I have recently started to see cloud disconnections when data quality drops to 25-29%. I am using an external antenna and selecting use of external antenna only in the firmware. Current PCB antenna used is a TE Connectivity 2118309 with a 20cm pigtail. Due to positioning of the Photon on the product would like to consider using a similar antenna but with a long tail that could be positioned in a more suitable location - what is the longest tail that can be used or would a stick antenna perform better.
There isn't a set maximum length to the antenna cable, though it's best to stick with the cable that the antenna came with and not extend it. If the antenna comes with a long antenna cable, it was presumably designed to use it, and that does not depend on the Particle device.
I do use the duck-style dipole antennas on Wi-Fi devices and they work well. I use the kind where you panel mount the SMA connector on the enclosure and the antenna screws onto it. A SMA to U.FL pigtail connects to the Particle device inside the box.
Of course all of these are not valid with the Particle certification, so you'd need to undergo intentional and unintentional radiator certification to use these antennas in most cases.
I just wanted to say that there are u.fl cable extenders on ebay and elsewhere. This does not solve your certification problem, of course.