Steve Jobs’s attempt to close the door on criticism of Apple Inc.’s latest iPhone ignited another debate: Are competing smartphones just as prone to reception problems or does Apple have a unique design flaw?

Jobs, in a news conference Friday, conceded that reception on the new iPhone 4 can be degraded by the way a user holds the device but insisted that the problem, which occurs in areas with relatively weak cellular coverage is shared by other smartphones.

His arguments were swiftly rejected by competitors, including RIM and Motorola Inc., which said they have deliberately avoided Apple’s approach of locating antennas on the phone’s edge. Some other cellular-industry veterans also called Apple’s antenna design a mistake, noting that it creates a uniquely sensitive spot on the lower left side that causes signals to degrade when touched with a hand or a finger.

“The proof of that 1 8 it being a flaw 3 8 is that it is so easily fixed,” said Martin Cooper, an engineer who helped lead Motorola’s development of handheld cellphones in the 1970s. He noted that Apple has said adding a case largely addresses the attenuation issue, and predicted the company will find a simple way to isolate antennas in future models to eliminate the problem without a case.

Antenna experts agree with Jobs’s statements that all cellphones are affected to some degree by a user’s touch. For that reason, designers typically go to considerable lengths to minimize the antenna’s contact with the hand, in some cases adding duplicate antennas to reduce the potential signal weakening that engineers call attenuation.

But some experts and competitors questioned the validity of Apple’s tests of competing products, which it posted on its website. They said the science behind the tests wasn’t clear and the visual evidence–a drop in coverage bars on the phones’ screens–isn’t very meaningful since the software that measures signals isn’t standardized.

Jason Lohn, a professor of electrical engineering at Carnegie Mellon University, said the competing phones Apple used in its videos “don’t have the antenna’s bare metal exposed to the hand, so I’d be surprised if the effect of the hand would be as pronounced as it was on the iPhone 4.”

Sanjay Jha, Motorola’s co-CEO, said Sunday his company’s tests showed that the amount radio signals decline on the iPhone 4 when a user’s hands touch the sensitive spot on the phone’s edge is significantly greater than the attenuation when other smartphones are held.

Not all experts think the iPhone 4’s design is a bad idea. Haim Harel, president and founder of wireless chip maker Magnolia Broadband, said Apple may have created a design “breakthrough” if it can perfect external antennas for cellphones.

The approach, which saves space for other functions in a phone, was also praised by Franz Birkner, CEO of antenna technology maker Rayspan Corp. “But the challenge of innovation is it inevitably comes with risk,” he said.

An Apple spokeswoman declined to comment Sunday, referring back to Jobs’s comments Friday. The CEO said the problem had been “blown out of proportion” by the media, stating that about one half of 1 percent of users had complained about it. Still, Apple offered free cases or “bumpers” to alleviate the problem.

Concern about the iPhone 4’s reception was elevated last week when Consumer Reports said it couldn’t recommend the product after conducting its own reception tests.

After Jobs’s presentation Friday, the organization called the offer of free cases “a good first step” but said it isn’t a long-term solution, and that it still didn’t recommend the phone.

Attensity360, which studies consumer sentiment on blogs and other social media, said Job’s press conference appeared to be a qualified success. In the two days after the press conference, the amount of negative conversation about the iPhone 4 on such English-language sites fell 30 percent compared with the prior seven days, the firm found.

But some consumers aren’t satisfied. Jason Wong, a 29-year-old college math lecturer in Southern California, said he plans to return his iPhone 4. “I feel it is kind of stupid to have paid $200 for a phone that I can’t hold in the natural way,” he said.

By Don Clark, Niraj Sheth, and Geoffrey A. Fowler. Joann S. Lublin contributed to this article.