In an extraordinarily thorough review of the iPhone 4, Anandtech has gone into the reception issues of the iPhone 4’s antenna in the greatest depth yet.
Mapping the bar representation of the signal strength against the actual signal strength, Anandtech found that over 40% of the signal strength is represented by 5 bars. You need to love over 40dB of signal to go from five to four bars, but only 10db to go from four to three.
On average, the iPhone 4 loses 26.4dB of signal when tightly held in your hand, and 19.8dB of signal when held naturally… a significant jump over the 3GS when it comes to signal degradation when held, thanks to Apple’s decision to allow the iPhone 4’s antenna to come in direct contact with human skin. Since over 40dB of signal strength is measured in the fifth bar, that means that if you’re in an area of great AT&T service, you might not notice a drop at all… but it’s still happening. If you’re in an area of worse coverage, though, the signal drop will be dramatically visible.

