The impedance is around 73 ohms, and the directivity is 2.15 dBi.
4
Designing a Half Wave Dipole Antenna on a PCB
This design note describes the antenna design process for the Zlight2 reference design, a 40
mm PCB disc that is used to demonstrate the capabilities of the CC2530 when used as a
wireless LED controller. This places some constraints on the use of the basic dipole antenna
structure, the main ones being that the antenna will be placed close to the edge of the board,
and it will be curved with a radius of 20 mm. The width of the trace does have an impact on
the antenna performance, particularly the bandwidth. In this design note we will stick to a width
of 300 um (12 mils), and refer the reader to more thorough papers on antenna theory if they
wish to probe deeper into this particular property of the dipole. Placing the antenna close to
the edge means that the effective dielectric constant will be harder to determine, as a 2.5D
EM simulation tool such as Momentum will not accurately capture the effects of our finite
substrate. Changing the shape of the antenna from a straight line into a curve means that the
radiation pattern will not be the same as that of an ideal dipole.
As a first step, we simulate a close-to-ideal dipole using ADS Momentum to verify that the
simulation setup yields credible results. Then we add substrate, curvature and a ground plane
in order to model the final board more accurately. Finally, we add some length to be able to
manually fine tune the antenna length on the first prototypes. This latter step is a
precautionary one, given the uncertainty in the effective dielectric constant seen by the
antenna.
The prototype testing and optimization process involves trimming the antenna to the right
length, and then adding matching and filtering components to obtain a good trade off of high
output power at the carrier frequency and low harmonics.
5
5.1
Simulating the Antenna
Step 1: Close to Ideal
Figure 1. A 300 um Trace in Free Space
The structure shown in Figure 1 is composed of two arc segments, each with a width of 300
um, a radius of 3 m and an angular length of 0.56 degrees. The substrate definition is set to
free space above and below the trace. Figure 2 shows the simulated return loss (S11) using
ADS momentum.
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Figure 2. Simulated S11. Substrate: Free Space
Figure 2 shows a 1 dB return loss variation in our band of interest (2400 MHz to 2480 MHz),
and an impedance of about 78 ohms.
5.2
Step 2: Add Substrate
The layout used in Section 5.1 is simulated again, this time on an 800 um FR4 substrate. The
resonance frequency has now dropped to around 1.86 GHz, and we need to shorten the arcs
to 0.416 degrees to move our pass band back to 2.45 GHz.
Figure 3. Simulated S11. Substrate: 800 um FR4
Adding a substrate has the effect of lowering the impedance of our antenna to 51 ohms, which
also explains the very low return loss given that we used 50 ohm ports in these simulations.
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5.3
Step 3: Adding Curvature
Figure 4. Curved Half Wave Dipole
The structure shown in Figure 4 is composed of two arc segments, each with a width of 300
um, a radius of 20 mm and an angular length of 64 degrees.
Figure 5. S11 Looking into the Curved Dipole
The impedance drops slightly (from ~51 ohms to ~45 ohms) as the shape is changed from an
almost straight trace to a curved trace with a 20mm radius.
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5.4
Step 4: Adding a Ground Plane
A ground plane is added as shown in Figure 6. This further reduces the impedance to around
37 ohms, but does not change the frequency response by much.
Figure 6. A Ground Plane is Added to Improve the Accuracy of the Board Model
Figure 7. S11 Looking into the Curved Dipole with Added Ground Plane
We will use this pattern as a starting point for our prototypes. The length of each of the two
arcs is 2π·20mm·64/360 = 22.3 mm, thus the total length of the dipole is about 45 mm.
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6
Designing the Prototypes
In vacuum (or air), a half wavelength at 2.44 GHz is 61.5 mm. Our first simulations suggested
a length of 2 times 29.3 mm (0.56 degrees with a radius of 3000 mm). When we added the
800 um FR4 substrate, this dropped to 2 times 21.8 mm (0.416 degrees) for the straight
dipole and 2 times 22.3 mm for the curved dipole. Since the effects of a finite substrate is not
taken into account by our tool (ADS Momentum), we will extend the antenna slightly to allow
us to physically cut the traces back to the optimum length by experimentation. We know that
the optimum length will be somewhere between 2·22.3 mm and 2·29.3 mm since a finite
substrate will give an effective dielectric constant somewhere between that of an infinite
substrate and that of no substrate at all.
With that in mind, we choose an arc length of 75 degrees. This corresponds to an antenna
length of 2·26.2 mm.
Figure 8. First Prototype Layout with Tuning Marks and Filtering Components
To facilitate the antenna tuning, we add some silk print markings to make it easier to relate the
end result back to the layout in our CAD tool. We also add a simple series/shunt filtering
network to help suppress harmonics if needed (see Figure 8).
7
7.1
Prototype Testing and Optimization
Antenna Length Tuning
With the prototypes back, the first step is to tune the antenna to the right length. This is done
by mounting 0 ohms resistors as series elements in the filtering network, and leaving the
shunt element empty. Then, the emitted RF power at 2.44GHz is measured in an antenna test
chamber while the antenna length is reduced by half a notch at a time. Since the information
of interest is the signal strength relative to that of the previous measurement, it would also
have been possible to measure range or RSSI when communicating between two Zlight2
boards.
