2.4-GHz Inverted F Antenna Design and Performance
Application Report by Texas Instruments
Abstract
This document describes a printed-circuit board (PCB) design that can be used with all 2.4-GHz transceivers and transmitters from Texas Instruments™. The maximum gain is measured to be +3.3 dBi, and the overall size requirements for this antenna are 25.7 × 7.5 mm. Thus, this antenna is compact, low cost, and high performance.
1 Description of Inverted F Antenna Design
Because the impedance of the inverted F Antenna (IFA) is matched directly to 50 Ω, no external matching components are required.
1.1 Implementation of 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 a gerber file or a 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, avoid placing components or having a ground plane close to the end points of the antenna. If the CAD tool being used does not support importing gerber or DXF files, see Figure 1 and Table 1.
[Diagram showing the layout of the Inverted F Antenna (IFA) with dimensions labeled H1 through H9 and W1, W2, L1 through L8. It indicates a 50-ohm feed point and a ground plane.]
| Dimension | Value | Dimension | Value |
|---|---|---|---|
| H1 | 5.70 mm | W2 | 0.46 mm |
| H2 | 0.74 mm | L1 | 25.58 mm |
| H3 | 1.29 mm | L2 | 16.40 mm |
| H4 | 2.21 mm | L3 | 2.18 mm |
| H5 | 0.66 mm | L4 | 4.80 mm |
| H6 | 1.21 mm | L5 | 1.00 mm |
| H7 | 0.80 mm | L6 | 1.00 mm |
| H8 | 1.80 mm | L7 | 3.20 mm |
| H9 | 0.61 mm | L8 | 0.45 mm |
| W1 | 1.21 mm |
Because there is no ground plane beneath the antenna, the PCB thickness will have little effect on the performance. The results presented in this document are based on an antenna implemented on a PCB with a 1-mm thickness.
2 Results
All of the results presented in this section are based on measurements performed with CC2430DB [1].
2.1 Radiation Pattern
[Diagram illustrating the relationship between the antenna's physical orientation and the three principal radiation pattern planes: XY plane, XZ plane, and YZ plane. Each plane shows a coordinate system (X, Y, Z) relative to the antenna.]
[Polar plot showing vertical polarization in the XY plane for the 2.4-GHz Inverted F Antenna. Center Frequency (CF): 2450.000 MHz. Gain scale: 4 dB/div. Reference Level (Ref Lev): ... dBm.]
[Polar plot showing horizontal polarization in the XY plane for the 2.4-GHz Inverted F Antenna. Center Frequency (CF): 2450.000 MHz. Gain scale: 4 dB/div. Reference Level (Ref Lev): ... dBm.]
[Polar plot showing vertical polarization in the XZ plane for the 2.4-GHz Inverted F Antenna. Center Frequency (CF): 2450.000 MHz. Gain scale: 2 dB/div. Reference Level (Ref Lev): +3.3 dBm.]
[Polar plot showing horizontal polarization in the XZ plane for the 2.4-GHz Inverted F Antenna. Center Frequency (CF): 2450.000 MHz. Gain scale: 3 dB/div. Reference Level (Ref Lev): -1.5 dBm.]
[Polar plot showing vertical polarization in the YZ plane for the 2.4-GHz Inverted F Antenna. Center Frequency (CF): 2450.000 MHz. Gain scale: 2 dB/div. Reference Level (Ref Lev): ... dBm.]
[Polar plot showing horizontal polarization in the YZ plane for the 2.4-GHz Inverted F Antenna. Center Frequency (CF): 2450.000 MHz. Gain scale: 5 dB/div. Reference Level (Ref Lev): +11 dBm.]
2.2 Reflection
[A graph titled 'Measured Reflection at Feed Point of Antenna'. The Y-axis represents Measured S11 in dB, ranging from 0 to -25 dB. The X-axis represents Frequency in GHz, ranging from 2.0 to 3.0 GHz. The red line shows the reflection coefficient, dipping to approximately -25 dB around 2.4 GHz, indicating good impedance matching and a bandwidth of over 300 MHz for less than 10% reflection.]
2.3 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 maximum hold function on a spectrum analyzer, the variation in output power across frequency can easily be measured.
3 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 IFA.
| Property | Value |
|---|---|
| Gain in XY plane | 1.1 dBi |
| Gain in XZ plane | 3.3 dBi |
| Gain in YZ plane | 1.6 dBi |
| Reflection | < -15 dB |
| Antenna size | 25.7 × 7.5 mm |
4 References
- CC2430DB Reference Design
Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from B Revision (April 2008) to C Revision:
- Changed +3.3 dB to be +3.3 dBi (Page 1)
- Changed dB in Table 2 to dBi (Page 11)
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