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An RF part of a dual antenna Wi-Fi module from Aliexpress.

enter image description here

We can see coax sockets connected in parallel to printed antennas. Two external antennas and corresponding pigtails are part of the package.

Unfortinately the vendor does not clarify the following, so let me ask experienced engineers here:

  1. If we assume the circuit is matched when no external antennas are connected, is it correct that connecting them will ruin the matching?
  2. If I connect external antennas permanently, with no intent of ever using printed ones, do I need to cut traces after the sockets to improve RF matching?
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  • \$\begingroup\$ There are RF connectors which have a feed-through connection that gets disconnected when you plug something into the socket. But I couldn't say if these ones have that feature, or if such a thing even exists in something that small. \$\endgroup\$
    – brhans
    Commented Jun 17 at 13:37
  • \$\begingroup\$ Doubt it is a switching connector. I've used MM8130-2600RA2 from Murata but I don't think they make U.FL connectors that switch like that. Looks like some fly by night engineering job. Is the vendor responsive? \$\endgroup\$
    – drkntz
    Commented Jun 17 at 19:03

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This looks like a design mistake. As you correctly noted, you can't build something where "something 50Ω connected or not" makes no difference to matching.

Your phrasing

dual antenna Wi-Fi module from Aliexpress

at least suggests you're also suspicious of the design quality.

If we assume the circuit is matched when no external antennas are connected, is it correct that connecting them will ruin the matching?

I honestly don't see how such a matching would look, but we can assume that the open connector simply acts as (bad) antenna, with a lot of reflections, and the rest of the energy leaves through the PCB antenna. That is a very optimistic assumption. More realistically, someone slapped these on the board, tried it and said "good enough".

If I connect external antennas permanently, with no intent of ever using printed ones, do I need to cut traces after the sockets to improve RF matching?

Impossible to know. Probably not. You'd be building a series capacitor of pretty undefined impedance with a cut in an antenna trace, and you'll end up sending RF energy somewhere in your board you don't want to. If it works with the external antenna plugged in, and you don't want to replace the device, leave it alone.

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  • \$\begingroup\$ Would a cut e.g. 1mm wide be a capacitor RF conductive enough to cause troubles? It will have capacitance of a tiny fraction of a picofarad, but also allow reverting with a solder blob if the performance decreases. \$\endgroup\$
    – ololo ololo
    Commented Jun 17 at 12:52
  • \$\begingroup\$ impossible to tell. You would need to measure this with the exact substrate configuration, and with the exact position of the cut – if it really ends up looking like an "open", congratulations, you've built an RF stub; that is a component. Leave it alone. \$\endgroup\$
    – Marcus Müller
    Commented Jun 17 at 13:07
  • \$\begingroup\$ Ok, much thanks. \$\endgroup\$
    – ololo ololo
    Commented Jun 17 at 13:10

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