Figure 49: Gnd Pin Of The Earphone Use To Be The Fm Antenna Interface - SimCom SIM800L Hardware Design

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The earphone is often used for FM antenna, as an example, the pin GND of the 3.5mm earphone is connected to
the FM interface. The circuit is recommended as following:
R303
GND
FM_ANT
C303 C304
Moudle

Figure 49: GND pin of the earphone use to be the FM antenna interface

Normally, R303 are 0Ω,C303\C304 are not mounted.
SIM800L(MT6261)_Hardware Design_V1.01 49 2016-07-07