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In my laptop, the WIFI card was connected to the 2 antennas by IPEX MHF4 connections, which look like tiny coax connectors. Unfortunately the antenna connectors got broken, so I have removed them and soldered the antenna wires directly onto the WIFI card connectors (1/2 in the picture.)

It worked, but the WIFI signal strength now is much worse. I can rule out that the wires got mixed or cold solder joints.

Why that loss of signal strength? Is that because the impedance no longer matches (guess it is 50 Ohms?)?

If it is a problem with the impedance, could I buy a preconfigured cable with IPEX 4 connectors, cut the cable off after a few centimeters and solder it to the antenna cable? I would not like to replace the whole antenna because it requires me to disassemble sensitive parts of my laptop.

Is the length of the antenna cable relevant (must it have an exact length?)?

By the way, do the inner and outer pin of that connector carry the same signal, or is that shielded? The antenna cable is just 1 wire, so not shielded, but just to make sure - or is the outer ring connected to ground?

WIFI card

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    \$\begingroup\$ You'll have to post a picture of your work, details matter on how the coax is connected to the board. Generally, the center conductor carries the RF signal and the braid is shield and the return path for the RF current (gross simplification, it is a coaxial transmission line). Some uFL connectors have switch contacts on the center pin, so there can be 4 pads on the PCB (shield, shield, signal in, signal bypass) signal in -> signal bypass when nothing is plugged in. This is only some uFL connectors through. \$\endgroup\$
    – Dean Franks
    Commented Apr 19, 2021 at 2:10
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    \$\begingroup\$ "The antenna cable is just 1 wire, so not shielded" - umm, are you really sure about that? I think you need to take a closer look with some magnification... \$\endgroup\$
    – brhans
    Commented Apr 19, 2021 at 3:12
  • \$\begingroup\$ I will check the cables again, but I am almost 100% sure it is just 1 wire, unshielded. If I had seen a shielded wire it would have been instantly clear to me that the connector has shield/signal pins. Will crosscheck. \$\endgroup\$
    – Horst Walter
    Commented Apr 19, 2021 at 10:36

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It is quite normal since the 2.4GHz is… very sensitive to layout and changes of impedance.

First of all the uFL connector is horribly small but it always has two conductor, in the form of a microcoax cable. It is extremely small but there is a distinction between the center and the shielding.

The other issue is that the connector is precisely manufactured to keep a 50 ohm impedance at some frequency range. We are talking about micron precision manufacturing. Even if you successfully solder both the signal and the return conductors (good luck with that given the size of the connector pads) it will be probably not a 50 ohm impedance connection, and that will degrade the performance.

The correct repair would be to replace the broken uFL with another one and replug the antenna cable. If the pads are damaged the repair is of course a lot more complex.

By the way: the coax length is not critical, it just need to match the 50 ohm system.

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  • \$\begingroup\$ Thanks, as you pointed out, then my wiring is wrong. Strange that I still receive a signal (though weaker) as I have shorted the pins. \$\endgroup\$
    – Horst Walter
    Commented Apr 19, 2021 at 10:43

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