GSM Shield causing unexpected behavior

Recently I got an Arduino Uno R3 with a GSM Shield(this one:

Off my Arduino I was getting a good reading off of an analog source. I put the shield on top, but still turned off and I still get a good reading. I turn on the shield and the analog readings become incorrect. Is there anything about this shield that I need to turn on or off to make the analog readings work right?

This is entirely expected behaviour (!)

You have a radio transmitter (GSM module) right next to a sensitive analog circuit - this means you are injecting RFI (radio frequency interference) into the analog circuit. You have to keep the radio emissions away from your analog circuit - this means shielding and/or distance. You also need to pay close attention to layout within your analog circuit to make it less susceptible to RFI - this is rather a complex concept to get across in a short post though.

Briefly you should try and arrange your analog circuit on a ground plane, and ensure there are no loops (ie run a signal wire right next to its ground return - preferably in a twisted pair or as a shielded coax cable).

If the GSM shield can use a remote aerial this can help. You ideally want the entire circuit in a metal box with only the aerial sticking out.

Thanks for the response. The antenna doesn't seem to solve my issue, though I did find that the shield had it's own port for analog communication elsewhere, I plug into that one and it reads just fine now!

RFI can be elusive to deal with - some setups work nicely, others have a lot of interference, its all due to the complexities of antenna design (in fact, accidental antennas can be present in a particular piece of equipment by chance, which resonate at the particular RF frequency of interest. Sounds like you have a solution though :)

Incidentally GSM phones are particularly bad at generating interference because they use time-division multiplexing, sending pulsed packet streams which cause morse-code like interference on many audio devices.