FSC-BT1035 External Antenna/Disable Onboard Antenna

papricot

Hello,

I'm designing a custom carrier PCB for the FSC-BT1035 module and plan to use an external 2.4 GHz 50 ohm antenna connected to the RF_OUT pin via a 50 ohm trace and U.FL connector.

Could you please confirm:

If I connect RF_OUT directly to an external 50 ohm antenna, is the onboard antenna automatically disconnected?
If not, what is the correct method to disable the onboard antenna and use only the external antenna?
Is there a 0 ohm resistor on the module that must be removed or changed?
Is there a specific antenna-selection pad or configuration required?

Thank you for any clarification.

FAE_Gavin

papricot Our team is currently on holiday for the Chinese New Year (Spring Festival) . We will resume work on Tuesday, February 24, 2026.

During this period, forum replies may be delayed. We will get back to you as soon as possible after we return.

FAE_Gavin

papricot Thank you for your inquiry regarding the FSC‑BT1035 module and external antenna connection. Here are the answers to your questions:

1. No, the onboard antenna is not automatically disconnected
When you connect an external antenna to the RF_OUT pin, the onboard antenna is not automatically disabled.

2. The correct method is to change the soldering position of the *0Ω resistors* on the module to connect the RF_OUT pin (Pin 51) to the external antenna.

3. Both resistors (labeled 1 and 2) are 0Ω resistors. You can connect to different external antenna interfaces by modifying the position and orientation of these two resistors as follows:

Resistor 1 must be moved to the left to connect to the external antenna pad on the left side of the module.
Resistor 2 should be soldered horizontally to connect the RF_OUT pin (Pin 51) to the external antenna.

4. Feasycom offers three antenna specifications
You can contact Feasycom sales before shipment to confirm the default antenna configuration of your modules, or to specify the antenna type you require.

papricot

FAE_Gavin Perfect. Just to make sure I understand correctly, if I want to use an external antenna connected from the RF_OUT pin 51 then I will need to move both of the resistors you identified?

FAE_Gavin

papricot If you want to use the external antenna on RF_OUT pin 51, you only need to solder the resistor marked "2" in the diagram horizontally.

papricot

FAE_Gavin

To make sure my carrier PCB is correct:

When resistor 2 is rotated, does this fully disconnect the onboard antenna path and no additional changes need to be made to resistor 1?

Is RF_OUT (Pin 51) a 50 Ω single-ended port intended to feed an external 50 Ω antenna directly?

Do you recommend any external matching (Pi footprint) or a DC-block capacitor between RF_OUT and a U.FL connector, or is a direct 50 Ω trace acceptable?

FAE_Gavin

papricot 1.Resistor setting: Simply solder resistor 2 horizontally on the module. This switches the RF signal to Pin 51 and automatically disables the onboard antenna. No change to resistor 1 is needed.

2.RF_OUT Pin Characteristics

RF_OUT (Pin 51) is the antenna port – you must route a microstrip line from this pin.
This pin is a 50 Ω single‑ended port (with integrated balun) and can directly feed a 50 Ω antenna.

3. PCB Layout Requirements

Impedance control: The RF microstrip line must be designed as a 50 Ω controlled impedance line, referenced to Layer 2 (solid ground plane).
Matching reserve: Place a Pi matching circuit footprint (0 Ω series resistor + two NC capacitor pads) near the antenna connector for future tuning.