The iPhone 4 'death grip' (aka 'antennagate') antenna degradation effect is real and plastic cases don't help, or so say UK researchers from University of Bristol. You hold a handset (any handset), and you affect the signal.
This from the statement accompanying the research:
The results from the study indicate a 100-fold reduction in sensitivity of the device when held, or when the user's thumb is mimicked by phantom material. This de-tuning of the antenna was found not to significantly alter the shape of the radiation pattern, but dramatically worsened the electrical match between the antenna and the electronic circuitry.
What's worse are the findings that a plastic case may not help alleviate the effect:
Further tests concluded that providing a gap between the antenna surface and the phantom thumb using a layer of plastic electrical insulator did not restore the matching and operational sensitivity of the phone for the antennas under evaluation. Thus, some phone covers in the market place may not improve the situation.
Let's also play fair here. While the iPhone 4 was the handset that made 'Death Grip and 'Antennagate' household words, other handset are affected in similar ways - basically the human hand is a good attenuator, and the more of the antenna you cover up, the worse the problem becomes.
As Scotty would say, "You cannae change the laws of physics!"
It's worth noting that this research only examined the effect of the hand on external antennas and NOT the issue of the two elements of the antenna being bridged by by the user's hand, something that is alleviated by having the handset inside a case.
[UPDATE: I approached Prof Mark Beech for some clarification as to how the testing was done. Here's what I received back:
2005 consisting only of 4 slot antennas to investigate the complex interactions between the wireless channel, the antennas and interaction with the user. This 'MIMO enabled handset' was then used with our channel sounder to look at MIMO specific channel parameters.
We conducted further analysis, including measurements in our anechoic and application of the phantom thumb, following media interest in Smart Phone Wireless Connectivity summer 2010.
We did NOT test an iPhone4 or other products currently in the marketplace.
Note: If you're subscriber to the IEEE journal Antennas and Wireless Propagation Letters then you can find the research paper here.