How to check the M24LRxx antenna tuning

AN2972

18/25

Doc ID 15629 Rev 6

3

How to check the M24LRxx antenna tuning

The methods of antenna design described in the previous section may lead to an

inductance slightly different from the value that would offer optimum performance in the end

application. This is because the overall inductance of the antenna might slightly drift in the

application (with magnetic and ferromagnetic materials in the proximity of the antenna). It is

therefore necessary to run actual measurements of the resonant frequency of the antenna.

3.1

Antenna tuning measurements with a network analyzer

The tuning frequency of the M24LRxx antenna can be measured using a network analyzer

with a loop probe.

The RF electromagnetic field is generated by connecting a loop probe (like the 7405-901

Eaton/Alitech 6 cm loop) to the output of the network analyzer set in reflection mode (S11

measurement).

Figure 19. Measurement equipment

This equipment setup will directly display the system’s resonant frequency.

Experiments

frequency sweep range has to be set around this value, that is:

●

Start frequency: 5 MHz

●

End frequency: 20 MHz

●

Output power: –10 dBm

●

Measurement: reflection or S11

●

Format: log magnitude

Place the antenna within the field generated by the network analyzer + loop probe. The

resonant frequency corresponds to the minimum observed on the S11 measurement curve.

Network analyser

Loop probe

Antenna prototype with the M24LRxx

(represented as a capacitor)

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