Design Note DN0007 2.4 GHz Inverted F Antenna By Audun Andersen Keywords • • • • • • • 1 • • • • • • CC2400 CC2420 CC2430 CC2431 CC2500 CC2510 CC2511 CC2550 CC2520 CC2480 PCB Antenna 2.4 GHz Inverted F Antenna Introduction This document describes a PCB antenna design that can be used with all 2.4 GHz transceivers and transmitters from Texas Instruments. Maximum gain is measured to be +3.3 dB and overall size requirements for this antenna are 25.7 x 7.5 mm. Thus, this is a compact, low cost and high performance antenna. SWRU120B Page 1 of 14 Design Note DN0007 Table of Contents KEYWORDS.............................................................................................................................. 1 1 INTRODUCTION............................................................................................................. 1 2 ABBREVIATIONS........................................................................................................... 2 3 DESCRIPTION OF THE INVERTED F ANTENNA DESIGN ......................................... 3 3.1 IMPLEMENTATION OF THE INVERTED F ANTENNA .......................................................... 3 4 RESULTS........................................................................................................................ 4 4.1 RADIATION PATTERN .................................................................................................. 4 4.2 REFLECTION ............................................................................................................ 11 4.3 BANDWIDTH ............................................................................................................. 11 5 CONCLUSION .............................................................................................................. 12 6 REFERENCES.............................................................................................................. 13 7 GENERAL INFORMATION .......................................................................................... 14 7.1 DOCUMENT HISTORY................................................................................................ 14 2 Abbreviations CC2480 EM IFA ISM PCB Z-Accel ZigBee Processor Evaluation Module Inverted F Antenna Industrial, Scientific, Medical Printed Circuit Board SWRU120B Page 2 of 14 Design Note DN0007 3 Description of the Inverted F Antenna Design Since the impedance of the Inverted F Antenna is matched directly to 50 ohm no external matching components are needed. 3.1 Implementation of the Inverted F Antenna 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 CC2430DB reference design [1]. The gerber file is called “Inverted_F_Antenna.spl” and the DXF file is called “Inverted_F_Antenna.dxf”. If the antenna is implemented on a PCB that is wider than the antenna it is important to avoid placing components 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 1 and Table 1 can be used. Figure 1. IFA Dimensions H1 H2 H3 H4 H5 H6 H7 H8 H9 W1 5.70 mm 0.74 mm 1.29 mm 2.21 mm 0.66 mm 1.21 mm 0.80 mm 1.80 mm 0.61 mm 1.21 mm W2 L1 L2 L3 L4 L5 L6 L7 L8 0.46 mm 25.58 mm 16.40 mm 2.18 mm 4.80 mm 1.00 mm 1.00 mm 3.20 mm 0.45 mm Table 1. IFA Dimensions Since there is no ground plane beneath the antenna, PCB thickness will have little effect on the performance. The results presented in this design note are based on an antenna implemented on a PCB with 1 mm thickness. SWRU120B Page 3 of 14 Design Note DN0007 4 Results All results presented in this chapter are based on measurements performed with CC2430DB [1]. 4.1 Radiation Pattern Figure 2 shows how to relate all the radiation patterns to the orientation of the antenna. The radiation patterns were measured with CC2430 programmed to 0 dBm output power. XZ plane XY plane YZ plane Figure 2. How to Relate the Antenna to the Radiation Patterns SWRU120B Page 4 of 14 Design Note DN0007 Figure 3. XY Plane Vertical Polarization SWRU120B Page 5 of 14 Design Note DN0007 Figure 4. XY Plane Horizontal Polarization SWRU120B Page 6 of 14 Design Note DN0007 Figure 5. XZ Plane Vertical Polarization SWRU120B Page 7 of 14 Design Note DN0007 Figure 6. XZ Plane Horizontal Polarization SWRU120B Page 8 of 14 Design Note DN0007 Figure 7. YZ Plane Vertical Polarization SWRU120B Page 9 of 14 Design Note DN0007 Figure 8. YZ Plane Horizontal Polarization SWRU120B Page 10 of 14 Design Note DN0007 4.2 Reflection Figure 9. Measured Reflection at the Feed Point of the Antenna Figure 9 show that the IFA ensures less than 10 % reflection of the available power for a bandwidth of more than 300 MHz. A large bandwidth makes the antenna less sensitive to detuning due to plastic encapsulation or other objects in the vicinity of the antenna. 4.3 Bandwidth Another way of measuring the bandwidth after the antenna is implemented on a PCB and connected to a transmitter is to write test software that steps a carrier across the frequency band of interest. By using the “Max hold” function on a spectrum analyzer the variation in output power across frequency can easily be measured. Figure 10 shows how the output power varies on the IFA when the PCB is horizontally oriented and the receiving antenna has horizontal polarization. This measurement was not performed in an anechoic chamber thus the graph shows only the relative variation for the given frequency band. SWRU120B Page 11 of 14 Design Note DN0007 Bandwidth IFA 2.4 0 2.4835 Relative output power [dB] -10 -20 Output pow er -30 Low er band edge Upper band edge -40 -50 -60 2.2 2.3 2.4 2.5 2.6 2.7 2.8 Frequency [GHz] Figure 10. Bandwidth of IFA 5 Conclusion The PCB antenna presented in this document performs well for all frequencies in the 2.4 GHz ISM band. Except for two narrow dips, the antenna has an omni directional radiation pattern in the plane of the PCB. These properties will ensure stable performance regardless of operating frequency and positioning of the antenna. Table 2 lists the most important properties for the inverted F antenna. Gain in XY Plane Gain in XZ Plane Gain in YZ Plane Reflection Antenna Size 1.1 dB 3.3 dB 1.6 dB < -15 dB 25.7 x 7.5 mm Table 2. Summery of the Properties of the IFA SWRU120B Page 12 of 14 Design Note DN0007 6 References [1] CC2430DB Reference Design (swrr034.zip) SWRU120B Page 13 of 14 Design Note DN0007 7 General Information 7.1 Document History Revision SWRU120B SWRU120A SWRU120 Date 2008-04-04 2008-02-28 2007-04-16 Description/Changes Renamed CCZACC06 to CC2480 Added reference to CCZACC06 and CC2520 Initial release. SWRU120B Page 14 of 14 IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, modifications, enhancements, improvements, and other changes to its products and services at any time and to discontinue any product or service without notice. Customers should obtain the latest relevant information before placing orders and should verify that such information is current and complete. All products are sold subject to TI’s terms and conditions of sale supplied at the time of order acknowledgment. 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