Impedance matching

AN3359

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Doc ID 018585 Rev 1

3

Impedance matching

Meander-like PCB antenna can be tuned to the required 50 Ohm impedance by matching

the impedance circuitry with the

π topology. In Figure 2 the impedance matching area is

marked with a dashed line. Under nominal conditions, this antenna should exhibit

impedance very close to the required nominal impedance (50 Ohm).

To check the performance of this design, a sample antenna was manufactured (according to

the specifications covered by this document). Figure 4 shows this antenna.

Figure 4.

Part of the ZigBee module's PCB with Meander-like antenna (around

scale 4:1)

Assuming that the manufactured sample exhibits the expected performance (no impedance

matching necessary), the impedance matching circuitry was bypassed by two 100 pF

capacitors connected in series, as shown in Figure 5:

Figure 5.

Bypassing impedance matching circuitry - direct RF connection

All electrical parameters of the meander-like antenna have been measured at connection to

the Band Pass Filter with the frequency span covering frequencies from 2.4 GHz to 2.5

GHz.

Complex impedance of the antenna is shown in the Smith diagram in Figure 6: