I just started testing my M138 and not having much luck. Plugged into my laptop (so powered by USB-C), connected to the large ground plane and antenna (as supplied) with supplied SMA to UFL. Connected the GPS sticky label. When I check my RSSI (noise) it’s always between 70 and 80dB and needless to say, I can’t transmit (message remains in the output queue even after hours - and yes, I’ve checked the flyover scheduler to make sure sats are visible). I have a clear view of the sky. The docs say the noise figure should be better but how do you get it better?
It sounds like you are in a very noisy environment - electrically speaking? Could you be standing close to ‘noisy’ equipment, anything containing a motor for example (like an air conditioning unit)?
For best results, the antenna needs a clear view of the sky. And the antenna + ground plane should be at least a meter above ground level - or any solid surfaces. (Please see the recent Swarm blog post for more details: https://www.sparkfun.com/news/4860 )
Could your laptop be causing the interference? Could you try a longer USB cable so the modem and antenna are positioned away from the laptop?
I have my antenna connected to the modem via 15m of RG58 low-loss coax cable. The antenna is up on the roofline, the modem and laptop are in the warm and dry… It works well. Maybe you could try something similar?