I am trying to understand the general way how wireless devices (cell modem, wifi card, router, other) work that have a socket for an optional external antenna. The main goal is to increase signal strength in both Rx and Tx directions via the use of external antennas (if possible).

For example, I have a cell modem device with a pair of built-in trace antennas (not shown in the picture) and a pair of CRC9 sockets for external ones:

crc9 sockets

If I just plug in the cables without any soldering, how many antennas (per channel) would be in use and why?

Another case is the modem with PCB trace antennas and small pigtail sockets:

pigtail sockets

Are these small sockets used for the same purpose as above, i.e. connecting external antennas? Should I again just plug in a pair of cables with appropriate sockets?

Should I isolate the PCB-trace antennas in any way for proper external usage for any of these cases?

I suppose there would be 2 antennas (per channel: PCB trace and external one) connected in parallel in both cases. Am I correct? How does this kind of scheme work in general?

  • \$\begingroup\$ Schematics are required to properly answer this question without guesswork or opinions. \$\endgroup\$
    – Andy aka
    Commented Sep 5, 2022 at 11:49
  • \$\begingroup\$ Does the datasheet suggest removing some component, such as a 0 ohm resistor, before using an external antenna? \$\endgroup\$ Commented Sep 5, 2022 at 17:11

1 Answer 1


There's microwave switches. SoCs for highly integrated wireless devices often integrate these so you can have multiple antenna paths, but you can buy them as external switch ICs as well. "Checking which antenna gives the best channel, using that" is a common techique in transceivers anyway (it's actually a diversity method – selection combining).

But I could even imagine that these U.FL sockets are mechanically designed to be switching between different paths.

  • \$\begingroup\$ Yes. The best laptops have dual antenna for diversity selection in the common case of dropouts from Ricean multipath cancellation. But it's possible the external antenna is a 3G 4G LTE Dipole Antenna Wide Band 7dBi 698-2700Mhz Omni Directional GSM \$\endgroup\$ Commented Sep 5, 2022 at 12:04
  • \$\begingroup\$ yep, and you could just use the same switch to switch between internal and external antenna port. If no external antenna is connected, well, that channel's always going to be bad. \$\endgroup\$ Commented Sep 5, 2022 at 12:06
  • \$\begingroup\$ So is it correct that in case of hardware path switching only a single path can be active at the moment; but the SoC switching case is more "dynamic", depends on chip program and potentially allows simulatinous antenna paths usage? \$\endgroup\$
    – Boris
    Commented Sep 5, 2022 at 16:53
  • \$\begingroup\$ this is all speculative. Maybe the device also just lives with the mismatch. \$\endgroup\$ Commented Sep 5, 2022 at 16:57

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