http://www.anandtech.com/show/3794/t...one-4-review/2

1) Cupping tightly - This is the absolute worst case and involves
squeezing the phone very tightly, like people are doing online in
videos demonstrating all the bars going away. I squeeze the phone hard
and make sure my palms are sweaty as well. You'd never hold the phone
this way because it's physically painful.
2) Holding naturally, comfortably - This is just how one would hold the
phone typically in a relaxed way. Not squeezing it to purposefully
reduce signal, but making contact with the fingers and not an open
palm.
3) Resting atop an open, flat palm.
4) Holding naturally, but inside a case - In this situation the Bumper
for iPhone 4, an Otter Box for the 3GS, and a comparable generic case
for the Nexus One.
5) Pinching the top and bottom - Our baseline, virtually no
attenuation. Held only to keep the exact position constant. It's not
reported since this is considered ideal.
Signal Attenuation Comparison in dB - Lower is Better
*
Cupping Tightly Holding Naturally On an Open Palm Holding Naturally
Inside Case
iPhone 4
24.6 19.8 9.2 7.2
iPhone 3GS
14.3 1.9 0.2 3.2
HTC Nexus One
17.7 10.7 6.7 7.7
It's difficult to be exact about the data, since signal is very
sensitive to direction, ambient conditions, and cell breathing. To
generate these numbers, I measured at least 6 times and took the
average. The results are pretty self explanatory. Inside a case, the
iPhone 4 performs slightly better than the Nexus One. However,
attenuation gets measurably worse depending how you hold the phone.
Squeezing it really tightly, you can drop as much as 24 dB. Holding it
naturally, I measured an average drop of 20 dB.*
The drop in signal from cupping the device with a case on is purely a
function of us being "ugly bags of mostly water." A material which
happens to be pretty good at attenuating RF - thus increasing path loss
between the handset and cellular base station. There's nothing Apple
nor anyone else can do to get around physics, plain and simple. It's
something which demonstrably affects every phone's cellular reception.
Add in an external antenna you're essentially forced to touch and
bridge to another adjacent antenna while holding, and the signal
attenuation is even worse. The fact of the matter is that either the
most sensitive region of the antenna should have an insulative coating,
or everyone should use a case. For a company that uses style heavily as
a selling point, the latter isn't an option. And the former would
require an unprecedented admission of fault on Apple's part.
That's not all there is to the story, however.

--
Best Regards, Keith
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