So AnandTech used a clever hack to get an iPhone 4 to report actual signal strength instead of "bars," giving some quantitative data about how bridging the antennas can negatively affect signal strength. The analysis explains why not all users are affected by the problem, and further investigation also shows that the antenna design does in fact improve reception as long as the left-bottom area isn't bridged when holding the device.
Other mobile handsets definitely have antenna attenuation problems—when cupping an iPhone 3GS or Google Nexus One in the manner that causes problems with the iPhone 4, AnandTech measured 14dB and 18dB drops, respectively. However, the problem with the iPhone 4's external antenna is worse; bridging the antennas detunes 3G reception by a full 24dB.
As long as you have a signal strong enough to show five bars on your iPhone 4—between −51dBm and −91dBm—a 24dB drop in signal strength shouldn't drop your call or data connection. However, Apple only uses a very small range of signal strength for showing four or less bars, from −91dBm down to the usable signal cutoff of −113dBm. If your iPhone displays four bars or less, a 24dB attenuation will cut the signal below the useable limit.
That's definitely a significant problem—one that Apple has so far failed to address adequately, in our opinion—but there is good news in all of this. AnandTech also discovered that the iPhone 4 can actually use signal at the lower threshold far more reliably than any previous iPhone. "It's readily apparent that the new baseband hardware is much more sensitive compared to what was in the 3GS," according to Anand Shimpi. "The difference is that reception is massively better on the iPhone 4 in actual use."
Rumors have suggested that Apple has an update to iOS 4 coming soon that may address the issue, and an e-mail from Steve Jobs to a frustrated customer seems to support that theory. A reader indicated to us via e-mail that Apple may be using digitally adjustable solid state capacitors to tune the iPhone 4's antennas, which if true would make a software fix possible.
While Apple has yet to externally admit there is a problem, the company is definitely looking to beef up its on-staff expertise in antenna engineering—ironically posting job listings for mobile antenna engineers on the same day that the iPhone 4 problems were discovered. Until Apple does offer an official fix (assuming one can be made), using some type of case, or alternately covering the gap between antennas on the bottom left corner with some type of insulating material, is the only reliable solution if you're affected by this problem.
Source: ars technica