Apple iPhone 4's signal problem explainedThis article contains unofficial information.
A test was set up with the Apple iPhone 4, the 3GS and the Nexus One. Each phone was held in one of 4 different ways. The phones were cupped tightly, held naturally, on an open palm, and in a case held naturally. The results showed that when the phones were cupped tightly, the iPhone 4 had the lowest signal strength with the 3GS out performing the N-One and the iPhone 4. When held naturally or held in the palm of one's hand, the results were the same with the 3GS outperforming the Nexus One by a wide margin. In a rubber case, the results were a bit different. Although the Apple iPhone 3GS again had the best signal, the iPhone 4 slightly outperformed the Nexus One. Further testing revealed that the antenna on the iPhone 4 works better than the one on the 3GS when the signal is low. Areas where a call would drop on the 3GS would find the iPhone 4 able to hold the call. With a bumper case on the new iPhone, places that were previously dead zones were able to host a call and provide data. According to the tester, this proves the superiority of the new antenna. The problem is that while a rubber case would resolve most of the issues, handing them out would be an admission of guilt by Apple and that most likely is not going to happen. iPhone 4 owners might have to reach into their wallet to buy a rubber case-they shelled out enough for the phone that adding $30 to improve reception would be well worth it.
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source: AnandTech via BGR