On January 26, 2010 I wrote a post here titled How To Put Your iPhone Into Field Test Mode & Compare Signal Strength Bars To Actual Signal Levels. That post received a lot of attention including a mention in The Wall Street Journal Numbers Guy Carl Bialik's blog.
The blog by Carl was titled The Case of Apple and the Mysterious Bars and focused on the antenna problems iPhone 4 users were having. Before he wrote the piece Carl and I talked - discussing antennas, signal strength and measurement. We also talked about how Apple had disabled field test mode with the release of iOS 4.0 back on June 21. This meant if you had purchased a new IPhone 4 and thought you were having antenna problems, you had no way of determining your signal strength - all you had were bars to look at.
Here’s an edited section of my January 2010 post:
The iPhone has the ability to go into something called Field Test mode. Once in this mode you can look at signal strength in both signal bars (what we are all used to seeing) and also in dBm. To put an iPhone into Field Test mode just punch in the following “number “on the phone keypad, including the "star" and "pound sign" keys:
After you punch these numbers and symbols in, hit the Call button on the keypad and you'll end up with your iPhone screen looking like this:
Take a look at where you usually see the signal bars and you'll see a negative number - this is the actual cell signal strength your phone is receiving in dBm referenced to 1 mW. In the above screen shot I'm measuring -102 dBm. Touch that signal strength number once and it toggles to the familiar signal strength bars. Touch it again and it toggles back to the signal strength number.
What constitutes a good signal? Here's some rough signal strength guidelines:
Full Signal:-70 or lower
Optimal Signal:-70 to -75
Fair Signal:-75 to -85
Poor Signal: -85 or higher
Remember, as a negative number increases in its numeric value it is actually decreasing with reference to zero. This means a -70 dBm signal is stronger than a -85 dBm signal.
This is interesting to experiment with - check your signal strength in different locations see and how it correlates to the numbers of bars you are getting.
To exit out of Field Test mode on the iPhone just hit the Home button.
On September 8, Apple released iOS 4.1 and they’ve turned Field Test mode back on. You access it exactly the same way.
Once you’ve updated your iPhone to iOS 4.1 be sure to give Field Test mode a try.