6 2.4-GHz PCB Compact Antenna Measurement and Matching

Antenna matching described in this chapter is showing an example of cased 2.4-GHz PCB compact (15 mm by 6 mm) antenna (Figure 6-1). This antenna is popular for portable devices with space constrained boards as it has small geometrical sizes. Detailed antenna description can be found on document AN043.

Portable devices are usually made for in-hand operation. Plastic case and human’s body can detune the antenna. In such case, a final matching should be done including the case and human’s body, representing a real device’s operation condition. Figure 6-2 is showing measured SWR values in free space, in a plastic case and in the plastic case with in-hand placement.

Users can see the SWR value changes from 1:1.70 (in free space) up to 1:3.68 when antenna is cased and placed in hand.

For given impedance of 19.2 + j30.4 Ohm (at 2.4410 GHz) Smith calculator shows ideal components values for matching network 4-4A of 1.9 pF plus 3.5 nH.

It’s important to remember, the calculator gives the ideal values. But for matching job, especially with human’s body affecting the antenna, the final matching circuit can be different.

For matching job, the only capacitor of 1.8 pF installed. Measured impedance with plastic and in-hand placement shown on Figure 6-3.

Looking at measured marker 1 and Smith calculator’s chart gives an idea that parasitic capacitance is about 0.5 pF. This means the value can be adjust down to about 1.2 pF. Figure 6-4 shows measured impedance with 1.5 pF installed.

For this matching job even doesn’t necessary to install the inductor and it can be replaced by SMD jumper (Figure 6-5) as SWR value is 1:1.05.