Consumer Reports said careful testing has confirmed user reports that holding the phone over a particular spot drastically reduces the signal strength it receives.
APPLE Inc will hold a press conference on Friday to discuss the latest iPhone model amid complaints about its antenna and Consumer Reports magazine's refusal to endorse it until the problems get fixed.
Apple would not provide details on the nature of the event at the company's headquarters in Cupertino, Calif, other than to say it will involve the iPhone 4.
On Monday, Consumer Reports said careful testing has confirmed user reports that holding the phone over a particular spot drastically reduces the signal strength it receives. Covering the spot with duct tape or a case alleviates the problem.
Apple hasn't commented on Consumer Reports' finding yet. Company watchers are speculating that the company may give iPhone buyers its "Bumper" case, which normally costs $29.
The phone went on sale three weeks ago and outsold previous iPhone launches in its first three days, with 1.7 million units sold. Complaints about the signal strength soon followed.
In an early response, Apple acknowledged that holding the phone in a certain way impeded the wireless signal somewhat, but said this happens with many other phones.
It said the real reason for the apparent big drop in signal strength was that the company has been using an incorrect formula to convert signal strength into the bars displayed on the screen.
Consumer Reports said it tested other phones and found none to have significant loss of signal strength when held.
While complaints about the iPhone 4 have been driving headlines, not everyone has had such problems. Some people say they can't replicate the so-called "death grip" - and have even seen reception improve in San Francisco and other big cities where previous iPhones stuttered.
Shares of Apple fell $3.79, or 1.5 per cent, to $248.94 in midday trading on Thursday.
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