Ever since iPhone 4 antennagate, many iPhone users have wanted to know how good the reception of their device is. Of course, there are the standard bars at the top of the display that represent the strength of the signal. However, if the reception is not great and you're having to move your iPhone around to find a better signal, this only gives you very limited information on the actual strength. When this is the case, the trick below means you can replace the bars with decibel numbers to gain a much more accurate indication of the signal strength. Here's what you need to do to replace the bars with decibel numbers:
Step 1: Dial *3001#12345#* and press Call.
This will open the Field Test app. Here you can see information about the SIM card and the network towers in your vicinity. You will also notice that the bars have disappeared and you can now see the reception in decibels.
Step 2: Press and hold the power button until you get the option to slide to power off. Do not turn off the phone.
Step 3: Press and hold the home button until the iPhone returns to your home screen. By pressing the home button you force quit the Field Test app but you keep the decibel measurement.
You can now tap the numbers in the top right corner to switch to the bar representation of your reception. Tap again and you'll see the decibel measurement again.
So what do the numbers mean? The numbers you see represent the signal strength in decibels above a reference level of one milliwatt. The nearer the number is to zero, the better your reception is. A signal strength of -50 means that you have full reception while -130 means no reception at all. If you would like to get back to just displaying the bars, go into Field Test app and quit it by pressing the home button.