AN2972

How to design an antenna on a PCB

Doc ID 15629 Rev 6

17/25

2.5.3

Metal considerations

What does it happen when there is metal near the antenna?

When the antenna of the M24LRxx is close to metal, it changes its own resonance

frequency, as shown in Figure 18.

Figure 18. Effect of metal on the antenna frequency tuning

Due to the modification of the tuning frequency, when an antenna is close to metal, the

frequency tuning of a tag should be modified: tuned at 13.56 MHz in the metal environment.

In other words, when a tag is placed in a metal environment, the tuning frequency of the tag

in the open air must be set to compensate for future shift. The metal environment induces

eddy current, quality factor downgrading, frequency detuning and a modification of the field-

strength distribution.

As a conclusion, when an antenna, which has been tuned in order to operate in the open air,

is to operate near metal, it is necessary to redesign a new antenna which will be precisely

tuned with the global application.

Frequency tuning of an antenna close to metal

MS30739V1

Frequency tuning of an antenna in the open air

Legend

:

F [MHz)]

13.56

S11 log

magnitude [dB]

13.56 + x%

Table 3.

Antenna features

Features

Antenna 1: ANT1-M24LR16E

Antenna 2: ANT1-M24LR16E with 74 pF

in parallel of the antenna

Antenna size

45 mm x 75 mm

45 mm x 75 mm

Frequency tuning in the air

13.7 MHz

7.5 MHz

Frequency tuning stuck on the

25 MHz

14 MHz

Read range in the open air (1)

7.5 cm

0.5 cm

Read range close to metal (1)(2)

No detection

2.5 cm

Status

This antenna is tuned to operate

in the open air

This antenna is tuned to operate close to

metal

1.

The measurement has been done with DEMO-CR95HF-A.

2.

The measurement has been done as per an antenna stuck on the full metal table.