7 2.4-GHz PCB Inverted-F Antenna Measurement and Matching

A PCB inverted-F antenna is popular for Bluetooth®, Bluetooth® Low Energy, and ZigBee™ devices. It also can be used for Wi-Fi™ applications. Users can find the antenna's detailed description in the Application report DN007

The board was prepared for the tests and VNA calibration was done including matching network traces (refer to the Section 4.2).

Figure 7-1 PCB Inverted-F Antenna

The impedance and SWR of unmatched antenna are shown in Figure 7-2. Markers are showing low, mid, and high operation frequencies for Bluetooth Low Energy and ZigBee applications. At 2.4410 GHz, the impedance is 29.5 + j11.5 Ohm and SWR value is 1.83:1.

Figure 7-2 Measured Impedance of Unmatched PCB Inverted-F Antenna at 2.4410 GHz

Figure 4-4 shows that the network 4-4C and 4-4D can move the impedance to the center of the chart.

Online Smith chart calculators for the frequency at 2441 MHz and given impedance of 29.5 + j11.5 Ohm for network 4-4C gave values of 0.9 nH plus 1.1 pF. For small value capacitances (≤ 2 pF) usually recommended to pull back the ground plane a little on the top layer (not to affect the final capacitance and to use tight tolerances).

The network 4-4D with components values of 1.8 pF plus 3.9 nH was used. For matching job next steps were made:

1. The only capacitor of 1.8 pF was installed for checking how far the marker moves (Figure 7-3).
   Figure 7-3 Measured Impedance with only 1.8 pF Capacitor Installed
2. Installed inductor of 3.9 nH (1.8 pF and 3.9 nH)
3. Adjusted inductor’s value to 4.3 nH (1.8 pF and 4.3 nH)
4. Adjusted capacitor’s value to 2.0 pF (2.0 pF and 4.3 nH)

Figure 7-4 shows a final matched impedance with corresponded SWR and S11 levels. SWR value improved from 1:1.83 down to 1:1.08.

Figure 7-4 Measured Impedance, SWR, and S11 for Network 4-4D (with 2.0 pF and 4.3 nH)

Figure 7-5 Board with Final Matching Components Installed