I am just trying to find info about connecting an external antenna to the board. Is it possible to use some of the pins to solder to the coaxial cable and connect to the external antenna?
I want to use an antenna: 3db wifi antenna
If anyone can guide me please, that would be wonderful. What pins should I use and what code in micro python can I use for that purpose?
You can't!
There may be unofficial kludges, but unless the device was designed and certified with an external antenna would be unsupported (if not illegal).
The previous answers are a bit ridiculous. You would NOT need to get a Pi Pico 'recertified' if you modded it to allow the use of an external antenna. You can swap external antennas on your wifi hotspot or router, you can add a 'cantenna' or yagi to your router or the wifi card in your laptop. As long as you don't amplify the signal electronically over the FCC limits (see https://www.air802.com/fcc-rules-and-regulations.html) you would be hard pressed to exceed allowable EIRP. A 3db gain antenna is certainly not going to be a problem when added to the Pico with its max radio transmission wattage of around 125mw.
Replacing the onboard antenna by cutting the feed trace and soldering on a coax connector would require some fine soldering skills. They are very tiny, very fiddly, and the plastic melts VERY easily (https://www.digikey.com/en/products/detail/te-connectivity-amp-connectors/1909763-1/4729711). I would advise just cutting the u.FL connector off the end of the cable of your new antenna, stripping it back 1/8" or so, and soldering the center and shield directly to the feed and ground on the board, as close as possible to the radio chip. to the board. Also, this will disable to onboard antenna.
Changing only the antenna just dictates the shape of the transmission pattern. With higher gain antennas the signal will be more directional (stronger transmission and more sensitive receive in some directions, less in others). The total radiated power will be the same. So, pay attention to antenna placement for best effect.
The Pico W uses a separate chip to handle the rf portion of the wifi function: the CYW43439 wireless chip made by Infineon. Please refer to Infineon's website for the CYW43439 datasheet that has details re antenna connections and rf circuitry. NB that the CYW43439 does not include an antenna, but the RPi documentation informs us that the on-board antenna is licensed through ABRACON.
All of that said, WiFi antennas are mostly commodity items, and replacing the ABRACON-designed unit with an external antenna is certainly within the realm of possibility. Before undertaking such a modification however, you should realize that the Pico W's certification is based on the use of the standard, on-board antenna; meaning that you cannot legally sell (or even use in some cases) a modified Pico W without going through another certification procedure. Check with your local bureaucracy for details :)
