Using CC253X or CC254X with Dipole PCB Antennas By Espen Wium Keywords Half wave dipole RF Antenna Efficiency Gain TRP (Total Radiated Power) CC2530 CC2531 CC2533 CC2540 CC2541 1 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 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. SWRA421 Page 1 of 19
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 SWRA421 Page 2 of 19
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 Simulating the Antenna 5.1 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. SWRA421 Page 3 of 19
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. SWRA421 Page 4 of 19
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. SWRA421 Page 5 of 19
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. SWRA421 Page 6 of 19
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 Prototype Testing and Optimization 7.1 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. SWRA421 Page 7 of 19
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 2 nd 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 SWRA421 Page 8 of 19
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. L [nh] C [pf] 1.8 3.3 3.6 3.9 4.3 4.7 1 2.14 2.6 1.2 1.5 2.14 2.52 1.8 1.99 2.04 2.18 2.2 1.32 1.56 1.37 2.7-0.06-0.78-0.54-0.61 Table 1. TRP in dbm for Different Values of L and C L [nh] C [pf] 1.8 3.3 3.6 3.9 4.3 4.7 1-44.2-44.3 1.2-44.8 1.5 1.8-46.4-45.8 2.2-45.1 2.7-47.1 Table 2. 2 nd Harmonic Levels in dbm (max EIRP) L [nh] C [pf] 1.8 3.3 3.6 3.9 4.3 4.7 1-31.5-32.5 1.2-35.5 1.5-36.0 1.8-37.2-36.2 2.2-38.1 2.7-39.7 Table 3. 3 rd Harmonic Levels in dbm (max EIRP) SWRA421 Page 9 of 19
The most obvious observation is that the 2 nd harmonic level is more or less independent of the TRP and L values, but has a tendency to decrease with increasing capacitance to ground. This is most likely a result of the 2.7 pf cap being resonant close to the 2 nd harmonic, so the closer we get to 2.7 pf the better the decoupling/filtering. The 3 rd 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 with the inductor in the LC parallel resonant circuit). This gives us a safe level of the 2 nd harmonic, a 3 rd harmonic that is compliant with up to almost 50% duty cycle, and just above +2 dbm TRP. If we reduce the output power slightly, the 3 rd harmonic will pass FCC even with 100% duty cycle. TXPOWER = 0xF5 TXPOWER = 0xE5 TXPOWER = 0xD5 Sample TRP 2 nd 3 rd TRP 2 nd 3 rd TRP 2 nd 3 rd S1 2.35 S2 2.32 BTM -37.1-1.61 BTM -48.6 S3 2.21 S4 2.79-45.1-35.4 0.16 BTM -42.8-1.06 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 efficiency of at least 100*10-0.2 = 63% even though this design has not been tuned for 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 3- component 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. SWRA421 Page 10 of 19
9 General Information 9.1 Document History Revision Date Description/Changes SWRA421 2013.01.25 Initial release. SWRA421 Page 11 of 19
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 SWRA421 Page 12 of 19
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