I'm currently developing an internet of things device that utilizes a uBlox SARA-U260 gsm/3G modem.
In field testing, we have had significant software/firmware issues due to poor cell phone reception in our deployments in sub-saharan Africa.
I'm having a hard time duplicating the problems in the lab because the cell reception here is too good. Even if there is no antenna plugged into the modem, it is still able to connect to the cell network internet!
So, I'm looking for the best way to thwart my modem into thinking it has bad reception.
One thought I had was to cut the coax cable of the antenna and solder a resistor between the gnd shroud and the center conductor. Would this work? What size resistor would make sense? Probably a fairly low-value resistor (say 50 ohm?)
I'm confused about why the modem can still connect even without an antenna. I thought that during normal operation the modem must short the ground and center conductor when it is transmitting (hence the high current associated with a transmission burst).
Under normal circumstances, is the short locate inside the antenna? If so, wouldn't the center conductor of the coax always be at ground potential (i.e. during reception)?
Thanks for the replies. I have a big metal box sitting on my desk ready to mount the DUT (thanks for that acronym @Ali Chen ) tomorrow!