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Design Note DN041
Figure 9. TRP (in dBm) versus the Number of Notches Cut (x-axis)
Two samples were tested, and they both indicated that the optimum length would be achieved
by cutting two notches.
7.2
Filtering Harmonics
When fed from a differential source, the half wave dipole antenna will radiate odd harmonics
rather well, while even harmonics will not be transmitted. However, in practice, some of the
unwanted harmonic contents will be common mode, i.e. the currents and voltages at the two
antenna terminals have zero phase difference. This means that our antenna will also radiate
even harmonics of the carrier frequency unless our filter is able to suppress common mode
signals as well as differential signals.
The initial filter was a purely differential design, which proved to be insufficient to handle the
nd
2 harmonic. A new version was therefore made, where shunt components were connected
to ground to filter common mode signals. The antenna length was also updated according to
the results found in Section 7.1.
To keep component count, cost, and insertion loss to a minimum, the new filter is simply a
parallel resonant LC circuit (tuned to the carrier frequency) where the C is split into two
capacitors of double capacitance connected to ground.
Figure 10. Parallel Resonant Circuit with Differential as well as Common Mode Rejection
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Design Note DN041
Figure 11. The Final Layout of the Antenna and Filtering Components
After tuning, the total length of the antenna is now 2·22.6 mm, which is very close to the
simulated value found in Section 5.4.
The next step was to test different values of L and C to obtain a good trade off between output
power and harmonic suppression. The tables below show the measured TRP for different
values of L and C, and the corresponding harmonic levels.
C [pF]
1
1.2
1.5
1.8
2.2
2.7
1.8
3.3
2.14
L [nH]
3.6
3.9
1.99
1.32
-0.78
-0.06
2.14
2.04
1.56
-0.54
4.3
4.7
2.6
2.52
2.18
1.37
-0.61
Table 1. TRP in dBm for Different Values of L and C
C [pF]
1
1.2
1.5
1.8
2.2
2.7
1.8
L [nH]
3.6
3.9
4.3
4.7
-44.3
-44.8
-46.4
-45.8
-45.1
-47.1
nd
Table 2. 2
C [pF]
1
1.2
1.5
1.8
2.2
2.7
3.3
-44.2
1.8
Harmonic Levels in dBm (max EIRP)
3.3
-31.5
L [nH]
3.6
3.9
-37.2
4.3
4.7
-32.5
-35.5
-36.0
-36.2
-38.1
-39.7
rd
Table 3. 3 Harmonic Levels in dBm (max EIRP)
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Design Note DN041
nd
The most obvious observation is that the 2 harmonic level is more or less independent of the
TRP and L values, but has a tendency to decrease with increasing capacitance to ground.
nd
This is most likely a result of the 2.7 pF cap being resonant close to the 2 harmonic, so the
closer we get to 2.7 pF the better the decoupling/filtering.
rd
The 3 harmonic level seems to depend mostly on the carrier power, and since we want to get
it down towards -41.2 dBm to avoid strict duty cycle requirements from the FCC, we choose
an inductance of 3.9 nH and a capacitance of 1.5 pF (which is equivalent to 0.75 pF in parallel
nd
with the inductor in the LC parallel resonant circuit). This gives us a safe level of the 2
rd
harmonic, a 3 harmonic that is compliant with up to almost 50% duty cycle, and just above
rd
+2 dBm TRP. If we reduce the output power slightly, the 3 harmonic will pass FCC even with
100% duty cycle.
Sample
S1
S2
S3
S4
TXPOWER = 0xF5
TRP
2nd
3rd
2.35
2.32
BTM
-37.1
2.21
2.79
-45.1
-35.4
TXPOWER = 0xE5
TRP
2nd
3rd
0.16
BTM
-42.8
TXPOWER = 0xD5
TRP
2nd
3rd
-1.61
BTM
-48.6
-1.06
BTM
-46.0
Table 4: TRP and Harmonics Levels with 3 Different Power Settings
(TXPOWER = 0xF5, E5 and D5)
The available power from CC2531 when using power setting 0xF5 is +4 dBm, which means
that our measured TRP is less than 2 dB short of the efficiency reference. This gives an
-0.2
= 63% even though this design has not been tuned for
efficiency of at least 100*10
maximum TRP.
The part numbers of the 3.9 nH inductor and 1.5pF capacitors are:
• GRM1555C1H1R5CZ01 (0402 capacitor with C0G/NP0 dielectric from Murata)
• LQG15HS3N3S02 (0402 multilayer chip inductor from Murata)
A full measurement report on this antenna can be found in Appendix A: CTIA Report
8
Conclusion
A half wave dipole can be an excellent antenna choice for low cost RF nodes. A simple 3component filter is all that is needed to ensure regulatory compliance. The antenna designed
in this Design Note has an efficiency of more than 63%, and a directivity of 5.04 dBi.
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Design Note DN041
9
9.1
General Information
Document History
Revision
SWRA421
Date
2013.01.25
Description/Changes
Initial release.
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Appendix A: CTIA Report
With the board placed in the XZ-plane as shown in Figure 12, the radiation diagram was
measured in an antenna chamber. The reference level used to calculate efficiency and gain
was 0 dBm, which means that 4 dB needs to be subtracted from the gain given that the
available power from CC2531 is 4 dBm.
Z
X
Figure 12. Board Orientation During the Antenna Measurements
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